Chemistry 9701/53 — October/November 2014

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

The enthalpy change, $\Delta H_r$ , for the reaction between magnesium atoms and aluminium ions to produce magnesium ions and aluminium atoms cannot be determined directly. However both metals react with dilute sulfuric acid and, using the enthalpy changes for these reactions, it is possible to use Hess’ Law to determine $\Delta H_r$ .

You are to plan experiments that will enable you to determine the value of the enthalpy change for the reaction of 1 mol of magnesium with excess aqueous aluminium ions.

You are provided with:

0.30 g of magnesium ribbon,
0.30 g of aluminium foil,
1.00 mol dm $^{-3}$ sulfuric acid,
standard laboratory equipment.

(a)

• Write an equation for the reaction of magnesium and sulfuric acid.

• Write an equation for the reaction of aluminium and sulfuric acid.

(b)

For the experiment to measure the enthalpy change when magnesium reacts with sulfuric acid name:

(i)

the independent variable,

(ii)

the dependent variable.

(c)

In the experiment, 20.0 cm $^3$ of 1.00 mol dm $^{-3}$ sulfuric acid is used with 0.30 g of magnesium.

(i)

Calculate the minimum volume in cm $^3$ of sulfuric acid required to react completely with 0.30 g of magnesium.
[ $A_r$ : Mg, 24.3]

(ii)

Suggest why 20.0 cm $^3$ of sulfuric acid is used.

(d)

You are also provided with 0.30 g of aluminium.
Calculate the minimum volume of 1.00 mol dm $^{-3}$ sulfuric acid required to react completely with 0.30 g of aluminium.
[ $A_r$ : Al, 27.0]

(e)

Explain how you would carry out the experiment using the 0.30 g magnesium.

(i)

List the measurements you would make.

(ii)

How would you do the experiment to make sure the results obtained would be as accurate as possible?

(iii)

How would you use the measurements made in the experiment to determine the enthalpy change of the reaction between 1 mol of magnesium and sulfuric acid?

(f)

Write an ionic equation for the reaction between magnesium and aqueous aluminium ions.

If the enthalpy change of the reaction between magnesium and sulfuric acid is $\Delta H_1$ and that for aluminium and sulfuric acid is $\Delta H_2$ , use Hess’ Law to determine the enthalpy change, $\Delta H_r$ , for the reaction between magnesium and aqueous aluminium ions.

Then determine the enthalpy change for 1 mol of magnesium in terms of $\Delta H_r$ .

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