Determining the enthalpy change of a reaction
Aims
The purpose of this experiment is to determine the enthalpy change for the displacement reaction:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
By adding an excess of zinc powder to a known amount of copper(II) sulphate solution, and measuring the temperature change over a period of time, you can calculate the enthalpy change for the reaction.
Apparatus
Goggles
Bench mat
25cm3 pipette
Pipette filler
Polystyrene cup with lid
Weighing bottle
Spatula
Balance
Thermometer
Stop clock
Zinc powder
1.0M copper(II) sulphate solution
Methods
1. Pipette 25.0cm3 of the copper(II) sulphate solution into the polystyrene cup.
2. Weigh about 6g of zinc powder in the weighing bottle – as this is an excess, there is no need to be accurate.
3. Put the thermometer through the hole in the lid, stir, and record the temperature every half minute for 2½ minutes in the table below.
4. At precisely 3 minutes, add the zinc powder to the cup.
5. Continue stirring, and record the temperature for an additional 6 minutes in the table below.
makaeResults table with time AND temperature
Plot the temperature (vertical axis) against time (horizontal axis).
2. Extrapolate the curve back to 3.0 minutes to establish the maximum temperature rise as shown in the example below: ΔT time (minutes)temperature (°C) 0 246810
3. Calculate the enthalpy change, ΔH, for the quantities used, using the formula:
ΔH = m c ΔT where m = mass of solution (g)
c = specific heat capacity of water = 4.18 J g–1 K–1
ΔT = rise in temperature (K)
4. Calculate the enthalpy change for one mole of Zn and CuSO4(aq).
5. Calculate the maximum error for each piece of apparatus, and then the total overall apparatus error.
6. The accepted value for this reaction is –217 kJ mol–1.
Compare your result with this value by calculating the percentage error in your answer:
error = experimental value - accepted valueaccepted value x 100%
Evaluation
1. Compare your total apparatus error with your answer to part 6 above – is the apparatus error enough to account for any difference between the accepted value and your experimental value?
2. List some possible reasons for any difference between your value and the accepted value (these should not be the apparatus errors mentioned above).
3. Why do you think the temperature increases for a few readings after adding the zinc?
(Hint: the temperature does not go even higher if more zinc is used, or if the powder is finely divided).
Tuesday, April 21, 2009
Simple calorimetry to find the enthalpy of combustion of alcohols
Simple calorimetry to find the enthalpy of combustion of alcohols
Aims
You will use simple calorimetry estimate the enthalpy of combustion of an alcohol.
Apparatus
Goggles
Bench mat
Stand, boss, clamp
Thermometer
100cm3 measuring cylinder
Steel can
Digital balance
Access to spirit burners containing:
methanol, ethanol,
propanol or butanol
Method
1. Draw up a suitable table or tables to record your results.
2. Measure 100cm3 of water in the measuring cylinder.
Pour the water into the steel can and record its temperature.
3. Choose a spirit burner.
Record the name of the fuel, and the mass of the whole burner (including the lid and fuel inside).
4. Clamp the steel can, and set it up so that the spirit burner will fit comfortably under it.
5. Light the wick of the spirit burner, and put it under the steel can.
6. Stir the water gently with the thermometer, and watch the temperature.
When it has increased by 20°C, put the lid on the spirit burner to put the flame out.
Record the new mass of the whole burner (including the lid and fuel inside).
7. Using fresh water each time, repeat the experiment at least twice with the same fuel.
Analysis
1. Calculate the energy transferred to the water using the equation q = mcΔT
Assume that 1cm3 of water has a mass of 1g and c = 4.18 J g–1 K–1.
2. For each replicate experiment, perform the calculations described below:
a) Calculate the mass of fuel burnt.
b) Calculate the Mr of the fuel used. Use your answer to part a) to work out the amount of fuel burnt.
c) Work out the energy transferred to the water in kJ mol–1, and so the enthalpy of combustion.
3. Estimate the maximum errors in the using each piece of apparatus, and the total apparatus error.
Aims
You will use simple calorimetry estimate the enthalpy of combustion of an alcohol.
