Exothermic & endothermic reactions
By the end of this topic you'll tell the two types apart from a temperature change, give real examples of each, and stop muddling up which way the heat goes.
Part 1Two ways energy can move
Every chemical reaction involves an energy change with the surroundings — almost always you notice it as a temperature change. There are only two possibilities, and the names tell you everything.
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings goes up. ("Exo" = exit — energy exits the reaction.) An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings goes down. ("Endo" = into — energy goes into the reaction.)
Know these examples
- Exothermic
- Combustion (burning), neutralisation (acid + alkali), most oxidation reactions, and many displacement reactions. The mixture gets warmer.
- Endothermic
- Thermal decomposition (e.g. heating a metal carbonate) and the reaction of citric acid with sodium hydrogencarbonate. The mixture gets colder.
⚠ Watch out — "surroundings", not "the reaction"
The temperature you measure is the surroundings (the solution, the test tube, the air). In an exothermic reaction the surroundings get hotter; in an endothermic one they get colder. Don't say "the reaction gets hot" — say energy is transferred to the surroundings. And note: exothermic does not mean fast, and endothermic does not mean slow — that's a different idea.
A student mixes two solutions and the thermometer reading falls from 21 °C to 14 °C. The reaction is:
- AExothermic — it gave out energy
- BEndothermic — it took in energy from the surroundings
- CExothermic — combustion always warms up
- DNeither — no energy changed
Show answer
Part 2Everyday uses
These energy changes are genuinely useful. We package them up into hand warmers and cold packs.
A self-heating hand warmer uses an exothermic reaction (for example the oxidation of iron) to release energy and warm your hands. A sports cold pack (instant ice pack) uses an endothermic process — a salt such as ammonium nitrate dissolving in water takes in energy, so the pack goes cold and can be pressed onto an injury.
Worked example — naming the type from a use
An instant cold pack is squeezed; the chemicals mix and it turns icy cold. State whether the change is exothermic or endothermic and explain how you know.
Define an exothermic reaction in terms of energy and the surroundings.
A reaction that transfers energy to the surroundings, so the temperature of the surroundings increases.Give two examples of exothermic reactions.
Any two of: combustion (burning), neutralisation, oxidation reactions, many displacement reactions.Name an everyday use of an endothermic process and the type of substance involved.
A sports cold pack (instant ice pack) — a salt such as ammonium nitrate dissolving in water takes in energy, so it goes cold.A neutralisation reaction warms the beaker from 18 °C to 27 °C. Is it exothermic or endothermic?
Exothermic — the temperature rose, so energy was transferred out to the surroundings.
Measuring the temperature change
The required practical — how to measure an energy change and which variables you can fairly investigate.
Part 1The idea behind the experiment
To find out how much energy a reaction releases or takes in, you measure the temperature change of the solution. A bigger temperature rise means a more exothermic reaction; a temperature fall means an endothermic one.
You can investigate what affects the size of the temperature change — for example the concentration of an acid, the type of metal in a displacement reaction, the mass or type of substance used, or the volume of solution. To keep it a fair test, you change one variable at a time and keep the others the same.
Variables for this practical
- Independent
- The one you change — e.g. acid concentration, or which metal you add.
- Dependent
- What you measure — the temperature change (Δθ) of the solution.
- Control
- Kept the same — volume of solution, starting temperature, same cup and insulation.
⚠ Watch out — record the change, not just the highest reading
The result you want is the temperature change: highest (or lowest) temperature minus the starting temperature. Always take the starting temperature first. A polystyrene cup with a lid is used because it's a good insulator — it cuts down energy transferred to the surroundings, so your reading is more accurate.
Part 2The required practical
Temperature changes in reacting solutions
Aim: investigate the variables that affect the temperature change in reacting solutions, e.g. neutralisation (acid + alkali) or a displacement reaction.
- Use a measuring cylinder to add a known volume of one solution (e.g. 30 cm³ of hydrochloric acid) to a polystyrene cup standing in a beaker for support.
- Measure and record the starting temperature with a thermometer.
- Add the second reactant (e.g. sodium hydroxide solution), put on the lid, and stir gently.
- Record the highest temperature reached (or lowest, if endothermic).
- Calculate the temperature change: Δθ = highest − starting
- Repeat, changing only your chosen variable (e.g. a higher acid concentration), keeping everything else the same.
Control / improve: use a lid and an insulated polystyrene cup to reduce energy lost to the surroundings, and repeat each measurement to spot anomalies and find a mean. Keep the volume of solution constant so the comparison is fair.
Why is the reaction carried out in a polystyrene cup with a lid rather than a glass beaker?
- APolystyrene reacts with the acid to speed things up
- BIt's a good insulator, so less energy is transferred to the surroundings
- CIt makes the reaction more exothermic
- DGlass is too heavy to put on a balance
Show answer
What do you measure to find the energy change of a reaction in solution?
