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During chemical reactions energy transfer takes place to and from the surroundings. This transfer is usually in the form of thermal energy, i.e. heat. A chemical reaction can be either exothermic or endothermic.
Exo- means 'out; think about words like "exit". Exothermic reactions release energy to their surroundings. The picture shows an example of an exothermic reaction:
Explosions are exothermic reactions, but a reaction can be exothermic without exploding. Any increase in temperature indicates an exothermic reaction. If you were to hold a container where an exothermic reaction takes place, you would feel the heat and a thermometer inside the container would show an increase in temperature. Examples of exothermic reactions are combustion (burning) and neutralisation. Hand warming pads contain chemicals which undergo an exothermic reaction when pressed together.
Endo- means 'inside'; an endoscope is a camera a doctor puts inside a patient's body. Endothermic reactions take in energy from their surroundings. If you were to hold a container where an endothermic reaction takes place, you would feel your hands getting cold, as the reaction would take heat from the container and...even your hands if you kept holding it! A thermometer in the container would show a decrease in temperature.
In an endothermic reaction, the energy of the products is higher than the energy of the reactants. Examples of endothermic reactions are electrolysis, photosynthesis and thermal decomposition. In both these reactions, we need to keep putting energy into the reaction for it to keep going. In electrolysis the energy source is the battery, in photosynthesis the energy is light, and in thermal decomposition the energy source is a heat source (like a flame). Cold pads to relieve injuries can use chemicals which undergo an endothermic reaction.
An reaction profile is a sort of graph, comparing the energy of a set of chemicals before, during and after a chemical reaction. Before the reaction is on the left, and after the reaction is on the right. Energy is on the vertical axis; high energy is high up and low energy is low down.
In an exothermic reaction (on the left), the energy of the chemicals starts high and ends lower. The energy difference is released into the surroundings.
In an endothermic reaction (on the right), the energy of the chemicals starts low and ends higher. The energy difference is taken from the surroundings.
Both reaction profiles have a peak in the middle, where the energy of the chemicals is highest of all. This is because nearly all chemical reactions need some energy input so they can begin to happen. The energy needed to start a chemical reaction is called the activation energy- it's the energy needed to make the reaction active.
Making and breaking bonds
We can predict whether a reaction will be exothermic or endothermic by thinking about the bonds between atoms in a chemical. Energy must be supplied to break a bond between two atoms- think about two boxes held together by sticky tape. We have to do work (put energy in) to break them apart. When a bond forms between two atoms, energy is released. In an exothermic reaction, the products have more (or stronger) bonds than the reactants. In an endothermic reaction, the products have fewer (or weaker) bonds than the reactants.
If we have data for the energy in a chemical bond, we can be more quantitative than this. For example, think about the combustion of methane.
Step 1: Draw the molecules for the reactants and products
Step 2: Count the bonds
On the reactant side, there are 4 C-H bonds in CH4, and 2 O=O bonds in the two O2 molecules. On the product side, there are 2 C=O bonds in CO2, and 4 H-O bonds in the two H2O molecules. (- means a single covalent bond, = means a double covalent bond).
The energies for each of these bonds are:
|Energy (kJ / mol)||412||498||465||532|
You don't need to memorise this table- if you need these data in an exam, you will be given the data you need.
Step 3: Calculate the energy for the reactants and products separately
Start by writing down the bond counts, then substitute in the bond energy data. Brackets will make the calculation more reliable.
Energy of bonds in reactants = (4 x C-H) + (2 x O=O)
= (4 x 412) + (2 x 498) = 2 644 kJ / mol.
Energy of bonds in products = (2 x C=O) + (4 x H-O)
= (2 x 532) + (4 x 465) = 2 924 kJ / mol.
Step 4: Work out the energy change
Difference = 2924 - 2644 = 280 kJ / mol.
Since the reaction is exothermic, the bonds must have less energy at the end than at the start. We write the bond energy change as - 280 kJ / mol.
Reaction profiles help us visualise the difference between exothermic and endothermic reactions, and to compare reactions involving different amounts of energy; for example, an explosion releases a large amount of energy in a short time. Bond energy calculations are a way of predicting what will happen in a reaction- it's really important to label your numbers, so you don't mix up reactants and products.