Apparatus
Goggles
Bench mat
Stand, boss, clamp
Thermometer
100cm3 measuring cylinder
Steel can
Digital balance
Access to spirit burners containing:
methanol, ethanol,
propanol or butanol
Method
1. Draw up a suitable table or tables to record your results.
2. Measure 100cm3 of water in the measuring cylinder.
Pour the water into the steel can and record its temperature.
3. Choose a spirit burner.
Record the name of the fuel, and the mass of the whole burner (including the lid and fuel inside).
4. Clamp the steel can, and set it up so that the spirit burner will fit comfortably under it.
5. Light the wick of the spirit burner, and put it under the steel can.
6. Stir the water gently with the thermometer, and watch the temperature.
When it has increased by 20°C, put the lid on the spirit burner to put the flame out.
Record the new mass of the whole burner (including the lid and fuel inside).
7. Using fresh water each time, repeat the experiment at least twice with the same fuel.
Analysis
1. Calculate the energy transferred to the water using the equation q = mcΔT
Assume that 1cm3 of water has a mass of 1g and c = 4.18 J g–1 K–1.
2. For each replicate experiment, perform the calculations described below:
a) Calculate the mass of fuel burnt.
b) Calculate the Mr of the fuel used. Use your answer to part a) to work out the amount of fuel burnt.
c) Work out the energy transferred to the water in kJ mol–1, and so the enthalpy of combustion.
3. Estimate the maximum errors in the using each piece of apparatus, and the total apparatus error.
Thermometric titration
Thermometric titration
Your task
You will use thermometric titration to determine the concentration of hydrochloric acid. Neutralisation is an exothermic reaction and the maximum temperature is reached at the end-point.
To gain full marks, you should:
• complete the experiment without guidance
(but remember – always ask for help if you need it);
• work carefully and safely, making accurate measurements and detailed observations; and
• record all your results clearly in an appropriate way.
Use the following reagents in your experiment:
• 1.000M sodium hydroxide solution
• approx. 2M hydrochloric acid
Method
1. Read through the Methods, then construct suitable blank tables for your Results.
2. Transfer 50cm3 of sodium hydroxide solution to a polystyrene cup.
Allow it to stand for a few minutes, then record the temperature of the solution.
3. Add 5.0cm3 of hydrochloric acid from a burette to the cup.
Immediately stir the mixture with the thermometer and record its temperature.
Repeat until you have added a total of 50.0cm3 of acid.
Analysis
1. Plot a graph of temperature (vertical axis) against total volume of acid added (horizontal axis). Total volume of acid added (cm3)Temperature (OC)
Draw straight lines of best fit and extend them until they cross (see diagram right).
The point at which the two lines meet corresponds to the volume of acid needed for neutralisation and to the maximum temperature.
2. Use information from your graph to calculate the concentration of the acid.
Evaluation
Comment on your results, their accuracy, and the likely sources of error in the experiment. Consider the limitations of the experiment, and possible improvements to it.
Thermometric titration
Technician’s Notes
Per pupil:
1 x polystyrene cup
1 x burette and stand
1 x 25cm3 pipette
1 x pipette filler
1 x plastic filter funnel
1 x thermometer
Per class:
sodium hydroxide solution: 1.000M (allow about 200cm3 per student)
hydrochloric acid: 2M approx. (allow about 150cm3 per student)
ethanoic acid: 2M approx. (allow about 150cm3 per student)
*Health and Safety Notes
Hydrochloric acid -Corrosive.
Refer to Hazcards for correct method to prepare the 2M (approx.) solution.
Ethanoic acid
Corrosive.
Harmful vapour.
Refer to Hazcards for correct method to prepare the 2M (approx.) solution.
Sodium hydroxide solid and solutions
Sodium hydroxide is very caustic and forms strongly alkaline solutions.
Exercise care in handling - wear gloves and eye protection.
If spilt, wash with a lot of water.
