### Learning check

Once you have watched the video, check your learning with this quiz.

## Exothermic reactions

When hydrogen gas is combusted in oxygen (in the air), a lot of energy is released in the form of heat and light. This reaction is thus exothermic.

Heat (energy) is released.

• Example: Hydrogen is combusted in oxygen.
• 2H2(g) + O2(g) → 2H2O(g) + energy

When hydrogen gas reacts with oxygen gas, the activation energy is much smaller than the energy given off.

To start a reaction, some energy is always required to break the initial bonds.

• Activation energy = the energy required to start a reaction

In exothermic reactions, energy is always released to the surroundings.

## Endothermic reactions

Heat (energy) is absorbed.

Example: photosynthesis.

• 6CO2 + 6H2O + light → C6H12O6 + 6O2

During photosynthesis, the amount of energy given off when new bonds form in glucose and oxygen is much smaller than the activation energy. Thanks to this, the energy from the light is stored in the glucose and oxygen molecules.

In endothermic reactions, energy is always taken up from the surroundings.

## Two fun reactions

### Dissolving sodium hydroxide in water

When solid sodium hydroxide is dissolved in water, heat is released to the surrounding water, increasing its temperature.

$${\sf \text{NaOH(s)} \xrightarrow{\text{ H}_2\text{O }} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)} + energy}$$

• We write "+ energy" to the right since energy is released in the reaction.

When solid sodium hydroxide is dissolved in water, heat is given off to the surrounding water. This is because the products (the aqueous ions) have less energy than the reactant (solid sodium hydroxide).

### Dissolving ammonium nitrate in water

When solid ammonium nitrate is dissolved in water, heat is absorbed from the surrounding water, decreasing its temperature.

$${\sf \text{NH}_4\text{NO}_3\text{(s)} + energy \xrightarrow{\text{ H}_2\text{O }} \text{NH}_4^+\text{(aq)} + \text{NO}_3^-\text{(aq)}}$$

• We write "+ energy" to the left since energy is absorbed in the reaction.

When solid ammonium nitrate is dissolved in water, heat is taken up from the surrounding water. This is because the products (the aqueous ions) have more energy than the reactant (solid ammonium nitrate).

## Enthalpy

The energy content of a substance is called its enthalpy.

• Written $$H$$
• Has the unit J (and sometimes J/mol).

In a reaction, the enthalpies for the reacting substances change.

• The change is written $$\Delta H$$, and it is equal to $$H_{\text{products}} – H_\text{reactants}$$.

### An example

1 mole of carbon reacts with 1 mole of oxygen, forming 1 mole of carbon dioxide.

• C(s) + O2(g) → CO2(g) + 394kJ

What is $$\Delta H$$ for the reaction?

#### Solution

$$\Delta H = H_\text{products} – H_\text{reactants} = H_{\text{CO}_2} - (H_\text{C} + H_{\text{O}_2}) = -394\text{kJ}$$

Why the minus sign?

### The sign of $$\Delta H$$ for exo- and endothermic reactions

Since the products always have lower enthalpy than the reactants in an exothermic reaction, ∆H < 0.

Since the products always have higher enthalpy than the reactants in an endothermic reaction, ∆H > 0.

Exothermic reactions: $$\Delta H < 0$$

Endothermic reactions: $$\Delta H > 0$$

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