5.1 Exothermic & Endothermic Reactions

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.

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

Citric acid reacts with sodium hydrogencarbonate solution.

A student investigated how the temperature changed during this reaction.

The results they obtained are shown in Figure 1.

Figure 1

Describe a method for how these results could have been obtained.

An anomalous result was obtained when 1.2 g of citric acid had been added to the sodium hydrogencarbonate solution.

Suggest two reasons for the anomalous result.

Using data from Figure 1, explain how the graph shows that this is an endothermic reaction and approximately how much citric acid is needed for the reaction to reach completion.

Explain why the temperature increases when between 3.0 g and 4.0 g of citric acid is added to the sodium hydrogencarbonate solution.

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 reaction 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.

Sodium chloride dissolving in water is endothermic so the temperature of the water will decrease.

A student investigated the temperature decrease of water when different masses of sodium chloride were added to 100 cm³ of water.

Their results are shown in Table 1.

Table 1

Plot the results of Figure 1 and draw two straight lines of best fit through the points.

Estimate the temperature of the room that the investigation was carried out at.

Use the graph drawn on Figure 1 and show your working on the graph.

It is difficult to use the data in Table 1 to find the exact mass of sodium chloride that would give the minimum final temperature.

Suggest further experimental work that the student should do to make it easier to find the exact mass of sodium chloride that would give the minimum final temperature.

The student repeated the investigation but used 200 cm³ of water instead of 100 cm³.

What would you expect to happen to the minimum final temperature?

Explain your answer.

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 can react together and produce nitrogen oxide. 

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

The equation for the reaction is:     N₂ + O₂ → 2NO

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

Figure 1

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

Explain what the term activation energy means and comment on its significance 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 reaction showing clearly your working out including a balanced displayed formula equation of the reaction.

Use the bond energies to explain why this reaction is endothermic.