Chemical Reactions & Energy (WJEC GCSE Chemistry)

Skiers and mountaineers use hand warmers in very cold weather conditions. Hand warmers are small packets that produce heat when needed. Some ski clothing is specially designed to hold a hand warmer.

Table 1 describes how three types of hand warmers work.

Table 1

Table 2 shows some information about each type of hand warmer.

Table 2

[1]

Gloves need to be worn when using hand warmers                           

Boiling water is used to recharge battery powered hand warmers     

Some chemical reactions give out heat energy                                  

All hand warmers are reusable                                                           

[2]

Reason 1 ..........................................................................

Reason 2 ..........................................................................

An air-activated hand warmer contains several chemicals mixed together. One of these chemicals is iron.

When the pack is opened, a reaction takes place between iron and oxygen in the air, causing an increase in mass.

A student investigated the increase in mass of the opened pack over several hours.
The results are shown below.

[1]

2 hours

3 hours

4 hours

5 hours

[1]

Iron reacts with oxygen forming iron oxide until all the oxygen is used up
Heat formed expands the iron
Iron oxide loses oxygen, forming iron
Iron reacts with oxygen forming iron oxide until all the iron is used up

The equation shows the bonds which are broken and the bonds which are formed during the electrolysis of water.

The total energy needed to break the bonds in the reactant is 1856 kJ.

Calculate the energy needed to break one OH bond.

Energy = ............................... kJ

The total energy released when the bonds in the products are formed is 1370 kJ.

The bond energy for HH is 436 kJ.

Calculate the energy released when making one OO bond.

Energy = .............................. kJ

The energy profile diagram shows the reaction to be endothermic.

On the diagram below use the symbol (↕) to show the overall energy change for the reaction.

Ammonia is manufactured from hydrogen and nitrogen in the Haber Process.

The equation shows the bonds which are broken and the bonds which are formed during the manufacture of ammonia. 

The relevant bond energies are shown in the table.

Bond	Bond energy (kJ)
NN	945
H—H	436
N—H	391

[2]

Energy = ....................... kJ

[2]

Energy = ....................... kJ

[1]

Energy change = ....................... kJ

The manufacture of ammonia is an exothermic reaction. Put a tick (✔) in the box next to the energy profile diagram that shows an exothermic reaction.

Higher Tier Only

The table shows the percentage yield of ammonia under different pressure and temperature conditions.

Use the information in the table to answer parts (i) and (ii).

[1]

Ammonia is used in the manufacture of nitrogenous fertilisers. One example of a nitrogenous fertiliser is ammonium nitrate. Ammonium nitrate is formed by the reaction between an acid and ammonia.

ammonia + ....................  ammonium nitrate

[1]

[1]

[1]

Propane gas can be used as a fuel.

Balance the following equation that shows the combustion of propane.

C₃H₈   +   .....O₂   →   .....CO₂   +   4H₂O

Hydrogen gas can also be used as a fuel. The following equation shows what happens when it burns.

HH   HH   +   OO   →   HOH   HOH

The table below shows the bond energies involved in the reaction.

Bond	Energy (kJ)
HH	436
OO	495
HO	463

The energy required to break all the bonds in the reactants can be calculated as shown below:

   2 × HH bonds = 2 × 436 = 872

   1 × OO bond = 495

Energy required to break all the bonds in the reactants = 872 + 495 = 1367 kJ

A student investigated displacement reactions by measuring the temperature change when metals are reacted with a copper sulfate solution.

Table 1 shows the student’s results.

Table 1

Plot the data from the table above on Figure 1 as a bar chart. 

Figure 1

The student evaluated the data and came to the conclusion that the reactions between the metals and copper sulfate solution are endothermic.

Give two reasons why this conclusion is not correct.

The magnitude of the temperature change observed depends on the reactivity of the metal.

Describe a method that could determine the relative reactivity of an unknown metal compared to the four known metals, indicating how you would ensure the results are valid.

Draw and label a reaction profile for the reaction between zinc and copper sulfate solution.

Bromine reacts with methane in the presence of sunlight to produce bromomethane and hydrogen bromide.  

Figure 1 shows the displayed formulae for the reaction between methane and bromine.

Figure 1

Table 1 below shows the bond energies and the overall energy change for the reaction.

Table 1

State whether this reaction is endothermic or exothermic and explain why in terms of energy transfer.

Thunderstorms often produce lightning which can reach temperatures of up to 30,000 ^oC. 

