OCR Gateway GCSE Chemistry

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GCSEChemistryOCR GatewayTopic Questions5. Monitoring & Controlling Chemical Reactions5.1 Monitoring Chemical Reactions

5.1 Monitoring Chemical Reactions

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1a
Sme Calculator2 marks

A student does three titrations with dilute hydrochloric acid and potassium hydroxide solution.

Hydrochloric acid neutralises the alkali potassium hydroxide.

HCl (aq) + KOH(aq) → KCl (aq) + H2O(l)

Look at the apparatus she uses.

q19-paper-4-specimen-2016-ocr-gcse-chem

Look at the diagrams. They show parts of the burette during the first titration.   

q19-1-paper-4-specimen-2016-ocr-gcse-chem

Here is the student’s results table.

Titration number 1 2 3
Final reading (cm3)   37.5 32.1
Initial reading (cm3)   20.4 15.0
Titre (volume of acid added) (cm3)   17.1 17.1

a)
Using the diagrams and table, calculate the mean titre.

Explain your answer.

Answer = …………………………………. cm3

[2]

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1b
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b)
The student uses 25.0 cm3 of potassium hydroxide solution, KOH.
 
She also uses hydrochloric acid with a concentration of 0.100 mol / dm3.
 
Calculate the concentration, in mol / dm3, of the KOH (aq).
 
Answer = …………………………………. mol / dm3
 
[2]

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1c
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c)

   Use your answer to (b) to calculate the concentration of the KOH (aq) in g / dm3.

Answer = …………………………………. g / dm3
 
[2]

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2a2 marks

A student neutralises potassium hydroxide with dilute sulfuric acid in a titration experiment.

a)
Look at the student’s method for her experiment.
q21a-paper-4-nov-2020-ocr-gcse-chem

  • Measure 25.0 cm3 of 0.200 mol / dm3 potassium hydroxide into a conical flask using a pipette.
  • Add a few drops of universal indicator to the potassium hydroxide.
  • Fill the burette to above the 0.00 cm3 line with dilute sulfuric acid.
  • Quickly add the dilute sulfuric acid to the potassium hydroxide until the indicator changes colour.
  • Repeat the experiment.

    Describe and explain one improvement the student should make to her method to get a more accurate titration result.

[2]

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2b
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b)
The student repeats the experiment four times.

Look at the student’s results.

Titration number 1 2 3 4
Volume of acid (cm3) 25.75 23.60 23.70 23.65

i)
Calculate the accurate volume of the acid that reacts with the alkali.
Accurate volume of acid = .................................................. cm3 [2]

ii)
Look at the equation for the reaction between sulfuric acid and potassium hydroxide.

H2SO4 + 2KOH → K2SO4 + 2H2O

Use your answer from part (b)(i) to calculate the concentration of the dilute sulfuric acid,

H2SO4, that reacted with the 25.0 cm3 of 0.200 mol / dm3 potassium hydroxide.

Give your answer to 3 significant figures.

Concentration of dilute sulfuric acid = .......................................... mol / dm3 [4]

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3a
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a)
In an experiment, a mixture of ammonium chloride and calcium hydroxide is heated.

Ammonia gas, NH3, is made.

2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O

A student adds 5.00 g of ammonium chloride to an excess of calcium hydroxide.

Calculate the maximum volume of ammonia gas that could be made at room temperature and pressure.

One mole of a gas occupies 24 dm3 at room temperature and pressure.

Volume of ammonia gas = .................................................. dm3
[2]

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3b
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b)
In another experiment a student reacts sodium hydroxide solution with dilute hydrochloric acid.

NaOH + HCl → NaCl + H2O

i)
35.0 cm3 of 0.075 mol/dm3 hydrochloric acid, HCl, are added to 25.0 cm3 of 0.100 mol/dm3 sodium hydroxide solution, NaOH.

Use the information to determine which reactant is in excess.
[3]

ii)
To find the exact amount of dilute hydrochloric acid that reacts with 25.0 cm3 of the sodium hydroxide solution, the student does a titration.

Look at the student’s results. The rough titration is not shown.

  Titration 1 Titration 2 Titration 3 Titration 4
Final burette reading (cm3) 36.30 38.60 39.25 38.30
Initial burette reading (cm3) 0.00 2.80 4.05 2.10
Volume of acid used (cm3) 36.30 35.80 35.20 36.20

Use the student’s concordant results to calculate the mean volume of hydrochloric acid required.

Mean volume = .................................................. cm3

[2]

How did you do?

3c4 marks

Higher Only 

c)
In another titration 25.0 cm3 of potassium hydroxide solution, KOH, are titrated with 0.200 mol/dm3 sulfuric acid, H2SO4.

