Monitoring Chemical Reactions (OCR GCSE Chemistry A (Gateway))

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.

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.

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

The table shows her results.

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

Show how she calculated this value.

The equation for this reaction is 

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Water is a waste product in this reaction.

Calculate the atom economy for the reaction.

Give your answer to 1 decimal place.

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

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

Look at the apparatus she uses.

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

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

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.

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

Here is the student’s results table.

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]

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, M_r, of each compound are shown in the table.

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

Assume that water is a waste product

Answer = ………………………………… %

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.

Look at the equation for the reaction.

Mg + 2HCl MgCl₂ + 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.

The student predicts she should make 9.5 g of magnesium chloride, MgC₂.

She actually makes 7.9 g.

Calculate the percentage yield.

Give your answer to 3 significant figures.

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

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

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.

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.

Student B does a titration.

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.

Higher Only

Look at the display from the datalogger.

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]

Higher Only

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.

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]

Higher Only

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]

Higher Only

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]

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]