Heat Energy Changes in Chemical Reactions Practice Questions

Question 1

4 marks

A student investigated the heat energy changes in four different changes. Each change was carried out in a polystyrene cup and the temperature of the mixture was measured at the start and at the end.

experiment	change	initial temperature in °C	final temperature in °C
A	potassium nitrate dissolved in water	21.0	16.5
B	dilute nitric acid added to potassium hydroxide solution	20.5	27.9
C	zinc powder added to copper sulfate solution	21.0	32.4
D	sodium sulfate solution added to barium chloride solution	20.8	21.6

(a)

Calculate the temperature change in experiment B.

temperature change = .............................................................. °C

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(b)

State which experiment is endothermic. Give a reason for your answer.

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(c)

Name the type of reaction taking place in experiment C.

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Question 2

4 marks

Calcium carbonate decomposes when it is heated strongly.

$\mathrm{CaCO_3 \rightarrow CaO + CO_2}$

This reaction is endothermic.

(a)

Which statement about an endothermic reaction is correct?

Put a cross ( ☒ ) in the box next to your answer.

☐ A The reaction takes in heat energy and the temperature of the surroundings falls.

☐ B The reaction takes in heat energy and the temperature of the surroundings rises.

☐ C The reaction gives out heat energy and the temperature of the surroundings falls.

☐ D The reaction gives out heat energy and the temperature of the surroundings rises.

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(b)

State what is meant by the term activation energy.

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(c)

Draw a labelled reaction profile for the thermal decomposition of calcium carbonate on the axes below.

Your profile should show the reactants, the products and the activation energy.

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Question 0

5 marks

Ammonia is manufactured by reacting nitrogen with hydrogen.

$\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$

The table shows some bond energies.

bond	bond energy in kJ mol $^{-1}$
N≡N	945
H–H	436
N–H	391

(a)

Calculate the energy change for this reaction.

energy change = .............................................................. kJ mol $^{-1}$

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(b)

Use your answer to part (a) to state whether this reaction is exothermic or endothermic. Give a reason for your answer.

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Question 0

5 marks

A student added an excess of magnesium powder to 50 cm $^3$ of copper sulfate solution in a polystyrene cup. The temperature of the mixture was recorded every 30 seconds.

time in s	0	30	60	90	120	150
temperature in °C	20.5	29.8	38.4	44.2	43.1	41.8

(a)

Calculate the maximum temperature rise in this experiment.

maximum temperature rise = .............................................................. °C

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(b)

Explain how the results show that the reaction between magnesium and copper sulfate solution is exothermic.

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(c)

The student repeated the experiment using 50 cm $^3$ of copper sulfate solution of half the concentration. An excess of magnesium powder was used again.

Predict the maximum temperature rise in this experiment and explain your answer.

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Question 0

6 marks

A hand warmer contains a supersaturated solution of sodium ethanoate. When it is activated, the sodium ethanoate crystallises.

A cold pack contains ammonium nitrate and a bag of water. When the bag is burst, the ammonium nitrate dissolves in the water.

The table shows the temperatures measured when each pack was activated.

pack	starting temperature in °C	final temperature in °C
hand warmer	20.4	52.1
cold pack	20.6	4.2

Compare the energy changes taking place in the hand warmer and in the cold pack.

In your answer you should refer to the data in the table, the direction of heat energy transfer, bond breaking and bond making, and the shapes of the reaction profiles for the two changes.

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Question 0

4 marks

The figure below shows the reaction profile for a reaction between two gases.

(a)

Determine the activation energy for this reaction.

activation energy = .............................................................. kJ mol $^{-1}$

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(b)

Determine the overall energy change for this reaction. Include a sign in your answer.

overall energy change = .............................................................. kJ mol $^{-1}$

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(c)

The reaction can be speeded up using a catalyst.

Draw, on the figure above, the reaction profile for this reaction when a catalyst is used.

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Question 0

6 marks

Methane reacts with chlorine in the presence of ultraviolet light.

$\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}$

In this reaction, one C–H bond and one Cl–Cl bond are broken, and one C–Cl bond and one H–Cl bond are made.

The energy change for the reaction is $-103$ kJ mol $^{-1}$ .

Some bond energies are shown below.

bond	bond energy in kJ mol $^{-1}$
C–H	413
Cl–Cl	243
H–Cl	432
C–Cl	to be calculated

(a)

Explain, in terms of bond breaking and bond making, what the energy change of $-103$ kJ mol $^{-1}$ shows about this reaction.

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(b)

Calculate the bond energy of the C–Cl bond.

bond energy of C–Cl = .............................................................. kJ mol $^{-1}$

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Question 0

6 marks

A student investigated the neutralisation of dilute hydrochloric acid by sodium hydroxide solution.

25.0 cm $^3$ of dilute hydrochloric acid at 20.0 °C was placed in a polystyrene cup. Sodium hydroxide solution, also at 20.0 °C, was added in 5.0 cm $^3$ portions. After each addition the mixture was stirred and the maximum temperature was recorded.

volume of sodium hydroxide solution added in cm $^3$	5.0	10.0	15.0	20.0	25.0	30.0
maximum temperature in °C	22.8	25.5	28.2	31.0	30.1	29.0

(a)

Calculate the greatest temperature rise obtained in this investigation.

greatest temperature rise = .............................................................. °C

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(b)

Explain the pattern in the results shown in the table.

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(c)

The temperature rises recorded by the student were lower than the true values.

Explain why, and suggest one improvement to the method.

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Key Concepts in Chemistry

States of Matter & Mixtures

Chemical Changes

Extracting Metals & Equilibria

Groups in the Periodic Table

Rates of Reaction & Energy Changes

Fuels & Earth Science

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