Energy Transfers in Reactions

Energy in Chemical Reactions

Chemical reactions involve breaking and forming bonds between atoms. Energy is stored in these chemical bonds, known as chemical potential energy.

During a reaction, bonds in the reactants break, which requires energy to be absorbed. New bonds then form in the products, releasing energy. The overall energy change depends on the difference between the energy absorbed to break bonds and the energy released when new bonds form.

Energy is transferred between the reacting system and its surroundings, usually as heat. This transfer causes temperature changes that can be observed during the reaction.

For example, when methane burns, energy stored in methane and oxygen bonds is released to the surroundings as heat and light.

The amount of energy transferred is measured in kilojoules (kJ).

For instance, if a reaction releases 200 kJ of energy to the surroundings, the surroundings will get warmer by absorbing this energy.

Exothermic Reactions

Exothermic reactions transfer energy to the surroundings, usually as heat, causing the temperature of the surroundings to increase.

In these reactions, the total energy released when new bonds form is greater than the energy needed to break the original bonds.

Common examples include:

• Combustion: Burning fuels like petrol or natural gas releases energy used for heating or powering engines.
• Respiration: The process cells use to release energy from glucose.

Exothermic reactions often feel hot to the touch because they release heat.

For example, when magnesium burns in air, it reacts with oxygen to form magnesium oxide and releases a lot of heat and light:

$\text{Mg} + \text{O}_2 \rightarrow \text{MgO} + \text{energy}$

This reaction is exothermic because energy is given out to the surroundings.

Endothermic Reactions

Endothermic reactions absorb energy from the surroundings, usually as heat, causing the temperature of the surroundings to fall.

In these reactions, more energy is needed to break bonds than is released when new bonds form.

Examples include:

• Photosynthesis: Plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen.
• Thermal decomposition: Breaking down compounds by heating, such as heating calcium carbonate to form calcium oxide and carbon dioxide.

Endothermic reactions often feel cold because they take in heat from the surroundings.

For example, the thermal decomposition of calcium carbonate:

$\text{CaCO}_3 + \text{energy} \rightarrow \text{CaO} + \text{CO}_2$

Energy must be absorbed for this reaction to occur, so it is endothermic.

Reaction Profiles

Reaction profiles are energy level diagrams that show how the energy of the reactants and products changes during a reaction.

The vertical axis shows energy, and the horizontal axis shows the progress of the reaction.

Key features of reaction profiles include:

• Reactants energy level: The starting energy of the substances before the reaction.
• Products energy level: The energy of the substances after the reaction.
• Activation energy (Ea): The minimum energy needed to start the reaction, shown as a peak on the diagram.
• Overall energy change: The difference in energy between reactants and products.

In an exothermic reaction, the products have less energy than the reactants, so the overall energy change is negative (energy is released).

In an endothermic reaction, the products have more energy than the reactants, so the overall energy change is positive (energy is absorbed).

For example, a reaction profile for an exothermic reaction might look like this:

Reactants start at a higher energy level, the curve rises to the activation energy peak, then falls to a lower energy level for the products.

The difference in height between reactants and products shows the energy released.

Learning example: Consider a reaction where reactants have an energy of 500 kJ, products have 300 kJ, and the activation energy is 150 kJ.

The overall energy change is $300 - 500 = -200\text{ kJ}$, meaning 200 kJ of energy is released to the surroundings.

The activation energy is 150 kJ, which is the energy needed to start the reaction.