Your task
You will use thermometric titration to determine the concentration of hydrochloric acid. Neutralisation is an exothermic reaction and the maximum temperature is reached at the end-point.
To gain full marks, you should:
• complete the experiment without guidance
(but remember – always ask for help if you need it);
• work carefully and safely, making accurate measurements and detailed observations; and
• record all your results clearly in an appropriate way.
Use the following reagents in your experiment:
• 1.000M sodium hydroxide solution
• approx. 2M hydrochloric acid
Method
1. Read through the Methods, then construct suitable blank tables for your Results.
2. Transfer 50cm3 of sodium hydroxide solution to a polystyrene cup.
Allow it to stand for a few minutes, then record the temperature of the solution.
3. Add 5.0cm3 of hydrochloric acid from a burette to the cup.
Immediately stir the mixture with the thermometer and record its temperature.
Repeat until you have added a total of 50.0cm3 of acid.
Analysis
1. Plot a graph of temperature (vertical axis) against total volume of acid added (horizontal axis). Total volume of acid added (cm3)Temperature (OC)
Draw straight lines of best fit and extend them until they cross (see diagram right).
The point at which the two lines meet corresponds to the volume of acid needed for neutralisation and to the maximum temperature.
2. Use information from your graph to calculate the concentration of the acid.
Evaluation
Comment on your results, their accuracy, and the likely sources of error in the experiment. Consider the limitations of the experiment, and possible improvements to it.
Thermometric titration
Technician’s Notes
Per pupil:
1 x polystyrene cup
1 x burette and stand
1 x 25cm3 pipette
1 x pipette filler
1 x plastic filter funnel
1 x thermometer
Per class:
sodium hydroxide solution: 1.000M (allow about 200cm3 per student)
hydrochloric acid: 2M approx. (allow about 150cm3 per student)
ethanoic acid: 2M approx. (allow about 150cm3 per student)
*Health and Safety Notes
Hydrochloric acid -Corrosive.
Refer to Hazcards for correct method to prepare the 2M (approx.) solution.
Ethanoic acid
Corrosive.
Harmful vapour.
Refer to Hazcards for correct method to prepare the 2M (approx.) solution.
Sodium hydroxide solid and solutions
Sodium hydroxide is very caustic and forms strongly alkaline solutions.
Exercise care in handling - wear gloves and eye protection.
If spilt, wash with a lot of water.
Volumetric Analysis 3 To determine the relative molecular mass of a soluble base
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Introduction
In Volumetric Analysis 1 & 2 you prepared a standard solution of sodium carbonate and used it to standardise an unknown concentration of dilute hydrochloric acid. In this practical you will use your new-found skills to find out the relative molecular mass of an unknown group 1 carbonate – the mysterious "Substance Z". Group 1 carbonates are soluble in water (although Li2CO3 is only sparingly soluble) and will react with dilute hydrochloric acid according to the overall equation below:
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
If you know the amount of hydrochloric acid that will react with a known amount of Substance Z, you should be able to determine the Mr of Substance Z and so identify the group 1 element in it.
You will need to make careful notes about your experiment as you go along today.
Apparatus
Consult your notes from Volumetric Analysis 1 & 2 to decide upon the apparatus you need.
Make sure that your practical write-up includes the apparatus you use today.
Method
Consult your notes from Volumetric Analysis 1 & 2 and The Burette to remind yourself of the procedures needed for safe and accurate working.
Make sure that your practical write-up includes the methods you use today.
1. Weigh out accurately between 1.3g and 1.7g of Substance Z.
Record your weighings in a suitable form. Dissolve your weighed Substance Z in de-ionised water, and make up the solution to 250cm3 in a volumetric flask.
2. Clean your burette with de-ionised water and then with the standard 0.100M hydrochloric acid to be used for the titration.
3. Pipette 25cm3 of the Substance Z solution into a clean conical flask.
Using methyl orange indicator, titrate with the standard hydrochloric acid.
4. Repeat step 3 until concordant results are obtained.
Record your results as in Volumetric Analysis 2.
