Chemistry

16.1 - Exothermic & Endothermic Reactions

11 min

Exothermic reactions

Endothermic reactions

Activation energy

Labelling the profile

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Exothermic and endothermic reactions

In an exothermic reaction, heat is given out to the surroundings, so the temperature of the surroundings rises.
In an endothermic reaction, heat is taken in from the surroundings, so the temperature of the surroundings falls.

Energy is conserved

Energy can't be created or destroyed, only transferred.
In an exothermic reaction, the products hold less energy than the reactants because energy was released to the surroundings.
In an endothermic reaction, the products hold more energy than the reactants because energy was taken in from the surroundings.

Sign of the energy change

Negative (−) - Energy given out (exothermic).
Positive (+) - Energy taken in (endothermic).

	Exothermic reaction	Endothermic reaction
Energy is	transferred to the surroundings	taken in from the surroundings
Temperature of the surroundings	rises	falls
Compared with reactants, products	hold less energy	hold more energy

Pink arrows point out of an exothermic reaction beaker, then its thermometer reads higher (temperature of surroundings rises); blue arrows point into an endothermic reaction beaker, then it reads lower (temperature of surroundings falls).

Neutralisation is exothermic

When an acid reacts with a base or an alkali to produce a salt and water, heat is given out to the surroundings.

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Activation energy

For particles to react, they must collide with sufficient energy.
The activation energy is the minimum amount of energy colliding particles must have for a reaction to occur. In the diagrams below, it's the height of the peak above the reactants' level.

Why there's a minimum energy

Chemical reactions require bonds in the reactants to break, which takes energy.

Size of activation energy

Small activation energy - Reactions start easily at room temperature because many collisions already carry enough energy.
Large activation energy - Reactions require energy from outside (usually by heating) so particles collide with enough energy to react.

Two reaction profiles running from reactants over a peak down to products, each with an activation energy arrow from the reactants level: short under reacts at room temperature, much taller under needs heating.

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Reaction profiles

A reaction profile (or reaction pathway diagram) shows how the energy stored in the chemicals changes during a reaction.

The axes and levels

Vertical axis - Energy stored in the chemicals.
Horizontal axis - Progress of the reaction (from left to right).
Reactants and products are drawn as horizontal lines.

Reading direction from the levels

Exothermic - The products are at a lower energy level than the reactants because energy is transferred to the surroundings.
Endothermic - The products are at a higher energy level than the reactants because energy is taken in from the surroundings.
The vertical gap between the reactants and products represents the overall energy change.

Activation energy on a profile

The curve rises from the reactants to a peak before falling to the products.
The activation energy is the height of the curve measured from the reactants' level to the top of the peak.

Drawing a fully labelled profile

Axes - Energy on the vertical axis, progress of reaction on the horizontal axis.
Levels - Reactants on the left, products on the right (products lower for exothermic, higher for endothermic).
Curve - Starts at reactants, rises to a peak above both levels, then meets the products.
Labels - An arrow from the reactants' level to the peak for activation energy, and an arrow between the reactants' and products' levels for the overall energy change.

Two reaction profiles, each with an activation energy arrow up to the peak and an overall energy change arrow between levels: products sit below reactants under exothermic reaction and above under endothermic reaction.

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Worked example - Drawing a reaction profile

A reaction is carried out in which the reactants store more energy than the products. Draw and label a reaction profile, marking the activation energy and the overall energy change, and state whether the reaction is exothermic or endothermic.

Step 1 - Draw and label the axes

Plot energy on the vertical axis and progress of the reaction on the horizontal axis.

Step 2 - Mark the reactant and product levels

Draw a horizontal line for reactants on the left. Draw a lower horizontal line for products on the right, since reactants store more energy.

Step 3 - Draw the curve

Draw a continuous curve starting from the reactants, rising to a peak above both levels, and falling to the products.

Step 4 - Mark the activation energy

Draw an arrow pointing vertically from the reactants' level up to the peak and label it activation energy.

Step 5 - Mark the overall energy change

Draw an arrow from the reactants' level down to the products' level and label it overall energy change.

Step 6 - Classify the reaction

Because the products are lower in energy than the reactants, energy has been transferred to the surroundings.

The reaction is exothermic.

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Reading and labelling ΔH on a profile

On a reaction profile, the enthalpy change ( $Δ H$ ) is the vertical gap between the reactants' level and the products' level.

Direction of the arrow

Exothermic - The arrow points down from reactants to products ( $Δ H$ is negative).
Endothermic - The arrow points up from reactants to products ( $Δ H$ is positive).

Calculating ΔH from a profile

$Δ H = energy of products - energy of reactants$

Reactants at 250 kJ, products at 100 kJ:

$\begin{matrix} Δ H & = 100 - 250 \\ = - 𝟏𝟓𝟎 kJ (exothermic) \end{matrix}$
Reactants at 100 kJ, products at 250 kJ:

$\begin{matrix} Δ H & = 250 - 100 \\ = + 𝟏𝟓𝟎 kJ (endothermic) \end{matrix}$

Under exothermic reaction, a down arrow from reactants at 250 to products at 100 reads ΔH = -150 kJ; under endothermic reaction the levels swap and an up arrow reads ΔH = +150 kJ.

key terms

TermDefinition

Exothermic reactionA reaction in which heat is given out to the surroundings, so the temperature of the surroundings rises.

Endothermic reactionA reaction in which heat is taken in from the surroundings, so the temperature of the surroundings falls.

Activation energyThe minimum amount of energy that colliding particles must have to react.

Reaction profileA diagram showing how the energy stored in the chemicals changes during a reaction.

Enthalpy changeThe thermal energy (heat) transferred to or from the surroundings during a reaction. It is given in kJ, or in kJ/mol for each mole that reacts.

end of notes

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16.2 - Bond Energy CalculationsNext up