Energetics, Reaction Profiles and Bond Energies
Energy Changes in Reactions
Every chemical reaction involves an energy change. Energy is either transferred to the surroundings or absorbed from them. The key principle is that energy is transferred, never created or destroyed. Two categories describe the direction of this transfer: exothermic and endothermic. Understanding which type a reaction belongs to — and the bond-level explanation for why — is central to chemistry at this level.
Exothermic reactions transfer energy to the surroundings — the temperature of the surroundings increases. Endothermic reactions take in energy from the surroundings — the temperature of the surroundings decreases.
Exothermic reactions include combustion (burning fuels), neutralisation (acid + alkali), and many oxidation reactions. Everyday examples include hand warmers and self-heating cans. Endothermic reactions include thermal decomposition and photosynthesis. Sports cold packs use the endothermic dissolution of ammonium nitrate in water to absorb heat from an injury.
Measuring Energy Changes
You can identify whether a reaction is exothermic or endothermic by measuring the temperature change of the surroundings — usually a solution in a polystyrene cup. If the temperature rises, the reaction is exothermic. If the temperature falls, the reaction is endothermic. A polystyrene cup acts as an insulator to reduce heat loss, giving a more accurate reading. Record the starting temperature, the maximum or minimum temperature reached, and calculate the change.
In everyday life, exothermic reactions are exploited wherever heat is needed without external power: hand warmers contain iron powder that oxidises slowly in air, self-heating food cans use the exothermic reaction between calcium oxide and water, and combustion of fuels in engines releases energy to do useful work. Endothermic reactions are used in sports cold packs and in some industrial processes where absorbing heat from the surroundings is beneficial. Understanding the direction of energy transfer helps engineers design safer, more efficient processes.
In your practical work (PAG C8), you may measure energy changes by mixing solutions in a polystyrene cup and recording the temperature change every 30 seconds. The maximum temperature rise (or fall) gives the energy transferred. Key sources of error include heat loss through the sides of the container and incomplete insulation. Using a lid reduces heat loss further. Always record your starting temperature before mixing and take multiple readings to identify any anomalies.
Reaction Profiles
A reaction profile is a diagram that shows the energy levels of reactants and products as a reaction proceeds. The vertical axis shows energy; the horizontal axis shows the progress of the reaction (the reaction pathway). It does not show time — only relative energy levels.
For an exothermic reaction: the products are at a lower energy level than the reactants. The profile shows the energy rising from reactants to a peak (the activation energy barrier), then falling to a level lower than the start. For an endothermic reaction: the products are at a higher energy level than the reactants. The profile shows the energy rising to a peak then settling above the starting level. In both cases, label four things: reactant energy level, product energy level, the activation energy (reactants to the peak), and the overall energy change (reactants to products).
Activation energy is the minimum energy colliding particles need for a reaction to occur. On a reaction profile it is shown as the height from the reactant energy level to the top of the energy barrier (the peak). A catalyst lowers the activation energy, providing an alternative pathway — shown on the profile as a lower peak, while reactant and product energy levels remain unchanged.
Bond Energy Calculations (Higher Tier)
During a chemical reaction, bonds in the reactants must be broken (this requires energy — endothermic step) and new bonds in the products are formed (this releases energy — exothermic step). The overall energy change depends on the balance between these two processes. Bond energy values are given in kJ/mol and represent the energy to break one mole of a particular bond.
Step 1: Add up the total energy required to break all bonds in the reactants. Step 2: Add up the total energy released when all bonds in the products are formed. Step 3: Calculate the overall energy change:
$$\text{energy change} = \text{energy to break bonds} - \text{energy released making bonds}$$
If the answer is positive, the reaction is endothermic. If negative, the reaction is exothermic. Worked example: for \(H_2 + Cl_2 \rightarrow 2HCl\): breaking bonds = H–H (436 kJ) + Cl–Cl (242 kJ) = 678 kJ. Making bonds = 2 × H–Cl (431 kJ) = 862 kJ. Energy change = 678 − 862 = −184 kJ. Negative, so the reaction is exothermic — more energy is released making bonds than is required to break them.
Breaking bonds requires energy (endothermic). Making bonds releases energy (exothermic). A negative overall energy change means exothermic; a positive value means endothermic.