Thermochemistry: understanding energy changes

Every chemical reaction is also an energy event. Thermochemistry is the chapter that studies that energy, where it comes from, where it goes, and how we measure it. Students often treat it as a formula to plug numbers into, but the calculations only make sense once you understand what is physically happening. This explainer builds the intuition first, then shows how the numbers follow.

Exothermic and endothermic: which way the heat flows

There are only two possibilities, and the whole chapter hangs on telling them apart.

• An exothermic reaction releases heat to the surroundings. The temperature of the mixture and its container rises. Combustion, neutralisation and most displacement reactions are exothermic.
• An endothermic reaction absorbs heat from the surroundings. The temperature falls and the mixture feels cold. Dissolving certain salts and some thermal decompositions are endothermic.

The single most reliable clue in an experiment is the thermometer: heat out means temperature up (exothermic), heat in means temperature down (endothermic). The thermochemistry chapter returns to this test again and again.

Where the energy actually comes from

Here is the idea that makes everything click: breaking bonds absorbs energy, and forming bonds releases energy. During a reaction, the bonds in the reactants must first be broken (energy in) and new bonds in the products are then made (energy out). The overall heat change is simply the difference between the two.

• If more energy is released making new bonds than was absorbed breaking old ones, the reaction is exothermic overall.
• If less energy is released than was absorbed, the reaction is endothermic overall.

This is why you never have to memorise which reactions are exothermic, you can reason it from the bonds. The heat change of a reaction, given the symbol ΔH, is the net of these two steps.

Energy level diagrams: a picture of ΔH

An energy level diagram turns this idea into a drawing. The vertical axis is energy; reactants sit on one level and products on another.

• In an exothermic reaction, the products are drawn lower than the reactants, because energy has been released to the surroundings. ΔH is negative.
• In an endothermic reaction, the products are drawn higher than the reactants, because energy has been absorbed. ΔH is positive.

Between the two levels there is always a hump, the activation energy, the minimum energy needed to start the reaction by breaking the first bonds. The downhill or uphill drop after the hump is ΔH. Getting the direction of that arrow right, and its sign, is worth easy marks in Paper 2.

The four heats you must know

SPM names four specific heat-of-reaction quantities, each defined per mole:

• Heat of neutralisation, heat released when an acid and an alkali react to form one mole of water.
• Heat of displacement, heat released when one mole of a metal is displaced from its salt by a more reactive metal.
• Heat of precipitation, heat released when one mole of a precipitate forms.
• Heat of combustion, heat released when one mole of a substance is completely burnt in oxygen.

All four are exothermic, so all four carry a negative ΔH. Combustion of alcohols and the heat of neutralisation are the two most examined experiments.

The calculation, and what each part means

Measuring a heat change comes down to one formula for the heat gained or lost by the solution:

Q = mcθ

where m is the mass of solution in grams, c is its specific heat capacity (taken as about 4.2 J g⁻¹ °C⁻¹, the value for water), and θ is the temperature change. Two standard assumptions make this workable: the solution has the density of water (1 g cm⁻³, so 1 cm³ weighs 1 g) and behaves thermally like water. To turn Q into the molar heat of reaction, divide by the number of moles reacting: our step-by-step heat of reaction per mole guide covers this cleanly.

A worked example

Suppose 50 cm³ of 1.0 mol dm⁻³ hydrochloric acid is mixed with 50 cm³ of 1.0 mol dm⁻³ sodium hydroxide, and the temperature rises by 6.8 °C. The total mass of solution is 50 + 50 = 100 g.

First the heat released: Q = mcθ = 100 × 4.2 × 6.8 = 2856 J.

Then the moles of water formed: moles of acid = 0.050 dm³ × 1.0 mol dm⁻³ = 0.05 mol, which produces 0.05 mol of water. So the heat of neutralisation = 2856 ÷ 0.05 = 57 120 J, or about 57.1 kJ per mole of water. Because heat was released, we write ΔH = −57.1 kJ mol⁻¹. You can rehearse the whole procedure in the determining the heat of neutralisation experiment.

Making it click

The trap in this chapter is doing the arithmetic without the meaning. Train yourself to state, for every question, whether heat flows in or out, whether the temperature rose or fell, and whether ΔH is therefore positive or negative, before you touch Q = mcθ. Once the physical story and the number agree, careless sign errors disappear. If energy level diagrams or the mole step still trip you up, a short online one-to-one lesson can make them concrete with a worked case or two, our teachers teach in English from RM50 an hour, with a paid one-hour trial to begin.