Chapter 15: Chemical Energetics

Secondary Chemistry Topic Revision: Energy Change

Energy Change: Exothermic vs Endothermic

Definition of enthalpy change: the amount of heat energy given out or taken in during a reaction, and is given by the equation:

∆𝐻 = 𝑇𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒔 − 𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝒓𝒆𝒂𝒄𝒕𝒂𝒏𝒕

Type of reaction	Exothermic	Endothermic
Definition	A chemical reaction that gives out energy (often in the form of heat) to the surrounding, resulting in a rise in temperature in surroundings.	A chemical reaction that takes in energy (often in the form of heat) from the surroundings, causing a drop in temperature in the surroundings.
Energy level Diagram
Energy profile Diagram
Enthalpy Change, ΔH	Negative (<0)	Positive (>0)
Examples of Reactions	Combustion reactions Neutralisation Respiration Corrosion of metals	Photosynthesis Dissolution of ammonium nitrate crystals Decomposition of limestone

Common mistake #1: Assuming that energy level diagram and energy profile diagram is the same.

Energy level diagram will only show the energy levels of both the reactants and products, as well as the enthalpy change. Energy profile diagram also shows the energy levels of reactants and products, enthalpy change and the activation energy.

Common mistake #2: Using a double-headed arrow when labelling activation energy or enthalpy change.

For activation energy, the arrowhead will always be pointed upwards, whereas for enthalpy change, the arrowhead should point from reactants to products. Thus, for exothermic reactions, as the energy level of products is lower than that of reactants, the arrowhead will point downwards, while for endothermic reactions, it will point upwards instead.

Bond-breaking is an endothermic process as energy is taken in or absorbed to break a bond.

For a chemical reaction to occur, bonds in the reactants must be broken so that new bonds can be formed in the product.

Bond energy measures the strength of a chemical bond. The stronger the bond, the higher the bond energy. For hydrogen gas, the bond energy is 436 kJ/mol. This means that 436 kJ of energy is absorbed to break 1 mol of H-H bonds. Take note that for the chemical equation above, the enthalpy change is positive as energy is taken in.

𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑒𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑐ℎ𝑎𝑛𝑔𝑒: ∆𝐻 = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑏𝑜𝑛𝑑 𝑏𝑟𝑒𝑎𝑘𝑖𝑛𝑔 (𝒑𝒐𝒔𝒊𝒕𝒊𝒗𝒆) + 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑏𝑜𝑛𝑑 𝑓𝑜𝑟𝑚𝑖𝑛𝑔 (𝒏𝒆𝒈𝒂𝒕𝒊𝒗𝒆)

• Exothermic (negative ΔH): if energy released in bond forming is more than energy absorbed for bond breaking.
• Endothermic (positive ΔH): if energy absorbed for bond breaking is more than energy released in bond forming.

1. The activation energy of the forward reaction
2. The activation energy of the reverse reaction
3. Enthalpy change of the forward reaction
4. Enthalpy change of the reverse reaction

• The activation energy of the forward reaction will be the energy change between the reactant of the forward reaction (N₂ + 3H₂) and E₂.
• The activation energy of the reverse reaction will be the energy change between the reactant of the reverse reaction (2NH₃ or E₁) and E₂.
• Enthalpy change of the forward reaction will be from the reactant of the forward reaction to the product of the forward reaction (E₁).
• Enthalpy change of the reverse reaction will be from the reactant of the reverse reaction (E₁) to the product of the reverse reaction (N₂ + 3H₂).

Explanation in
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