Heat Energy Changes in Chemical Reactions (Edexcel GCSE Chemistry)

When solid ammonium chloride is shaken with water, a colourless solution forms and the temperature changes from 20°C to 16°C.

Give the name of the type of heat change occurring.

A student carries out an experiment to measure accurately the temperature changes when different metals are added to iron(II) sulfate solution. The method for the experiment is:

• measure 25cm³ of iron(II) sulfate solution and pour into a container

• record the initial temperature of the solution

• add excess magnesium ribbon

• record the highest temperature of the mixture

• repeat the experiment using excess copper turnings, then using excess zinc foil.

i) State a suitable container for the iron(II) sulfate solution in this experiment.

(1)

ii) State what the student should do to the mixtures during the experiment.

(1)

iii) Figure 5 shows the results obtained by the student.

Figure 5

Explain the order of reactivity of the metals magnesium, copper and zinc using the results.

(2)

iv) Explain how the student could improve the method to make a fairer comparison of the temperature change produced by the different metals.

(2)

v) The iron(II) sulfate solution contained 6.2 g of iron(II) sulfate in 50 cm³ of solution. Calculate the concentration of the iron(II) sulfate solution in g dm^–3.

(2)

concentration = .............................................................. g dm^–3

Which statement describes activation energy?

Higher Only

Each of these substances forms ions in solution.

One mole of the following substances is dissolved in 1 dm³ of water. Which solution contains the greatest number of ions?

When sodium hydroxide solution is neutralised with an acid there is a temperature change.

A student is given dilute hydrochloric acid and dilute ethanoic acid of the same concentration in mol dm⁻³.

Devise a plan to compare the temperature changes produced when sodium hydroxide solution is neutralised with each of these two acids.

Hydrogen reacts with chlorine to form hydrogen chloride.

H₂ (g) + Cl₂ (g) → 2HCl (g)

The reaction is exothermic. Draw and label the reaction profile diagram for this reaction, identifying the activation energy.

Higher Only

The energies of some bonds are shown in Figure 13.

Figure 13

Hydrogen reacts with chlorine to form hydrogen chloride.

H₂ (g) + Cl₂ (g) → 2HCl (g)

Calculate the energy change, in kJ mol^–1, for the reaction of 1 mol of hydrogen gas, H₂ with 1 mol of chlorine gas, Cl₂, to form 2 mol of hydrogen chloride gas, HCl.

Higher Tier Only

Chlorine reacts with methane in sunlight.

 CH₄ + Cl₂  → CH₃Cl + HCl

The diagram below shows the displayed formulae for the reaction of chlorine with methane.

The table below shows the bond energies and the overall energy change in the reaction. 

What is the value of X?

Higher Tier Only

How much energy is needed to break all the bonds in 0.025 g of nitrogen gas?

The relative atomic mass, A_r, of N is 14.

Students are investigating exothermic and endothermic reactions. They are finding the temperature change in 50 cm³ water when a solid dissolves in it. The apparatus is shown in Figure 1.

Figure 1

The steps needed to carry out this experiment are P, Q, R, S and T. They are shown below.

They are not in the correct order.

P pour the 50 cm³ water into the polystyrene cup
Q add the solid to the water and stir
R measure 50 cm³ water using a beaker
S measure the initial temperature of the water
T measure the final temperature of the solution when all the solid has dissolved

Write the steps in the correct order, from left to right.

The dissolving of this solid in water is an exothermic change. The experiment is repeated a number of times.

Compared with the initial temperature of the water, the final temperature of the solution is

State how step R could be changed to measure the volume of water more accurately.

Figure 2 shows a cold pack.

When the pack is squeezed hard the inner bag bursts. Then the pack goes cold.

i) Explain why the pack goes cold.

(2)

ii) Give the reason why the pack can be used only once.

(1)

The graph below shows a reaction profile for the reaction between methane and oxygen. 

What does the label X represent?

Which is true about chemical energy changes?

A student poured 50 cm³ water into a beaker and measured the water’s temperature.

Figure 7

The student added 1.00g calcium chloride to the water, stirred the mixture and then recorded the temperature.

Give the name of the apparatus that could be used to measure 1.00 g of calcium chloride.

The student’s results were

      temperature of water at start       = 21°C

      temperature of mixture after stirring  = 32°C

Explain, using these results, the type of heat energy change that occurs when calcium chloride dissolves in water.

Calcium chloride is hazardous to health.

i) Which hazard symbol would be expected to be seen on a container of calcium chloride?

