Practicals Chemistry AQA A-Level Exam with Questions and Answers – 100%
Solved
8. Measuring the EMF of a cell
Method - ✔✔✔-1. Clean a piece of copper and zinc using fine grade sandpaper.
2. Degrease the metal using cotton wool and propanone.
3. Place the copper into a 100cm3 beaker with 50cm3 of 1moldm-3 of copper sulphate
4. Place a strip of Zinc into a 100cm3 beaker with 50cm3 of 1moldm-3 of Zinc Sulphate.
5. use a strip of filter paper soaked in potassium nitrate for the salt bridge
6. connect the 2 half cells by connecting the metals to crocodile clips attached to leads joined to
the voltmeter.
9. Titration Curves - Investigating how pH changes in WA/SB and SA/WB
Method - ✔✔✔-1. Transfer 25cm3 of acid to a conical flask using a volumetric pipette.
2. measure the initial pH of the acid with a pH meter
4. Add the alkali from the burette in small amounts e.g. 2cm3 at a time. Noting the volume
added.
Stir mixture to equalise the pH
5. measure and record the pH to one d.p.
,Practicals Chemistry AQA A-Level Exam with Questions and Answers – 100%
Solved
6. when approaching end point add alkali in smaller volumes, drop by drop until the alkali is in
excess.
During reflux what precautions do you take? - ✔✔✔-NEVER SEAL THE CONDENSER - build up of
pressure from gas could cause apparatus to break and explode.
Anti-bumping granular added in distillation and reflux to prevent uneven boiling and stop the
liquid hitting the sides of the flask - smaller bubbles instead of larger bubbles.
Errors in the method
Why would the enthalpy change be different to that in the reaction? - ✔✔✔-Heat loss/heat
gain from the surroundings - temp change lower/higher
Energy can be absorbed by the apparatus.
incomplete reaction/ slow reaction.
Errors in this reaction: CALORIMETRY COMBUSTION - ✔✔✔-Heat loss to the surroundings- heat
loss from evaporation of water
Incomplete combustion
Evaporation of Fuel after weighing - lowers it's mass
heat capacity of the calorimeter is not included
Errors: What does the method assume about the solutions? - ✔✔✔-The solutions have the
same specific heat capacity as water.
, Practicals Chemistry AQA A-Level Exam with Questions and Answers – 100%
Solved
how do you reduce percentage uncertainty in a titration? - ✔✔✔-- replacing any measuring
cylinders with pipettes and burettes which have a lower apparatus uncertainty.
- increase the vol and conc. of the substance in the conical flask
- decrease the concentration of the sol in the burette so a larger volume will be needed.
How else can you improve accuracy of a acid base titration? pH curves - ✔✔✔-By maintaining a
constant temperature.
Measuring Enthalpy Change - Combustion - flame calorimetry - ✔✔✔-1. measure the mass of
the spirit burner containing the alcohol/fuel
2. Measure a volume of water e.g. 50cm3 and add it to a copper container- calorimeter.
3. Measure the initial temperature of the water
4. burn the spirit burner under the calorimeter until a specific temperature has been reached
e.g. a temp change of 50 degrees Celsius.
stirring the water to keep the temp increase uniform.
5. Once this temp has been reached, extinguish the spirit burner and measure it's final mass-
use this to workout the mass lost.
6. use this mass to work out how much mass of alcohol reacted- and then use q=mc∆T.
Solved
8. Measuring the EMF of a cell
Method - ✔✔✔-1. Clean a piece of copper and zinc using fine grade sandpaper.
2. Degrease the metal using cotton wool and propanone.
3. Place the copper into a 100cm3 beaker with 50cm3 of 1moldm-3 of copper sulphate
4. Place a strip of Zinc into a 100cm3 beaker with 50cm3 of 1moldm-3 of Zinc Sulphate.
5. use a strip of filter paper soaked in potassium nitrate for the salt bridge
6. connect the 2 half cells by connecting the metals to crocodile clips attached to leads joined to
the voltmeter.
9. Titration Curves - Investigating how pH changes in WA/SB and SA/WB
Method - ✔✔✔-1. Transfer 25cm3 of acid to a conical flask using a volumetric pipette.
2. measure the initial pH of the acid with a pH meter
4. Add the alkali from the burette in small amounts e.g. 2cm3 at a time. Noting the volume
added.
Stir mixture to equalise the pH
5. measure and record the pH to one d.p.
,Practicals Chemistry AQA A-Level Exam with Questions and Answers – 100%
Solved
6. when approaching end point add alkali in smaller volumes, drop by drop until the alkali is in
excess.
During reflux what precautions do you take? - ✔✔✔-NEVER SEAL THE CONDENSER - build up of
pressure from gas could cause apparatus to break and explode.
Anti-bumping granular added in distillation and reflux to prevent uneven boiling and stop the
liquid hitting the sides of the flask - smaller bubbles instead of larger bubbles.
Errors in the method
Why would the enthalpy change be different to that in the reaction? - ✔✔✔-Heat loss/heat
gain from the surroundings - temp change lower/higher
Energy can be absorbed by the apparatus.
incomplete reaction/ slow reaction.
Errors in this reaction: CALORIMETRY COMBUSTION - ✔✔✔-Heat loss to the surroundings- heat
loss from evaporation of water
Incomplete combustion
Evaporation of Fuel after weighing - lowers it's mass
heat capacity of the calorimeter is not included
Errors: What does the method assume about the solutions? - ✔✔✔-The solutions have the
same specific heat capacity as water.
, Practicals Chemistry AQA A-Level Exam with Questions and Answers – 100%
Solved
how do you reduce percentage uncertainty in a titration? - ✔✔✔-- replacing any measuring
cylinders with pipettes and burettes which have a lower apparatus uncertainty.
- increase the vol and conc. of the substance in the conical flask
- decrease the concentration of the sol in the burette so a larger volume will be needed.
How else can you improve accuracy of a acid base titration? pH curves - ✔✔✔-By maintaining a
constant temperature.
Measuring Enthalpy Change - Combustion - flame calorimetry - ✔✔✔-1. measure the mass of
the spirit burner containing the alcohol/fuel
2. Measure a volume of water e.g. 50cm3 and add it to a copper container- calorimeter.
3. Measure the initial temperature of the water
4. burn the spirit burner under the calorimeter until a specific temperature has been reached
e.g. a temp change of 50 degrees Celsius.
stirring the water to keep the temp increase uniform.
5. Once this temp has been reached, extinguish the spirit burner and measure it's final mass-
use this to workout the mass lost.
6. use this mass to work out how much mass of alcohol reacted- and then use q=mc∆T.
