Chemical energetics

In every chemical reaction, energy is transferred. The reaction is either exothermic or endothermic, depending on which way the energy moves.

• An exothermic reaction 放热反应 gives out thermal energy 热能 to the surroundings 环境. So the temperature of the surroundings goes up.
• An endothermic reaction 吸热反应 takes in thermal energy from the surroundings. So the temperature of the surroundings goes down.

Examples of exothermic reactions are combustion 燃烧 (burning a fuel) and neutralisation 中和 (an acid reacting with an alkali). An example of an endothermic reaction is the thermal decomposition 分解 of a compound (breaking it down using heat).

The amount of thermal energy transferred in a reaction is called the enthalpy change 焓变. It is written as $\Delta H$ and has these signs:

• $\Delta H$ is negative for an exothermic reaction, because energy leaves the chemicals.
• $\Delta H$ is positive for an endothermic reaction, because energy is taken in.

Particles do not react every time they meet. The activation energy 活化能 ($E_a$) is the smallest amount of energy that colliding 碰撞 particles 粒子 must have before they can react. It is like a hill the particles must get over before the reaction can happen.

A reaction pathway diagram 反应进程图 shows how the energy changes as a reaction happens. Energy is on the vertical axis, and the progress of the reaction is on the horizontal axis.

• The line starts at the energy level of the reactants 反应物.
• It rises over a 'hill' — the height of this hill is the activation energy $E_a$.
• It then falls or rises to the energy level of the products 生成物.
• The gap between the reactant level and the product level is the enthalpy change $\Delta H$.

For an exothermic reaction, the products are lower than the reactants, so energy is given out and $\Delta H$ is negative.

For an endothermic reaction, the products are higher than the reactants, so energy is taken in and $\Delta H$ is positive.

A reaction involves breaking the bonds in the reactants and making new bonds in the products.

• Bond breaking 断键 takes in energy, so it is an endothermic step.
• Bond making 成键 gives out energy, so it is an exothermic step.

The bond energy 键能 is the energy needed to break one mole of a particular bond. You can use bond energies to find the enthalpy change:

If more energy is given out making bonds than is taken in breaking bonds, the reaction is exothermic ($\Delta H$ negative). If less energy is given out, it is endothermic ($\Delta H$ positive).

Worked example

For the reaction $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$, use these bond energies (in kJ/mol): H–H $= 436$, Cl–Cl $= 242$, H–Cl $= 431$.

Bonds broken: one H–H and one Cl–Cl $= 436 + 242 = 678$.

Bonds made: two H–Cl $= 2 \times 431 = 862$.

The answer is negative, so this reaction is exothermic.

• Exothermic gives out heat, so the surroundings warm up and $\Delta H$ is negative; endothermic takes in heat, so the surroundings cool and $\Delta H$ is positive.
• Bond breaking takes energy in (endothermic); bond making gives energy out (exothermic). Use $\Delta H = (\text{energy to break bonds}) - (\text{energy released making bonds})$.
• If more energy is released making bonds than is used breaking them, the reaction is exothermic. Always check the sign of your final answer.
• On a reaction pathway diagram, an exothermic reaction ends lower than it starts; an endothermic one ends higher. The height of the hill is the activation energy.