Cambridge O Level Chemistry · Syllabus 5070 · Chemical Energetics
Enthalpy Change
What is Enthalpy Change?
The transfer of thermal energy during a chemical reaction, given the symbol delta H; it is negative when the reaction transfers thermal energy to the surroundings and positive when the reaction takes thermal energy in from the surroundings.
This definition is part of the Chemical Energetics chapter in Cambridge O Level Chemistry.
Enthalpy Change in context
Chemical energetics is the study of the thermal energy transferred when a chemical reaction takes place. An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases; an endothermic reaction takes in thermal energy from the surroundings, so their temperature decreases. The transfer of thermal energy during a reaction is called the enthalpy change, \(\Delta H\): it is negative for an exothermic reaction because energy leaves the system, and positive for an endothermic reaction because energy enters it. The thermometer sits in the surroundings, so whatever it does, the system did the opposite.
Every chemical reaction both breaks bonds and makes bonds. Bond breaking is endothermic, because separating bonded atoms works against the attraction holding them together and needs an energy input; bond making is exothermic, because forming a bond releases energy. The overall enthalpy change is the balance: \(\Delta H\) equals the total energy taken in to break bonds minus the total energy given out when bonds form. If more energy is given out than taken in, the reaction is exothermic and \(\Delta H\) is negative; if less, it is endothermic and \(\Delta H\) is positive. Activation energy, \(E_\mathrm{a}\), is the minimum energy colliding particles must have to react.
Questions students ask about Enthalpy Change
Why is the enthalpy change negative for an exothermic reaction?
Because the sign is a direction label, not a statement that energy is negative. There is no negative energy. The minus sign records that thermal energy left the system and went into the surroundings. If a reaction has \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), then 184 kJ of thermal energy is transferred to the surroundings for every mole of reaction; the sign says which way it went and the number says how much. On a reaction pathway diagram the products sit below the reactants.
What is activation energy, and how is it shown on a reaction pathway diagram?
Activation energy, \(E_\mathrm{a}\), is the minimum energy that colliding particles must have to react. Every reaction, exothermic or endothermic, has this barrier. On a reaction pathway diagram it is the arrow drawn from the reactants level up to the peak of the curve, never from zero and never from the products level. The enthalpy change, \(\Delta H\), is a different arrow: it runs between the reactants level and the products level, and the peak plays no part in it. Same start, different finish.
Does breaking a bond release energy?
No, and this is the single most damaging misconception in the topic. Bonded atoms attract one another, so pulling them apart works against that attraction and needs an energy input: bond breaking is endothermic. Energy is released when new bonds form: bond making is exothermic. Every reaction does both, and the sign of the overall enthalpy change depends only on which total is bigger, the energy taken in to break bonds or the energy given out when bonds are made.
How do you calculate the enthalpy change of a reaction from bond energies?
Draw the structures and make a bond inventory for both sides, remembering that a coefficient such as \(2\mathrm{O_2}\) means two \(\mathrm{O{=}O}\) bonds and that a molecular formula counts atoms, not bonds. Total the energy taken in to break every bond in the reactants, total the energy given out when every bond in the products forms, then subtract: \(\Delta H\) equals bonds broken minus bonds made. Use the row of the table that matches the bond, so \(\mathrm{O{=}O}\) not \(\mathrm{O-O}\). Reversing the subtraction turns every exothermic answer into an endothermic one.
Does a large activation energy mean a large enthalpy change?
No. The two are independent. The activation energy says how much energy colliding particles need to cross the barrier; the enthalpy change says where the products ended up relative to the reactants. A strongly exothermic reaction can have a very large barrier, and a barely endothermic one can have a small barrier, so two reactions with very different peaks can have exactly the same \(\Delta H\). You cannot infer either quantity from the other, and a question that gives only one does not let you conclude the other.