After cleaning and clearing away, determine the identity of Substance Z as described overleaf.
Copyright © 2003 Nigel Saunders N-ch1-37 Copyright © 2003 Nigel Saunders N-ch1-37
Analysis
As in Volumetric Analysis 2, 1 mole of X2CO3 will react with 2 moles of HCl (see equation below):
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
1. Calculate the number of moles of HCl there were in your mean titre.
2. Calculate the number of moles of HCl that would react with the entire 250cm3 of Substance Z solution.
3. Work out the number of moles of X2CO3 were there in the 250cm3 of Substance Z solution.
You now know:
•the mass of X2CO3 in your Substance Z solution; and
•the number of moles of X2CO3 in your Substance Z solution.
4. Calculate the mass of one mole of X2CO3.
5. What is Substance Z, and why? Copyright © 2003 Nigel Saunders N-ch1-37
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Technician's Notes
Prior to practical
Sodium carbonate*
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110oC for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "Substance Z - Harmful".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Burettes
Please check the cleanliness and correct functioning of the burettes.
Per class
Sodium carbonate solid (see above). Allow approx. 2.5g per student.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
0.100M hydrochloric acid* ( a good home-made solution should suffice for this practical).
Allow 200cm3 per student.
Methyl orange indicator solution (the more bottles the better).
Per student
(Normally found in lab anyway)
1 x pair of safety goggles
1 x bench mat
2 x 100cm3 beaker
2 x 250cm3 beaker
1 x 250cm3 conical flask
1 x glass funnel (check that it will enter the neck of the volumetric flask easily) Copyright © 2003 Nigel Saunders N-ch1-37
Per student
(Additional apparatus to put out)
1 x glass rod (long)
1 x 250cm3 volumetric flask with stopper to fit
1 x 25cm3 bulb pipette
1 x pipette filler (check correct functioning)
1 x burette (see overleaf)
1 x burette stand
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x small spatula
1 x white tile
1 x small plastic filter funnel
1 x copy of N-ch1-37 (student guide to practical)
*Health and Safety Notes
Hydrochloric acid
Corrosive.
Use pre-prepared standard solution, or refer to Hazcards for correct method to prepare an accurate 0.100M solution.
Sodium carbonate (solutions and solid)
Sodium carbonate solutions are alkaline and therefore caustic.
Exercise care in handling - wear eye protection and, if spilt, wash with a lot of water.
To determine the relative molecular mass of a soluble base
Introduction
In Volumetric Analysis 1 & 2 you prepared a standard solution of sodium carbonate and used it to standardise an unknown concentration of dilute hydrochloric acid. In this practical you will use your new-found skills to find out the relative molecular mass of an unknown group 1 carbonate – the mysterious "Substance Z". Group 1 carbonates are soluble in water (although Li2CO3 is only sparingly soluble) and will react with dilute hydrochloric acid according to the overall equation below:
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
If you know the amount of hydrochloric acid that will react with a known amount of Substance Z, you should be able to determine the Mr of Substance Z and so identify the group 1 element in it.
You will need to make careful notes about your experiment as you go along today.
Apparatus
Consult your notes from Volumetric Analysis 1 & 2 to decide upon the apparatus you need.
Make sure that your practical write-up includes the apparatus you use today.
Method
Consult your notes from Volumetric Analysis 1 & 2 and The Burette to remind yourself of the procedures needed for safe and accurate working.
Make sure that your practical write-up includes the methods you use today.
1. Weigh out accurately between 1.3g and 1.7g of Substance Z.
Record your weighings in a suitable form. Dissolve your weighed Substance Z in de-ionised water, and make up the solution to 250cm3 in a volumetric flask.
2. Clean your burette with de-ionised water and then with the standard 0.100M hydrochloric acid to be used for the titration.
3. Pipette 25cm3 of the Substance Z solution into a clean conical flask.
Using methyl orange indicator, titrate with the standard hydrochloric acid.
4. Repeat step 3 until concordant results are obtained.
Record your results as in Volumetric Analysis 2.