2KOH + H2SO4 → K2SO4 + 2H2O

24.80 cm3 of sulfuric acid are needed to neutralise 25.0 cm3 of the potassium hydroxide solution.

Calculate the concentration of the potassium hydroxide solution in mol/dm3.

Concentration = .......................................... mol/dm3

[4]

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1a2 marks

A student neutralises an alkali with an acid in a titration experiment.

Complete the word equation for the reaction of an acid with an alkali.

acid + alkali → ............................... + ...............................

How did you do?

1b2 marks

Higher Only

Look at the student’s method for her experiment.

q20b-paper-2-nov-2020-ocr-gcse-chem

• Measure 25.0 cm3 of alkali solution into a conical flask using a pipette.
• Add a few drops of universal indicator to the alkali solution.
• Fill the burette to above the 0.0 cm3 line with acid.
• Quickly add the acid to alkali until the indicator changes colour.
• Repeat the experiment until consistent results are obtained.

Describe and explain one improvement the student should make to her method to get a more accurate titration result.

How did you do?

1c
Sme Calculator2 marks

The student repeats the experiment three times.

Look at the student’s results.

Titration number 1 2 3
Volume of acid (cm3) 25.75 23.60 23.70


Calculate the accurate volume of the acid that reacts with the alkali.

Accurate volume of acid = .................................................. cm3 

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2a4 marks

Higher Only

A student does a titration with an acid and an alkali.

He uses dilute sulfuric acid, sodium hydroxide solution and an indicator solution.

q21-paper-2-june-2018-ocr-gcse-chem

The student’s method is:

  • Use a measuring cylinder to pour 25.0 cm3 of sodium hydroxide solution into a conical flask
  • Add a few drops of an indicator to the sodium hydroxide solution
  • Use a burette to add dilute sulfuric acid to the sodium hydroxide solution until the indicator changes colour.

The student wants to get a more accurate value for how much acid reacts with 25.0 cm3 of sodium hydroxide solution.

Describe and explain how the student could improve his experiment to get a more accurate value.

How did you do?

2b
Sme Calculator2 marks

Another student does a titration. She also uses dilute sulfuric acid, sodium hydroxide solution and an indicator solution.

The table shows her results.

Titration number 1 2 3 4
Volume of acid (cm3) 26.4 25.2 25.6 25.4

The student decides that the best value for the mean (average) volume of acid is 25.4 cm3.

Show how she calculated this value.

How did you do?

2c
Sme Calculator3 marks

The equation for this reaction is 

H2SO4 + 2NaOH → Na2SO4 + 2H2O

  Relative formula mass, Mr
H2SO4 98
NaOH 40
Na2SO4 142
H2O 18

Water is a waste product in this reaction.

Calculate the atom economy for the reaction.

Give your answer to 1 decimal place.

Answer = .....................................

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3a2 marks

A student does three titrations with dilute hydrochloric acid and potassium hydroxide solution.

Look at the apparatus she uses.

17a

She uses a pipette to measure out the 25.0 cm3 of potassium hydroxide solution.

q17a-paper-2-specimen-2016-ocr-gcse-chem

Describe and explain one safety precaution that she should use with the pipette.

How did you do?

3b1 mark

In her first titration the student measures the initial volume of hydrochloric acid in the burette.

She slowly adds the acid until the potassium hydroxide is just neutralised.

She then measures the volume of the hydrochloric acid again.

Describe how she can tell when the potassium hydroxide solution is just neutralised.

How did you do?

3c3 marks

Look at the diagrams. They show parts of the burette during the first titration.

q17c-paper-2-specimen-2016-ocr-gcse-chem

Here is the student’s results table.

Titration number 1 2 3
final reading in cm3   37.5 32.1
initial reading in cm3   20.4 15.0
titre (volume of acid added) in cm3   17.1 17.1

i)

Complete the table by recording the burette readings from the diagrams.

[2]

ii)
The student thinks the mean titre is 17.1 cm3.

Is she correct?

Explain your answer.
[1]

How did you do?

3d
Sme Calculator2 marks

The student does another titration to make a fertiliser called potassium nitrate, KNO3.

Look at the equation for the reaction she uses. 

KOH + HNO3 → KNO3 + H2O
 

The relative formula masses, Mr, of each compound are shown in the table.

Compound Formula Relative formula mass
potassium hydroxide KOH 56.1
nitric acid HNO3 63.0
potassium nitrate KNO3 101.1
water H2O 18.0

What is the atom economy for the reaction to make potassium nitrate?

Assume that water is a waste product

Answer = ………………………………… %

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4a1 mark

A student added 2.4 g of magnesium to hydrochloric acid. She observed that no magnesium was left when the reaction was complete.