The temperature change of the solution — highest (or lowest) temperature minus the starting temperature.Give the independent and dependent variables when investigating the effect of acid concentration.
Independent: the acid concentration (what you change). Dependent: the temperature change (what you measure).State two things you should keep the same to make it a fair test.
Any two of: volume of solution, starting temperature, the same cup/insulation, and the same way of stirring.The acid starts at 19 °C and reaches a highest reading of 31 °C. State the temperature change and the type of reaction.
Δθ = 31 − 19 = 12 °C rise — the reaction is exothermic.
Reaction profiles
Reading energy-level diagrams, finding the overall energy change, and what activation energy really is.
Part 1Drawing the energy of a reaction
A reaction profile is a diagram showing the energy of the reactants and products as a reaction goes from start to finish. The energy of the reactants is on the left, the products on the right.
In an exothermic reaction the products have less energy than the reactants — energy has been released to the surroundings, so the line goes down. In an endothermic reaction the products have more energy than the reactants — energy has been taken in, so the line ends higher up.
Part 2Activation energy
Even an exothermic reaction needs a little energy to get going. The activation energy is the minimum energy that reacting particles must have to react — it's the energy needed to break the bonds and start the reaction. On the profile it's the size of the "hump": the gap from the reactants up to the top of the curve.
⚠ Watch out — two different gaps
Don't confuse the two measurements on a profile. The activation energy is from the reactants up to the top of the hump. The overall energy change is from the reactants straight across to the products. They are not the same gap. Every reaction has a hump — even exothermic ones — so the line always goes up first, then to its final level.
On a reaction profile, the products are drawn higher than the reactants. What does this tell you?
- AThe reaction is exothermic — energy was released
- BThe reaction is endothermic — energy was taken in
- CThere is no activation energy
- DThe reaction cannot happen
Show answer
On an exothermic reaction profile, are the products higher or lower than the reactants?
Lower — the products have less energy because energy has been transferred to the surroundings.Define activation energy.
The minimum energy that reacting particles must have for a reaction to happen (the energy needed to start breaking bonds).Where is the activation energy shown on a reaction profile?
It's the gap from the reactants up to the top of the hump (the peak of the curve).How can you tell from a profile whether a reaction is endothermic?
The products are higher than the reactants, so the overall energy change is an increase — energy was taken in.
Bond energies
Higher tier — why breaking bonds takes energy, why making them gives it back, and how to work out the overall energy change.
Part 1Breaking takes in, making gives out
A chemical reaction is really just a rearranging of bonds: old bonds in the reactants break, and new bonds in the products form. Each step involves energy.
Breaking bonds is endothermic — energy must be supplied to pull bonded atoms apart. Making bonds is exothermic — energy is released when new bonds form. Whether the whole reaction is exothermic or endothermic just depends on which of these is bigger.
The calculation
- overall = (energy to break bonds) − (energy to make bonds) recall
- "break minus make" — energy in to break the bonds of the reactants, minus energy out when the bonds of the products form. A negative answer means exothermic; a positive answer means endothermic. (Bond energies are always provided in the question.)
Higher tier — energy change from bond energies
You may be given the bond energies (in kJ/mol) and asked to calculate the overall energy change. The method is always the same: add up the energy needed to break all the bonds in the reactants, add up the energy released when all the bonds in the products form, then do break − make.
If more energy is released making bonds than is used breaking them, the answer is negative and the reaction is exothermic. If breaking the bonds costs more than making them gives back, the answer is positive and the reaction is endothermic.
Part 2Working it out
Worked example — H₂ + Cl₂ → 2HCl
Calculate the overall energy change. Bond energies (kJ/mol): H–H = 436, Cl–Cl = 242, H–Cl = 431.
⚠ Watch out — count every bond, and watch the sign
Multiply each bond energy by how many of that bond there are (look at the balanced equation — "2HCl" means two H–Cl bonds). Do break − make in that order. A negative overall value is exothermic; a positive value is endothermic. Getting the sign the wrong way round is the most common mistake here.
A reaction needs 950 kJ to break the reactant bonds and releases 1100 kJ making the product bonds. The overall energy change is:
- A+150 kJ, endothermic
- B−150 kJ, exothermic
- C+2050 kJ, endothermic
- D−1100 kJ, exothermic
Show answer
Is breaking bonds exothermic or endothermic, and why?
Endothermic — energy must be supplied (taken in) to break bonds and pull the atoms apart.Is making bonds exothermic or endothermic?
Exothermic — energy is released (given out) when new bonds form.Write the rule for calculating the overall energy change from bond energies.
overall = energy to break bonds − energy to make bonds (break − make). Negative = exothermic; positive = endothermic.Breaking bonds needs 600 kJ; making bonds releases 540 kJ. Find the overall change and state the type.
overall = 600 − 540 = +60 kJ → endothermic (positive, more energy used breaking than released making).