After cleaning and clearing away, determine the identity of Substance Z as described overleaf.
Copyright © 2003 Nigel Saunders N-ch1-37 Copyright © 2003 Nigel Saunders N-ch1-37
Analysis
As in Volumetric Analysis 2, 1 mole of X2CO3 will react with 2 moles of HCl (see equation below):
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
1. Calculate the number of moles of HCl there were in your mean titre.
2. Calculate the number of moles of HCl that would react with the entire 250cm3 of Substance Z solution.
3. Work out the number of moles of X2CO3 were there in the 250cm3 of Substance Z solution.
You now know:
•the mass of X2CO3 in your Substance Z solution; and
•the number of moles of X2CO3 in your Substance Z solution.
4. Calculate the mass of one mole of X2CO3.
5. What is Substance Z, and why? Copyright © 2003 Nigel Saunders N-ch1-37
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Technician's Notes
Prior to practical
Sodium carbonate*
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110oC for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "Substance Z - Harmful".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Burettes
Please check the cleanliness and correct functioning of the burettes.
Per class
Sodium carbonate solid (see above). Allow approx. 2.5g per student.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
0.100M hydrochloric acid* ( a good home-made solution should suffice for this practical).
Allow 200cm3 per student.
Methyl orange indicator solution (the more bottles the better).
Per student
(Normally found in lab anyway)
1 x pair of safety goggles
1 x bench mat
2 x 100cm3 beaker
2 x 250cm3 beaker
1 x 250cm3 conical flask
1 x glass funnel (check that it will enter the neck of the volumetric flask easily) Copyright © 2003 Nigel Saunders N-ch1-37
Per student
(Additional apparatus to put out)
1 x glass rod (long)
1 x 250cm3 volumetric flask with stopper to fit
1 x 25cm3 bulb pipette
1 x pipette filler (check correct functioning)
1 x burette (see overleaf)
1 x burette stand
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x small spatula
1 x white tile
1 x small plastic filter funnel
1 x copy of N-ch1-37 (student guide to practical)
*Health and Safety Notes
Hydrochloric acid
Corrosive.
Use pre-prepared standard solution, or refer to Hazcards for correct method to prepare an accurate 0.100M solution.
Sodium carbonate (solutions and solid)
Sodium carbonate solutions are alkaline and therefore caustic.
Exercise care in handling - wear eye protection and, if spilt, wash with a lot of water.
Volumetric Analysis 2
Volumetric Analysis 2
To standardise hydrochloric acid
Introduction
In the last practical you prepared a standard solution of sodium carbonate.
Today, you will use it to find the concentration of dilute hydrochloric acid by titration.
This process is known as standardising the hydrochloric acid.
The reaction between sodium carbonate and hydrochloric acid takes place in two stages:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq) (1)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) (2)
Two indicators are needed to cover both stages:
•in stage 1, phenolphthalein is most suitable, and will respond to the pH change associated with the formation of sodium hydrogencarbonate, NaHCO3.
•in stage 2, methyl orange is most suitable, and will respond to the pH change associated with the final formation of sodium chloride, NaCl.
As a result, this practical gives you experience of titration using two different indicators (the phenolphthalein colour change is easy to spot, whereas the methyl orange colour change is quite difficult to judge).
Apparatus
Goggles
Bench mat
100cm3 beaker
250cm3 beaker
250cm3 conical flask
25cm3 bulb pipette
Pipette filler
Burette
Burette stand and holder
Plastic filter funnel
White tile
Teat pipette
Access to:
your standard sodium carbonate solution
dilute hydrochloric acid to standardise
phenolphthalein indicator solution
methyl orange indicator solution
Methods
1. Transfer a 25cm3 aliquot (portion) of your sodium carbonate solution to a 250cm3 capacity conical flask. Add a few drops of phenolphthalein indicator solution.
2. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from pink to colourless. Note the titre, then add a few drops of methyl orange.
3. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from yellow to red. Note the second titre.
4. Repeat steps 1 - 3 until concordance (i.e. until the readings are the same or within 0.1cm3).
Tabulate your titrations as described in The Burette sheet. You will need two sets of tables.
