C5 Energy changes

Energy is conserved in a reaction: it is transferred, never created or destroyed.

In an exothermic reaction, energy is transferred to the surroundings, so the temperature of the surroundings goes up. Examples include combustion (burning fuels), many oxidation reactions (like iron powder oxidising in a single-use hand warmer), and neutralisation.

In an endothermic reaction, energy is taken in from the surroundings, so the temperature of the surroundings goes down. Examples include thermal decomposition (which needs continuous heating to keep going) and the reaction between citric acid and sodium hydrogencarbonate.

Uses: hand warmers and self-heating cans (e.g. calcium oxide reacting with water) use exothermic reactions. Instant cold packs for sports injuries use an endothermic change. To judge a use, look at the size of the temperature change, how long it lasts, whether it can be reused, cost and safety.

A student mixes two solutions at 21.0 °C. The highest temperature reached during the reaction is 29.5 °C. What type of reaction is this, and what is the temperature change?

The temperature of the surroundings increased, which means energy was transferred outwards to the surroundings. Therefore, it is an exothermic reaction.

Temperature change = final temperature − initial temperature.

Temperature change = 29.5 − 21.0 = 8.5 °C.

The 'surroundings' include the solution, the container and the air, not just the air.

A common mistake is thinking an endothermic reaction gets hot because it 'takes in heat'. It takes energy from the surroundings, so they feel cold.

Because energy is conserved, if a reaction is exothermic (giving energy out), the products must store less energy than the reactants by exactly the amount given out. If it is endothermic, the products store more energy than the reactants.

A reaction profile (or energy level diagram) plots the energy of the substances against the progress of the reaction. It shows the energy of the reactants, a curved hump, and the energy of the products.

The activation energy is the minimum energy colliding particles need to react. It is the energy put in to start the reaction.

In an exothermic reaction, the products have less energy than the reactants because energy is given out to the surroundings. In an endothermic reaction, the products have more energy than the reactants because energy is taken in.

An exothermic profile shows energy given out. An endothermic profile shows energy taken in. The activation energy is always measured upwards from the reactants line.

A reaction profile shows the reactants at 150 kJ, the highest point of the curve at 320 kJ, and the products at 60 kJ. What is the activation energy, and is the reaction exothermic or endothermic?

Activation energy = highest point − reactants = 320 − 150 = 170 kJ.

The products (60 kJ) are lower than the reactants (150 kJ), so energy has been given out. The reaction is exothermic.

The overall energy change is 150 − 60 = 90 kJ given out.

Always measure the activation energy from the reactants line to the top of the curve. Never measure from the bottom of the axis, and never measure from the products line.

The overall energy change is just the difference between the reactants and the products. It does not depend on how high the peak is.

A reaction with a very large activation energy might need a spark or a flame to start, even if it is exothermic overall (like burning a fuel).

During a chemical reaction, old bonds break and new bonds form.

Breaking bonds is an endothermic process (it takes in energy).

Making new bonds is an exothermic process (it releases energy).

A reaction is exothermic overall if more energy is released making bonds than is taken in.

To calculate the overall energy change:

1. Count the number and type of every bond in the reactants and add up their bond energies (energy in).

2. Count every bond in the products and add up their bond energies (energy out).

3. Subtract the energy out from the energy in (overall energy change = energy in − energy out).

An energy level diagram for the reaction of H₂ + Cl₂ to form 2HCl. The arrows show energy taken in to break bonds in the reactants to form separate atoms, energy released when new bonds form to give the products, and the overall energy change.

Hydrogen reacts with chlorine: H₂ + Cl₂ → 2HCl. The bond energies are H–H 436 kJ/mol, Cl–Cl 243 kJ/mol and H–Cl 432 kJ/mol. What is the overall energy change?

Energy in (bonds broken) = 436 (H–H) + 243 (Cl–Cl) = 679 kJ/mol.

Energy out (bonds made) = 2 × 432 (H–Cl) = 864 kJ/mol.

Overall energy change = energy in − energy out = 679 − 864 = −185 kJ/mol. The negative sign shows the reaction is exothermic.

Always multiply the bond energy by the number of bonds in the molecule and the balancing number in front of the molecule (e.g. 2H₂O has 4 O–H bonds).

A negative overall energy change means the reaction is exothermic; a positive overall energy change means it is endothermic.

Energy is never used up or created; it is just transferred.

This investigates how a variable affects the temperature change of a reaction. One example is measuring the maximum temperature reached when different volumes of sodium hydroxide (alkali) are added to dilute hydrochloric acid (an exothermic neutralisation).

1. Measure 30 cm³ of dilute hydrochloric acid into a polystyrene cup.

2. Stand the cup inside a glass beaker for stability.

3. Record the starting temperature of the acid.

4. Add 5 cm³ of sodium hydroxide to the cup.

5. Put a lid on, stir, and record the highest temperature reached.

6. Repeat adding 5 cm³ portions up to 40 cm³.

7. Repeat the experiment and calculate means.

Independent variable: volume of sodium hydroxide added.

Dependent variable: temperature change (highest − starting temperature).

Control variables: volume and concentration of acid, concentration of alkali, type of cup and lid, starting temperature.

Apparatus: a thermometer, or a temperature probe and data logger for higher resolution (e.g. 0.1 °C).

Hydrochloric acid and sodium hydroxide are irritants. Wear eye protection and wipe up spills.

A polystyrene cup and lid are good thermal insulators. They reduce energy loss so the temperature reading is more accurate.

The biggest error source is energy loss to the surroundings. Using a glass beaker instead results in a smaller temperature rise because glass conducts heat better.

When plotting highest temperature against volume, the crossing point of the lines of best fit shows the exact volume needed to neutralise the acid.

After neutralisation, adding more alkali cools the warm mixture down because no more reaction happens.