At these high temperatures, oxygen and nitrogen in the air react together to produce nitrogen oxide. 

N₂ (g) + O₂ (g) → 2NO (g)

This is an endothermic reaction which has a very high activation energy.

The energy profile for the reaction is shown in Figure 1.

Figure 1

Explain how the energy profile shows that this reaction is endothermic.

Explain what the term activation energy means, in terms of particle theory.

The bond energies involved in the reaction are shown in the Table 1 below.

Calculate the total energy change for the combustion of nitrogen to form nitrogen oxide.

Explain, in terms of bond making and breaking, why this reaction is endothermic. 

The reaction between hydrogen and chlorine to give hydrogen chloride can be represented by the following equation.

H–H   + Cl–Cl   →   H–Cl   H–Cl

The relative amounts of energy needed to break the bonds shown are given in the table below.

Using the bond energy values in the table, calculate:

Using your answer to part (a), state whether this reaction is endothermic or exothermic and explain why in terms of bond breaking and bond making.

The equation shows the bonds which are broken and the bonds which are formed in the production of sulfur trioxide.

The bond energy of an SO bond is 523 kJ.

The total energy needed to break the bonds in the reactants is 2587 kJ. Calculate the energy needed to break an O=O bond.

Energy = .................................................... kJ

Calculate the overall energy change for the reaction.

Overall energy change =.................................................... kJ

Complete the energy profile for the reaction.

The reaction between methane and oxygen occurs as shown:

CH₄       +        2O₂       →           CO₂         +        2H₂O

      methane          oxygen             carbon dioxide           water

The structural formulae shown below more clearly represents the bonds in each molecule involved in the reaction:

In the three stages shown at (a), (b) and (c) below, calculate the net energy transfer when 1 mole of methane reacts with oxygen.

Complete the table by identifying the type and number of bonds broken and bonds formed during the reaction.

Calculate the total energy changes involved in the process of breaking and forming these bonds using the bond energies in the table.

Bonds broken = .................... kJ per mole

Bonds formed = .................... kJ per mole

Describe in as much detail as you can what the results from part (b) tell you about the overall reaction.

‘Hot cans’ are designed to heat the food inside them when it is to be eaten. The heat is generated by mixing calcium oxide with water.

The following diagram shows the cross-section of a ‘hot can’.

During a trial reaction, the temperature reached 50°C but a temperature of 70°C is required to properly heat the food.

Suggest a change that could have been made and explain how this would lead to the can reaching the higher temperature.

When chemical reactions take place bonds are broken and new bonds are formed.

Explain, in terms of bond making and breaking, why some reactions are exothermic.

Methanol is an alcohol that will undergo a combustion reaction.

The chemical equation for is:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

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.

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

Complete the energy 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

Endothermic reactions can be used in everyday life. One use is in sports injury packs.

A student looked at the reaction profiles of three different reactions to decide which reaction would be the most suitable to be used in a sports injury pack.

The reaction profiles being investigated are shown in Figure 1.

Figure 1

Suggest which of the reactions would be the most suitable for using in a sports injury pack.

Give reasons for your answer. 

The student thought that the reaction would need to have a low activation energy in order to be effective.

Define activation energy and explain how activation energy can be shown on a energy profile.

Explain why a reaction with a lower activation energy would be more suitable in a sports injury pack than a reaction with a high activation energy.

Some students did an experiment to find the temperature change when hydrochloric acid reacts with sodium hydrogencarbonate.

The results are in the table.

Sodium hydrogencarbonate is used as baking powder for making cakes. When the cake mixture is baked the sodium hydrogencarbonate decomposes.

The equation for the reaction is:
2 NaHCO₃ (s)  Na₂CO₃ (s) + H₂O (g) + CO₂ (g)

The equation for the reversible reaction of hydrogen and iodine to form hydrogen iodide is shown below.

H₂ (g) + I₂ (g)  2HI (g)

The energy profile for this reaction is shown below.

The forward reaction forms hydrogen iodide from hydrogen and iodine.

The exothermic reaction of ethene and bromine is shown below.

C₂H₄ + Br₂ → C₂H₄Br₂ 

State the colour change that would be seen during this reaction.

Draw the energy profile for the exothermic reaction of ethene and bromine. Include labelled arrows to show the overall energy change and activation energy.

The bond energies and overall energy change for the reaction of ethene and bromine are shown in the table below

Use the information to calculate the bond energy for the Br–Br bond.