5. After tidying away, do the calculations described overleaf.
Copyright © 2003 Nigel Saunders N-ch1-36 N-ch1-36 (N.S. 2003)
Calculations
1. Calculate the Mr of Na2CO3.
Ar (Na) = 23 Ar (C) = 12 Ar (O) = 16
2. Look back at the accurate mass of sodium carbonate you used in the last practical.
Using your answer to step 1, calculate the number of moles of Na2CO3 that you dissolved in 250cm3 of water during Volumetric Analysis 1.
3. Use your answer to step 2 to calculate the number of moles of Na2CO3 in the 25cm3 transferred to the conical flask.
Stage 1 Phenolphthalein results
4. The equation for the first stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
From the equation, you can see that 1 mole of Na2CO3 will react with 1 mole of HCl.
How many moles of HCl will react with the number of moles of Na2CO3 calculated in step 3?
5. The answer to step 4 tells you how many moles of HCl were in your first mean titre.
Divide this number by the volume of the first mean titre: this is the concentration of HCl in mol dm−3.
Stage 2 Methyl orange results
6. The equation for the second stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
From the equation, you can see that 1 mole of NaHCO3 will react with 1 mole of HCl.
The number of moles of NaHCO3 is equal to the number of moles of Na2CO3.
How many moles of HCl will react with the number of moles of NaHCO3 calculated in step 3?
7. The answer to step 6 tells you how many moles of HCl were in your second mean titre.
Divide this number by the volume of the second mean titre: this is also the concentration of HCl.
8. Your answers to steps 5 and 7 should be identical. Comment on your findings. N-ch1-36 (N.S. 2003)
Volumetric Analysis 2
To standardise hydrochloric acid
Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results
1. Phenolphthalein indicator (first part of each run)
Burette reagent
Approx. 0.075M hydrochloric acid
Conical flask reagent
Standard sodium carbonate solution
Indicator
Phenolphthalein
To standardise hydrochloric acid
Introduction
In the last practical you prepared a standard solution of sodium carbonate.
Today, you will use it to find the concentration of dilute hydrochloric acid by titration.
This process is known as standardising the hydrochloric acid.
The reaction between sodium carbonate and hydrochloric acid takes place in two stages:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq) (1)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) (2)
Two indicators are needed to cover both stages:
•in stage 1, phenolphthalein is most suitable, and will respond to the pH change associated with the formation of sodium hydrogencarbonate, NaHCO3.
•in stage 2, methyl orange is most suitable, and will respond to the pH change associated with the final formation of sodium chloride, NaCl.
As a result, this practical gives you experience of titration using two different indicators (the phenolphthalein colour change is easy to spot, whereas the methyl orange colour change is quite difficult to judge).
Apparatus
Goggles
Bench mat
100cm3 beaker
250cm3 beaker
250cm3 conical flask
25cm3 bulb pipette
Pipette filler
Burette
Burette stand and holder
Plastic filter funnel
White tile
Teat pipette
Access to:
your standard sodium carbonate solution
dilute hydrochloric acid to standardise
phenolphthalein indicator solution
methyl orange indicator solution
Methods
1. Transfer a 25cm3 aliquot (portion) of your sodium carbonate solution to a 250cm3 capacity conical flask. Add a few drops of phenolphthalein indicator solution.
2. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from pink to colourless. Note the titre, then add a few drops of methyl orange.
3. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from yellow to red. Note the second titre.
4. Repeat steps 1 - 3 until concordance (i.e. until the readings are the same or within 0.1cm3).
Tabulate your titrations as described in The Burette sheet. You will need two sets of tables.
5. After tidying away, do the calculations described overleaf.
Copyright © 2003 Nigel Saunders N-ch1-36 N-ch1-36 (N.S. 2003)
Calculations
1. Calculate the Mr of Na2CO3.
Ar (Na) = 23 Ar (C) = 12 Ar (O) = 16
2. Look back at the accurate mass of sodium carbonate you used in the last practical.
Using your answer to step 1, calculate the number of moles of Na2CO3 that you dissolved in 250cm3 of water during Volumetric Analysis 1.