The student transferred the solution to an evaporating basin. She heated the solution using a Bunsen burner and evaporated all the water.

Explain how you can tell from the student’s observation that the hydrochloric acid was in excess.

How did you do?

4b1 mark

Look at the equation for the reaction.

Mg + 2HCl rightwards arrowMgCl2 + H2

The student knows the reaction is complete when there is no magnesium left.

Use the equation to explain one other way the student could tell that the reaction was complete.

How did you do?

4c
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The student predicts she should make 9.5 g of magnesium chloride, MgCl2.

She actually makes 7.9 g.

Calculate the percentage yield.

Give your answer to 3 significant figures.

Answer = ..................................... 

How did you do?

4d1 mark

Write down one reason, other than a mistake, why the student may have obtained a percentage yield of less than 100%.

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5a4 marks

Student A does a titration with an acid and an alkali.

He uses dilute sulfuric acid, sodium hydroxide solution and an indicator solution.

The diagram shows the apparatus he uses.

q20-paper-4-june-2018-ocr-gcse-chem

The student adds sodium hydroxide solution from the burette to the sulfuric acid until the indicator changes colour.

He then adds a few more drops of sodium hydroxide to be certain the sulfuric acid is neutralised.

He takes the final volume reading on the burette to find out how much acid reacts with 25.0 cm3 of sodium hydroxide solution.

Describe and explain how the student could improve his experiment to get a more accurate value.

How did you do?

5b1 mark

Student B does a titration.

q20b-paper-4-june-2018-ocr-gcse-chem


Sodium hydroxide solution is slowly added to the beaker of dilute sulfuric acid.

The pH probe is connected to a datalogger.

Suggest how student B’s method is better than student A’s.

How did you do?

5c2 marks

Higher Only

Look at the display from the datalogger.

q20c-paper-4-june-2018-ocr-gcse-chem

i)
What is the pH value when 15 cm3 of alkali has been added?


Answer = .....................................

[1]

ii)

What volume of alkali is needed to exactly neutralise the sulfuric acid?

Answer = ..................................... cm3

[1]

How did you do?

5d
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Student B does another experiment.

This time she uses:
• 20.0 cm3 of dilute hydrochloric acid in the beaker
• sodium hydroxide solution of concentration 0.200 mol / dm3 in the burette.

Look at student B’s results.

Titration number 1 2 3 4
Final burette reading (cm3) 26.9 27.6 27.0 28.2
Initial burette reading (cm3) 0.5 2.5 1.2 3.2
Titre (volume of alkali used) (cm3) 26.4 25.1 25.8 25.0

i)
Student B decides to only use the results from titration numbers 2 and 4.

Explain why.

[1]

ii)
Look at the equation for the reaction between hydrochloric acid, HCl, and sodium hydroxide, NaOH.
 
HCl + NaOH → NaCl + H2O
 
Calculate the concentration of hydrochloric acid in mol / dm3.

Use the average titre, in cm3, from titration numbers 2 and 4.

Give your answer to 2 significant figures.


Answer = ..................................... mol / dm3

[4]

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6a
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A student dissolves 0.6 g of zinc sulfate in 250 cm3 of water.

i)
Calculate the volume of the water in dm3.

Answer = ..................................... dm3

[1]

ii)
Use your answer to part (a)(i) to help you calculate the concentration of the zinc sulfate in g / dm3.

Answer = ..................................... g / dm3

[1]

How did you do?

6b
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Zinc reacts with sulfuric acid. Zinc sulfate and hydrogen gas, H2, are made.

 
Zn + H2SO4 → ZnSO4 + H2

i)
Calculate the amount of hydrogen gas, in mol, that could be made from 3.27 g of zinc.

Answer = ..................................... mol

[2]

ii)
Use your answer to part (b)(i) to calculate the volume of hydrogen gas produced at room temperature and pressure.

One mole of any gas occupies 24 dm3 at room temperature and pressure.

Answer = ..................................... dm3

[2]

How did you do?

6c
Sme Calculator6 marks

Hydrogen can be made by reacting methane with steam.

 
CH4 + H2O → CO + 3H2
 

The atom economy for this process is 17.6%.

Hydrogen can also be produced by the decomposition of ammonia.

This reaction requires a catalyst.

 
2NH3 → N2 + 3H2

i)
Calculate the atom economy for the production of hydrogen from ammonia.

Give your answer to 3 significant figures.

Answer = ..................................... %

[3]

ii)
Suggest other factors, apart from atom economy, that must be considered when deciding which reaction pathway to choose for the manufacture of hydrogen.
[3]

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