Exothermic and Endothermic Reactions

In some reactions, the energy that must be absorbed to break the bonds in the reactants, is less than the energy that is released when the new bonds of the products are formed. This means that in the overall reaction, energy is released as either heat or light. This type of reaction is called an exothermic reaction.

Definition: Exothermic reaction

An exothermic reaction is one that releases energy in the form of heat or light.

Another way of describing an exothermic reaction is that it is one in which the energy of the products is less than the energy of the reactants, because energy has been released during the reaction. We can represent this using the following general formula:

\[\text{Reactants} → \text{Products} + \text{Energy}\]

In other reactions, the energy that must be absorbed to break the bonds in the reactants, is more than the energy that is released when the new bonds in the products are formed. This means that in the overall reaction, energy must be absorbed from the surroundings. This type of reaction is known as an endothermic reaction.

Definition: Endothermic reaction

An endothermic reaction is one that absorbs energy in the form of heat or light.

Another way of describing an endothermic reaction is that it is one in which the energy of the products is greater than the energy of the reactants, because energy has been absorbed during the reaction. This can be represented by the following general formula:

\[\text{Reactants} + \text{Energy} → \text{Products}\]

The difference in energy (E) between the reactants and the products is known as the heat of the reaction. It is also sometimes referred to as the enthalpy change of the system. This is represented using \(\Delta \text{H}\)

Tip:

\(\Delta\) is read as delta and means a change in. You may recall this symbol from physics.

Optional Experiment: Endothermic and exothermic reactions - Part 1

Apparatus and materials

You will need:

citric acid
sodium bicarbonate
a polystyrene cup
a lid for the cup
thermometer
glass stirring rod
scissors

Tip:

You can get polystyrene cups with lids from coffee shops or fast food stores. Cardboard cups will also work fine. Some of the lids will have a hole for a straw, which is useful for this experiment.

Note that citric acid is found in citrus fruits such as lemons. Sodium bicarbonate is actually bicarbonate of soda (baking soda), the baking ingredient that helps cakes to rise.

Method

If your lid does not have a hole for a straw, then cut a small hole into the lid.
Pour some citric acid \((\text{C}_{6}\text{H}_{8}\text{O}_{7})\) into the polystyrene cup, cover the cup with its lid and record the temperature of the solution.
Stir in the sodium bicarbonate \((\text{NaHCO}_{3})\), then cover the cup again.
Immediately record the temperature, and then take a temperature reading every two minutes after that. Record your results.

The equation for the reaction that takes place is:

\(\text{C}_{6}\text{H}_{8}\text{O}_{7}\text{(aq)} + 3\text{NaHCO}_{3}\text{(s)} → 3\text{CO}_{2}\text{(g)} + 3\text{H}_{2}\text{O(}ℓ\text{)} + \text{Na}_{3}\text{C}_{6}\text{H}_{5}\text{O}_{7}\text{(aq)}\)

Results

Time (\(\text{mins}\))	0	2	4	6
Temperature (\(\text{℃}\))

Plot your temperature results on a graph of time (\(x\)-axis) against temperature (\(y\)-axis).

Discussion and conclusion

What happens to the temperature during this reaction?
Is this an exothermic or an endothermic reaction? (Was energy taken in or given out? Did the temperature increase or decrease?)
Why was it important to keep the cup covered with a lid?

Optional Experiment: Endothermic and exothermic reactions - Part 2

Apparatus and materials

Vinegar
steel wool
thermometer
polystyrene cup and plastic lid (from previous experiment)

Method

Put the thermometer through the plastic lid, cover the cup and record the temperature in the empty cup. You will need to leave the thermometer in the cup for about 5 minutes in order to get an accurate reading.
Soak a piece of steel wool in vinegar for about a minute. The vinegar removes the protective coating from the steel wool so that the metal is exposed to oxygen.
Take the thermometer out of the cup. Keep the thermometer through the hole of the lid.
After the steel wool has been in the vinegar, remove it and squeeze out any vinegar that is still on the wool. Wrap the steel wool around the thermometer and place it (still wrapped round the thermometer) back into the cup. The cup is automatically sealed when you do this because the thermometer is through the top of the lid.
Leave the steel wool in the cup for about 5 minutes and then record the temperature. Record your observations.

Results

You should notice that the temperature increases when the steel wool is wrapped around the thermometer.

Conclusion

The reaction between oxygen and the exposed metal in the steel wool is exothermic, which means that energy is released and the temperature increases.

Exothermic and Endothermic Reactions