3. Use your answer to step 2 to calculate the number of moles of Na2CO3 in the 25cm3 transferred to the conical flask.
Stage 1 Phenolphthalein results
4. The equation for the first stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
From the equation, you can see that 1 mole of Na2CO3 will react with 1 mole of HCl.
How many moles of HCl will react with the number of moles of Na2CO3 calculated in step 3?
5. The answer to step 4 tells you how many moles of HCl were in your first mean titre.
Divide this number by the volume of the first mean titre: this is the concentration of HCl in mol dm−3.
Stage 2 Methyl orange results
6. The equation for the second stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
From the equation, you can see that 1 mole of NaHCO3 will react with 1 mole of HCl.
The number of moles of NaHCO3 is equal to the number of moles of Na2CO3.
How many moles of HCl will react with the number of moles of NaHCO3 calculated in step 3?
7. The answer to step 6 tells you how many moles of HCl were in your second mean titre.
Divide this number by the volume of the second mean titre: this is also the concentration of HCl.
8. Your answers to steps 5 and 7 should be identical. Comment on your findings. N-ch1-36 (N.S. 2003)
Volumetric Analysis 2
To standardise hydrochloric acid
Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results
1. Phenolphthalein indicator (first part of each run)
Burette reagent
Approx. 0.075M hydrochloric acid
Conical flask reagent
Standard sodium carbonate solution
Indicator
Phenolphthalein
Volumetric Analysis 1
Volumetric Analysis 1
To make a standard solution of sodium carbonate
Introduction
A standard solution is one whose concentration is known exactly. Standard solutions of liquids, for example acids, are easy to prepare and are usually supplied. Standard solutions of solids can be prepared by weighing a mass of solid, and dissolving it in a known volume of solution in a volumetric flask. Today, you are going to prepare a standard solution of sodium carbonate to use later in another practical.
Apparatus
Goggles
Bench mat
100 cm3 beaker
250 cm3 beaker
250 cm3 volumetric flask with stopper
Filter funnel
Glass rod
Teat pipette
Spatula
Label
De-ionised water
Anhydrous sodium carbonate, Na2CO3(s) Copyright © Nigel Saunders N-ch1-35
Methods
Read through the Methods. Make a suitable blank results table, complete with units in the headings.
1. Using the ± 0.1g balance, weigh approximately between 1.2g and 1.4g of sodium carbonate into the small beaker. Do not record the mass.
2. Using the ± 0.01g balance, weigh the small beaker and its contents accurately.
Record this mass.
3. Transfer the contents of the small beaker into the large beaker.
Weigh the small beaker again using the ± 0.01g balance.
Record this mass.
The difference between the two accurate masses is the mass of sodium carbonate in your beaker.
4. Add de-ionised water cautiously down the side of the large beaker.
Use about 150cm3 of water, and swirl the beaker to mix the contents.
5. Stir using a glass rod to dissolve the solid completely.
6. Transfer the solution into the volumetric flask using the funnel.
Remember: pour down the glass rod;
remove the last drop of solution from the glass rod onto the funnel.
Wash the beaker, rod and funnel several times using de-ionised water from the wash bottle, letting the washings go into the flask.
7. Make up to the mark on the volumetric flask with de-ionised water.
Stopper firmly, and shake the flask thoroughly to mix the contents.
8. Label the flask clearly with your name, the date, and the contents of the flask. Copyright © 2003 N. Saunders N-ch1-35
Volumetric Analysis 1
To make a standard solution of sodium carbonate:
Technician's Notes
Prior to practical
Sodium carbonate
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110°C for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "sodium carbonate".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Requirements per class
Sodium carbonate solid (see above). Minimum of 3g per student approx.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
Requirements per student
1 x 250cm3 beaker (dry)
1 x 250cm3 volumetric flask with stopper to fit
1 x glass funnel (check that it will enter the neck of the volumetric flask easily)
1 x glass rod
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x teat pipette
1 x small spatula
1 x self-adhesive label
To make a standard solution of sodium carbonate
Introduction
A standard solution is one whose concentration is known exactly. Standard solutions of liquids, for example acids, are easy to prepare and are usually supplied. Standard solutions of solids can be prepared by weighing a mass of solid, and dissolving it in a known volume of solution in a volumetric flask. Today, you are going to prepare a standard solution of sodium carbonate to use later in another practical.
