GCSEChemistry: Combined ScienceOCR GatewayTopic Questions5. Monitoring & Controlling Chemical Reactions5.1 Monitoring Chemical Reactions

Monitoring Chemical Reactions (OCR Gateway GCSE Chemistry: Combined Science)

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1
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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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2a5 marks

A student investigates the rate of reaction between dilute hydrochloric acid and magnesium carbonate.

She wants to find out how the concentration of the acid changes the rate of reaction.

Look at the apparatus she uses.

q19-paper-4-nov-2020-ocr-gcse-chem

Carbon dioxide gas is given off in the reaction.

The student measures the loss in mass every 30 seconds for 5 minutes.

a)
Another student also investigates the rate of reaction between dilute hydrochloric acid and magnesium carbonate.

He also wants to find out how the concentration of the acid changes the rate of reaction. He uses a different method.

Describe the method he uses.

Include a labelled diagram of the set‑up of the apparatus he uses.

[5]

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2b8 marks
b)
The students now investigate the reaction between sodium thiosulfate and dilute hydrochloric acid.

They want to find out how temperature changes the rate of reaction.

Look at the diagram of their experiment.

Add dilute hydrochloric acid and start timing
q19b-paper-4-nov-2020-ocr-gcse-chem

A yellow solid is made during the reaction.

The students time how long it takes for the cross to disappear. This is the reaction time.

Look at their results.

Temperature (°C) Reaction time (s) Rate of reaction (s−1)
10 140 7 × 10−3
20 56 2 × 10−2
30 34 3 × 10−2
40 26 4 × 10−2
50 22 5 × 10−2

i)

Plot a graph of the results on the grid. Draw a line of best fit.

q19bi-paper-4-nov-2020-ocr-gcse-chem

[4]

ii)
What happens to the rate of reaction as the temperature increases?.

[1]

iii)

Explain your answer to (b)(ii).

Use ideas about collisions between particles in your answer.

[3]

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32 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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4
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Higher Only 

a)
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]

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1a2 marks

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

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

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

[2]

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1b2 marks

Higher Only

b)

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.

[2]

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1c
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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 [2]

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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.
a)
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.
[4]

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2b
Sme Calculator2 marks
b)
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.

[2]

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3a1 mark
a)
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.
[1]

How did you do?

3b3 marks
b)

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]

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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.

a)
Explain how you can tell from the student’s observation that the hydrochloric acid was in excess.
[1]

How did you do?

4b1 mark
b)
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.
[1]

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