Exothermic and Endothermic Reactions (HSC SSCE Chemistry): Revision Notes

Exothermic and Endothermic Reactions

Understanding energy changes in chemical reactions

Temperature changes are one of the key signs that a chemical reaction has occurred. Nearly all chemical reactions involve changes in both temperature and energy. Understanding these energy changes is crucial for chemistry students.

Energy from chemical reactions powers our daily lives. We use this energy to produce food, clothing and shelter, generate electricity, power transport, run manufacturing processes and provide entertainment. Currently, most electricity comes from burning coal, transport relies on burning petrol and diesel, and many homes burn natural gas for cooking.

Exothermic reactions

When a reaction releases heat energy to its surroundings, we call it an exothermic reaction. The word "exothermic" tells us that energy is exiting the system.

In an exothermic reaction, the test tube or container becomes hot. This happens because chemical energy decreases during the reaction, and the 'lost' chemical energy converts to heat, which warms the container and its contents.

Common examples of exothermic reactions

Combustion reactions:

• Burning of coal
• Burning of petrol

Synthesis reactions (direct combination reactions):

• $\text{Cu}$ or $\text{Fe} + \text{S}$
• $\text{H}_2 + \text{Cl}_2$
• $\text{P} + \text{Cl}_2$

Reactions of metals with water and acids:

• $\text{Li} + \text{H}_2\text{O}$
• $\text{Zn} + \text{H}_2\text{SO}_4$

Reactions of acids with bases and carbonates:

• $\text{H}_2\text{SO}_4 + \text{NaOH}$
• $\text{CaCO}_3 + \text{HCl}$

Dissolution reactions:

• Dissolving calcium chloride in water
• Dissolving sodium carbonate in water

Endothermic reactions

When a reaction absorbs heat energy from its surroundings, we call it an endothermic reaction. The word "endothermic" indicates that energy is entering the system.

In an endothermic reaction, the test tube or container becomes cold. This happens because the reaction needs to absorb heat, which gets converted into chemical energy. The reaction takes this heat from the container and its contents, causing them to cool down.

Common examples of endothermic reactions

Decomposition reactions:

• Decomposition of copper nitrate
• Decomposition of calcium carbonate

Precipitation reactions:

• $\text{MgCl}_2 + \text{Na}_2\text{CO}_3$ (some precipitation reactions)

Biological processes:

• Photosynthesis

Dissolution reactions:

• Dissolving potassium sulfate in water
• Dissolving ammonium chloride in water

Physical and chemical processes

Both exothermic and endothermic terms apply to physical processes as well as chemical reactions.

Investigation 14.1: Temperature changes in physical and chemical processes

Aim

To measure temperature changes for various chemical and physical processes and determine whether these processes are endothermic or exothermic.

Materials required

Chemicals:

• $45 \text{ mL}$ of $2 \text{ mol L}^{-1}$ $\text{HCl}$
• $3 \text{ g}$ $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$
• $5 \text{ cm}$ strip of $\text{Mg}$ ribbon
• $25 \text{ mL}$ of $2 \text{ mol L}^{-1}$ $\text{NaOH}$
• $4 \text{ g}$ $\text{NaOH}_{(s)}$
• $10 \text{ g}$ $\text{KNO}_{3(s)}$
• Water

Equipment:

• 2 medium test tubes
• 3 polystyrene (e.g. Styrofoam) cups
• $-10$ to $110°\text{C}$ thermometer or temperature probe and data logger
• $100 \text{ mL}$ measuring cylinder
• $10 \text{ mL}$ measuring cylinder
• $250 \text{ mL}$ beaker
• Stirring rod
• Bunsen burner
• Heat-proof mat
• Tripod
• Wire gauze
• Spatula
• Test-tube rack

Risk assessment

What are the risks in doing this investigation?	How can you manage these risks to stay safe?
$\text{SO}_{2(g)}$ is released in part A.	Part A must be performed in a fume cupboard. Check for students with asthma before performing the experiment.

Method

Part A: Reaction of sodium thiosulfate with hydrochloric acid

1. Pour $10 \text{ mL}$ $\text{HCl}$ into a medium test tube.
2. Measure the initial temperature of the $\text{HCl}$ using the thermometer or temperature probe.
3. Add one spatula of $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$ to the test tube.
4. Stir with the stirring rod, then record the highest or lowest temperature reached.

Part B: Reaction of magnesium with hydrochloric acid

1. Pour $10 \text{ mL}$ $\text{HCl}$ into a medium test tube.
2. Measure the initial temperature of the $\text{HCl}$ using the thermometer or temperature probe.
3. Place a $5 \text{ cm}$ strip of magnesium into the $\text{HCl}$.
4. Stir with the stirring rod, then record the highest or lowest temperature reached.

Part C: Dissolving sodium hydroxide in water

1. Pour $100 \text{ mL}$ water into a polystyrene cup.
2. Measure the initial temperature of the water using the thermometer or temperature probe.
3. Add $4 \text{ g}$ $\text{NaOH}_{(s)}$ to the polystyrene cup.
4. Stir with the stirring rod, then record the highest or lowest temperature reached.

Part D: Dissolving potassium nitrate in water

1. Pour $100 \text{ mL}$ water into a polystyrene cup.
2. Measure the initial temperature of the water using the thermometer or temperature probe.
3. Add $10 \text{ g}$ $\text{KNO}_{3(s)}$ to the polystyrene cup.
4. Stir with the stirring rod, then record the highest or lowest temperature reached.

Part E: Combustion of methane

1. Pour $100 \text{ mL}$ water into a beaker.
2. Measure the initial temperature of the water using the thermometer or temperature probe.
3. Place the beaker on wire gauze above a Bunsen burner.
4. Turn on the Bunsen burner.
5. After $5$ minutes, turn off the Bunsen burner and record the final temperature of the water.

Part F: Neutralisation of sodium hydroxide with hydrochloric acid

1. Pour $25 \text{ mL}$ of $2 \text{ mol L}^{-1}$ $\text{NaOH}$ into a polystyrene cup.
2. Measure the temperature of the $\text{NaOH}$ using the thermometer or temperature probe.
3. Measure $25 \text{ mL}$ of $2 \text{ mol L}^{-1}$ $\text{HCl}$ into a small beaker and record its temperature.
4. Calculate the average of the $\text{NaOH}$ and $\text{HCl}$ temperatures. This is the initial temperature for this experiment.
5. Pour the $\text{HCl}$ into the polystyrene cup containing the $\text{NaOH}$, stir and record the highest or lowest temperature reached.

Recording results

Create a results table to collect data about initial and final temperatures for each experiment part (A through F).

Analysis of results

Complete the following table using your experimental results:

Further analysis:

• Compare your results with those from other groups
• Comment on the reliability of your results
• Look for patterns in temperature changes
• Identify which processes are chemical versus physical

Drawing conclusions

For each part of the investigation, justify whether the process is:

• Chemical or physical
• Shows a positive (temperature increase) or negative (temperature decrease) change
• Endothermic or exothermic

Exam tips

Remember!