Apparatus
Goggles
Bench mat
100 cm3 beaker
250 cm3 beaker
250 cm3 volumetric flask with stopper
Filter funnel
Glass rod
Teat pipette
Spatula
Label
De-ionised water
Anhydrous sodium carbonate, Na2CO3(s) Copyright © Nigel Saunders N-ch1-35
Methods
Read through the Methods. Make a suitable blank results table, complete with units in the headings.
1. Using the ± 0.1g balance, weigh approximately between 1.2g and 1.4g of sodium carbonate into the small beaker. Do not record the mass.
2. Using the ± 0.01g balance, weigh the small beaker and its contents accurately.
Record this mass.
3. Transfer the contents of the small beaker into the large beaker.
Weigh the small beaker again using the ± 0.01g balance.
Record this mass.
The difference between the two accurate masses is the mass of sodium carbonate in your beaker.
4. Add de-ionised water cautiously down the side of the large beaker.
Use about 150cm3 of water, and swirl the beaker to mix the contents.
5. Stir using a glass rod to dissolve the solid completely.
6. Transfer the solution into the volumetric flask using the funnel.
Remember: pour down the glass rod;
remove the last drop of solution from the glass rod onto the funnel.
Wash the beaker, rod and funnel several times using de-ionised water from the wash bottle, letting the washings go into the flask.
7. Make up to the mark on the volumetric flask with de-ionised water.
Stopper firmly, and shake the flask thoroughly to mix the contents.
8. Label the flask clearly with your name, the date, and the contents of the flask. Copyright © 2003 N. Saunders N-ch1-35
Volumetric Analysis 1
To make a standard solution of sodium carbonate:
Technician's Notes
Prior to practical
Sodium carbonate
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110°C for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "sodium carbonate".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Requirements per class
Sodium carbonate solid (see above). Minimum of 3g per student approx.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
Requirements per student
1 x 250cm3 beaker (dry)
1 x 250cm3 volumetric flask with stopper to fit
1 x glass funnel (check that it will enter the neck of the volumetric flask easily)
1 x glass rod
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x teat pipette
1 x small spatula
1 x self-adhesive label
Estimating Errors in Chemistry
Copyright © 2003 Nigel Saunders N-ch1-24 Estimating Errors in Chemistry
Introduction
You must be able to estimate the size and importance of errors in practical work to gain full marks in your practical assessments. The table summarises what you need to do about errors in each Skill:
Skill
You must be able to:
Skill P
Planning
Organise the procedure to be followed, selecting appropriate techniques, reagents and apparatus, with due regard to precision of measurement and scale of working.
Skill I
Implementing
Make and record measurements to a degree of precision allowed by the apparatus used.
Skill A
Analysing
Identify sources of error, and recognise the limitations of experimental measurements.
Skill E
Evaluating
Assess the reliability of your data and the conclusions drawn from them, taking into account the errors in the data obtained.
Introduction
You must be able to estimate the size and importance of errors in practical work to gain full marks in your practical assessments. The table summarises what you need to do about errors in each Skill:
Skill
You must be able to:
Skill P
Planning
Organise the procedure to be followed, selecting appropriate techniques, reagents and apparatus, with due regard to precision of measurement and scale of working.
Skill I
Implementing
Make and record measurements to a degree of precision allowed by the apparatus used.
Skill A
Analysing
Identify sources of error, and recognise the limitations of experimental measurements.
Skill E
Evaluating
Assess the reliability of your data and the conclusions drawn from them, taking into account the errors in the data obtained.
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