3.2.1 Exothermic and endothermic reactions and reaction profiles

Exothermic reactions transfer energy to the surroundings

1. In an exothermic reaction, energy stored in the reacting chemicals is transferred to the surroundings.
   1. A thermometer in the reaction mixture shows a temperature rise.
2. Combustion of fuels, such as methane burning in a gas cooker, is exothermic.
3. Neutralisation of an acid by an alkali is exothermic, and so are most reactions of metals with acids.
4. Oxidation reactions, such as iron powder reacting with oxygen in a disposable hand warmer, are exothermic too.

Endothermic reactions take in energy from the surroundings

1. In an endothermic reaction, energy is taken in from the surroundings and stored in the products.
   1. A thermometer in the reaction mixture shows a temperature fall.
2. Thermal decomposition, such as heating calcium carbonate to make calcium oxide, is endothermic and needs continuous heating.
3. The reaction of citric acid with sodium hydrogencarbonate solution is endothermic, so the mixture feels cold.
4. Photosynthesis is endothermic, taking in energy from sunlight to make glucose.
5. Instant cold packs for sports injuries contain ammonium nitrate, which takes in energy from the surroundings as it dissolves in water.

Temperature change shows the type of reaction

1. Measure the starting temperature of the solution before adding the other reactant.
2. Record the highest or lowest temperature reached after mixing.
3. Calculate the temperature change by subtracting the starting temperature from the final temperature.
   1. A positive temperature change means the reaction is exothermic.
   2. A negative temperature change means the reaction is endothermic.
4. A reading of $21.0{}^{\circ }C$ rising to $28.5{}^{\circ }C$ gives a change of $+7.5{}^{\circ }C$, so that reaction is exothermic.

Reaction profiles show energy levels and activation energy

1. The vertical axis shows energy and the horizontal axis shows the progress of reaction.
2. A short horizontal line on the left shows the energy of the reactants, and one on the right shows the energy of the products.
3. A curve joins the two lines, rising to a peak before falling to the products.
   1. The rise to the peak shows that energy must be supplied before the reactants can change into products.
4. The activation energy is shown by an upward arrow from the reactants line to the top of the peak.
5. The overall energy change is shown by an arrow from the reactants line to the products line.

Comparing exothermic and endothermic reaction profiles

1. In an exothermic profile, the products are drawn lower than the reactants.
   1. The difference is the energy given out to the surroundings, so the energy change arrow points down.
2. In an endothermic profile, the products are drawn higher than the reactants.
   1. The difference is the energy taken in from the surroundings, so the energy change arrow points up.
3. Both types of reaction have an activation energy, so both profiles rise to a peak.
4. In an endothermic profile the peak is above the products, so the activation energy is always larger than the energy change.

3.2.2 Calculating energy change from bond energies

Bond breaking is endothermic and bond making is exothermic

1. Breaking a covalent bond needs energy to overcome the attraction between the shared electrons and the two nuclei, so bond breaking is endothermic.
2. Making a bond releases energy, so bond making is exothermic.
   1. Forming a bond releases the same amount of energy that is needed to break that bond.
3. In a reaction, bonds in the reactants break and new bonds form in the products.
4. Different bonds have different strengths, such as $436kJ/mol$ for $H-H$ and $243kJ/mol$ for $Cl-Cl$.

Energy change equals bonds broken minus bonds made

1. Calculate the energy change using: $$energy\; change=energy\; to\; break\; bonds-energy\; released\; making\; bonds$$
2. A negative answer means more energy is released making bonds than is taken in breaking them, so the reaction is exothermic.
3. A positive answer means more energy is taken in breaking bonds than is released making them, so the reaction is endothermic.
4. The answer is in kilojoules per mole, $kJ/mol$, for the equation as written.
5. Bond energies are average values taken from many different molecules, so the result is an estimate.

Count every bond broken and made, using balancing numbers

1. Draw the displayed formula of each substance to see every bond.
2. Multiply the bonds in each molecule by its balancing number in the equation.
   1. In $2H_{2}O$ there are four $O-H$ bonds, because each water molecule has two.
3. A double bond such as $O=O$ or $C=O$ has its own bond energy and counts as one bond.
   1. Each ${CO}_2$ molecule, $O=C=O$, contains two $C=O$ bonds.
4. List the bonds broken and the bonds made in two columns, so that no bond is missed.

Burning methane is exothermic as bond making releases more

1. Methane burns in oxygen: $${CH}_4+2O_2\to {CO}_2+2H_{2}O$$
2. The bonds broken are four $C-H$ bonds in methane and two $O=O$ bonds in the oxygen molecules.
3. The bonds made are two $C=O$ bonds in carbon dioxide and four $O-H$ bonds in the water molecules.
4. The bond energies are $C-H=413$, $O=O=498$, $C=O=805$ and $O-H=463kJ/mol$.

A positive energy change shows a reaction is endothermic

1. When water is split into hydrogen and oxygen, $2H_{2}O\to 2H_2+O_2$, four $O-H$ bonds are broken.
   1. Using $O-H=463kJ/mol$, the energy to break them is $4\times 463=1852kJ/mol$.
2. Two $H-H$ bonds and one $O=O$ bond are made.
   1. Using $H-H=436$ and $O=O=498kJ/mol$, the energy released is $(2\times 436)+498=1370kJ/mol$.
3. The energy change is $1852-1370=+482kJ/mol$, so the reaction is endothermic.
4. This matches the reaction profile of an endothermic reaction, in which the products sit higher in energy than the reactants.