(1)

☐	A
☐	B
☐	C
☐	D

ii) Give a safety precaution that the student should take during the experiment.

(1)

State one way in which the apparatus could be changed to reduce the amount of heat energy lost during the experiment.

The concentration of a calcium chloride solution is 12 g dm^–3.

Calculate the volume of this solution, in cm³, that contains 9.0 g of calcium chloride.

You must show your working.

The reaction profile for making nitrogen monoxide, NO, is shown below:

What is the activation energy for this reaction?

The concentration of a solution can be calculated using the equation

A student dissolved 9.25 g of ammonium chloride in water and made up the solution to a volume of 200 cm³

Use the equation to calculate the concentration of this solution in g dm⁻³.

Dissolving ammonium chloride in water is an endothermic process. Figure 17 shows part of the reaction profile for this process.

Figure 17

i) Explain how Figure 17 shows that dissolving ammonium chloride in water is an endothermic process.

(3)

ii) Complete the reaction profile in Figure 17 and label the activation energy.

(2)

A student used the equipment in Figure 18 to investigate whether electricity can pass through solid ammonium chloride and through ammonium chloride solution.

Figure 18

If an electrical current flows in the circuit, the lamp will light up.

Figure 19 shows the results of the investigation.

Figure 19

Explain the results of the investigation.

Ammonia gas is toxic.

i) Which symbol should be placed on a container of a toxic gas?

(1)

ii) Give one safety precaution that should be taken when working with toxic gases in the laboratory.

(1)

Which statement is true about exothermic and endothermic reactions?

Higher Tier Only

Methanol is an alcohol that will undergo a combustion reaction.

The chemical equation for is:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

The equation for the reaction can be also be shown as:

Table 1 shows the bond energies of different bonds.

Table 1

Use the bond energies in Table 1 to calculate the overall energy change for the combustion of methanol.

Higher Tier Only

Methanol is the first member of the homologous series of alcohols.

i) Write balanced chemical equations for the complete combustion of one mole of the next two members of the homologous series.

(2)

ii) Using your answer to part (i) and the information in part (a), explain how the overall energy change is affected as you move through the homologous series.

(4)

Explain why, in terms of bonds breaking and forming, the combustion of methanol is an exothermic reaction.

Complete the reaction profile for the combustion of methanol shown in Figure 1. You need to label:

• The products

• The overall energy change

• The activation energy

Figure 1

Higher Only

The table below has some enthalpy data for a different chemical reaction. Hydrazine, N₂H₄ can react with hydrogen peroxide in an exothermic reaction, as shown below. 

N₂H₄ + 2H₂O₂ → N₂ + 4H₂O   ΔH = -789 kJ / mol 

The structures of the reactants and products are shown. 

Using the reaction equation and the data in the table above, calculate the value of the N-H bond in hydrazine.

Bond energy of N-H bond = .......... kJ / mol

The atoms in hydrazine are joined together by covalent bonds.

Draw a dot cross diagram for hydrazine, N₂H₄.

When nitrogen and oxygen are heated to very high temperatures, they combine to form nitrogen monoxide and a lot of heat is absorbed in this reaction. 

Complete the reaction profile. You should include labels for activation energy and overall energy change. 

Higher Only

The balanced equation for the combustion of ethene is

C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (l)

Use the information in the table to calculate the following:

i) Energy required to break the bonds

(1)

ii) Energy released when bonds are made

(1)

iii) Energy change for the reaction

(1)

Complete the reaction pathway diagram for the reaction in part (a).

Label the activation energy and the overall energy change.    

Higher Only

The reaction for the production of ethanol from ethene is shown below.

The forward reaction is exothermic. 

C₂H₄ (g) + H₂O (g) CH₃CH₂OH (g)  

In terms of temperature and pressure, explain which conditions would give the best yield of ethanol. 

During a chemical reaction, bonds are broken in the reactant molecules, and bonds are formed to make product molecules. 

Which row shows the correct energy change for each process?

Hydrogen fluoride is formed from the reaction between hydrogen and fluorine. 

H₂  +  F₂  →  2HF

Figure 1 shows the bond energies for the bonds in the three molecules.

Figure 1

i) Calculate the energy change for the reaction. 

(4)

ii) Explain whether the reaction is exothermic or endothermic. 

(1)

Catalysts are used in a variety of different reactions. 

The reaction profile for an uncatalysed exothermic reaction is shown in Figure 2. 

Figure 2

Discuss what catalysts do, and their effect on the activation energy of a reaction. 

You should use some examples of catalysts you have met in chemistry. 

You can use Figure 2 to help illustrate your answer.