Energetics

All reactions can be classified in one of two categories: endothermic or exothermic. Endothermic reactions need to take in heat energy to get going and examples include frying an egg and photosynthesis. Exothermic reactions release heat energy and include combustion reactions.

Exothermic and endothermic

All reactions can be described as either exothermic or endothermic. Exothermic reactions release heat to the surroundings and can be detected by an increase in temperature of the surroundings. Endothermic reactions absorb heat energy and can be detected by a decrease in temperature of the surroundings.

We describe the change in heat energy during a reaction as a change in enthalpy. If the reaction is exothermic, the enthalpy change is negative because the reaction is giving out energy. If the reaction is endothermic, the enthalpy change is positive because the reaction absorbs energy.

Calorimetry experiments

We can calculate enthalpy change experimentally using calorimetry. It involves burning fuel underneath a beaker of water and measuring how much heat energy is transferred to the water by recording the increase in temperature. You can also measure the enthalpy change of a displacement or neutralisation reaction by mixing the contents in a polystyrene cup and recording the change in temperature.

To calculate the change in heat energy from a measured temperature change, use the equations:

Worked example: calorimetry

Comparing experimental values to published data

In a calorimetry experiment like the one described above, the data collected may differ from enthalpy values found in a data book for the following reasons:

• Non-standard conditions used

• Heat lost to the surroundings

• Heat absorbed by the apparatus

• Incomplete combustion

• Evaporation of fuel or water

To make calorimetry experimental data more accurate, you should try to minimise heat loss by using a lid or insulated beaker.

Reaction profiles

Reaction profiles show the amount of energy that a substance has at the start and end of a reaction. If you look at the reaction profile below and to the left, you can see that during an exothermic reaction the reactants lose energy as they form products. The difference in energy between the reactants and products is the enthalpy change which is negative. The initial increase in energy is the activation energy. Activation energy is the minimum amount of energy needed to get a reaction going.

The graph on the right shows an endothermic reaction. You can see from the diagram that the products have more energy than the reactants. This is because energy has been absorbed from the surroundings so the enthalpy change is positive.

Bond breaking is an endothermic process whereas bond making is exothermic. Remember: BENDO MEXO. An exothermic reaction is one in which more heat energy has been released making bonds than absorbed breaking bonds. In contrast, in an endothermic reaction, more heat energy has been absorbed breaking bonds that was released making new bonds.

Bond energies

You can also use bond energies to calculate the enthalpy change during a chemical reaction. Different bonds require different amounts of energy to be broken. For example, the bond between two chlorine atoms requires 242 kJ/mol of energy to break, whereas the bond between a hydrogen and chlorine atom requires almost double the energy (431 kJ/mol).

You may be given average bond energies in the exam and you’ll be expected to calculate the enthalpy change for the overall reaction using the following equation:

Worked example: bond energies

Answer:

With these questions you may find it useful to draw out the molecules to make sure you don’t miss out any bonds. Remember that when we have a big number in front of the molecule, we have to multiply the molecule by that number.

• Use the equation: enthalpy change = bonds broken - bonds formed

• Energy to break bonds in the reactants: (4 x 413) + (2 x 498) = 2648 kJ/mol

• Energy needed to form bonds in the products: (2 x 745) + (4 x 467) = 3358 kJ/mol

• Enthalpy change = 2648 - 3358 = -710 kJ/mol

• Enthalpy change is negative which means this is an exothermic reaction.

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