## C3.3 Calculating and explaining energy change

Knowing the amount of energy involved in chemical reactions is useful so that resources are used efficiently and economically. It is possible to measure the amount of energy experimentally or to calculate it.

Candidates should use their skills, knowledge and understanding to:
consider the social, economic and environmental consequences of using fuels
interpret simple energy level diagrams in terms of bond breaking and bond formation (including the idea of activation energy and the effect on this by catalysts)
evaluate the use of hydrogen to power cars compared to other fuels

‘HEADLINE’ SUMMARY OF TOPIC

Measuring energy change
Energy is measured in joules (J).
A simple water-containing calorimeter may be used to ‘capture’ the energy released (or absorbed) by a chemical reaction.
When using this type of calorimeter, use the formula Q= m c ΔT  to measure the energy change of a chemical reaction, where:
Q   Energy Change (J)
m   mass of water (g),
c   specific heat capacity of water, which is 4.2 J g−1 °C−1
ΔT temperature change (°C)

This means that 4.2 joules of heat energy will raise the temperature of 1g of water by 1°C. Endothermic reaction. A is activation energy. Energy needed for bond breaking (A) is more than the energy released by bond forming (B). Hence, the overall energy for the reaction (C) is a positive value.

Endothermic reaction:
A chemical reaction that absorbs heat from its surroundings.
The energy of the products is more than the energy of the reactants.
The energy needed to break existing bonds is greater than the energy released from forming new bonds.
The overall heat energy change for the chemical reaction, ΔH is, shown as a +ve value. Exothermic reaction. B is activation energy. Energy released by bond forming (C) is more than the energy needed by bond breaking (B). Hence, the overall energy for the reaction (A) is a negative value.

Exothermic reaction:
A chemical reaction that releases heat to its surroundings.
The energy of the products is more than the energy of the reactants.
The energy released from forming new bonds is greater than the energy needed to break existing bonds.
The overall heat energy change for the chemical reaction, ΔH is, shown as a −ve value.
Acid-alkali neutralisation and combustion of fuels (e.g. fossil fuels, hydrogen or alcohol) are examples of exothermic reactions. The fact that a combustion reaction generates heat energy makes it particularly useful as a fuel.

Activation energy and action of a catalyst

Activation energy is the minimal energy needed to initiate a chemical reaction.
Activation energy is equivalent to the energy needed to break all the bonds of the reactants to form intermediate particles. As the reaction proceeds, intermediate particles form new bonds and release energy to form the product.
If more energy is released during bond forming than bond breaking, then the overall reaction is exothermic.
Conversely, if more energy is needed to break bonds than released by bond breaking, then the overall reaction is endothermic.

A high activation energy makes the reaction slow. A low activation energy makes the reaction fast.

Catalysts speed up the rate of chemical reaction by lowering the activation energy and increasing the frequency of successful particle collisions.

Determining the overall energy of a chemical reaction using bond energies (for Higher tier candidates)

When calculating the overall energy change of a chemical reaction by using bond energies, apply the following rules:

Breaking bonds needs energy, but forming bonds releases energy
The numerical value of the bond energy is the same whether you are trying to break it or form it, but the heat change is +ve for breaking bonds and −ve for forming bonds.

For example, the O-H bond energy is 464 kJ/mol and a molecule of water only has two O-H bonds.

So, the energy needed to break all the bonds in a water molecule= 2 ×464 = + 928 kJ/mol
However, the energy needed to form all the bonds in a water molecule= 928 kJ/mol
The minus sign is important as this signifies heat energy is released by forming bonds.

To calculate the overall energy of a chemical reaction:
STEP 1: Add up all bond energies for ALL reactants (left-hand side of the equation). Where necessary, multiply the bond energy of a molecule by the number of molecules present in a balanced chemical equation.
This total will always be a positive heat energy value. Units are normally kJ per mole.

STEP 2: Add up all bond energies for ALL products (right-hand side of the equation). Where necessary, multiply the bond energy of a molecule by the number of molecules present in a balanced chemical equation.
Having determined this total value, now apply a minus sign (make it negative) to indicate that heat energy is released. Units are normally kJ per mole.

STEP 3: Apply the following formula,

Overall energy for a chemical reaction = Total energy of bond breaking + Total energy for bond forming.

Remember, that the total energy for bond forming is always a negative value.

Worked examples are shown in the following slideshow:

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Energy from reactions - Simple calorimetry (fuels or foods) The relative amounts of energy released when substances (such as fuels or foods) burn can be measured by simple calorimetry, eg by heating water in a glass or metal container. Using this simple calorimeter, the amount of energy released is calculated using the equation: Q = mc ΔT Q= the energy, is measured in joules (J) ΔT= change  