Energy Changes in Chemical Reactions

Topic: Energy Changes in Chemical Reactions

Basics of Energy Changes

• Energy is conserved in chemical reactions, meaning it cannot be created or destroyed, only transferred.
• Chemical reactions that release energy to the surroundings are described as exothermic.
• Examples of exothermic reactions include combustion, neutralisation and oxidation reactions.
• Chemical reactions that absorb energy from the surroundings are referred to as endothermic.
• Examples of endothermic reactions include thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.

Energy Profile Diagrams

• Energy profile diagrams depict the energy change during a reaction.
• The reactants usually start with a certain amount of energy (shown on the y-axis), which changes as the reaction progresses (shown along the x-axis).
• In an exothermic reaction, the energy of the products is less than that of the reactants. The difference in energy is released to the surroundings.
• In an endothermic reaction, the energy of the products is greater than that of the reactants. The difference in energy is absorbed from the surroundings.

Activation Energy

• The activation energy is the minimum amount of energy needed for a reaction to occur. It’s the energy needed to break the bonds in the reactants.
• A reaction will not occur unless the particles collide with sufficient energy to overcome the activation energy.
• Catalysts are substances that decrease the activation energy, thereby increasing the rate of reaction.

Measuring Energy Changes

• The energy change in a chemical reaction can be measured using a calorimeter.
• The heat absorbed or released in the reaction changes the temperature of a known mass of water.
• Using the specific heat capacity of water and the formula Q = mcΔT, where Q is heat, m is mass, c is specific heat capacity, and ΔT is change in temperature, you can calculate the energy change.

Hess’s Law

• Hess’s law states that the total enthalpy change for a reaction is the same no matter what route is taken.
• This means that if a reaction can be expressed as the sum of two or more other reactions, the sum of the energy changes in those reactions will equal the energy change of the initial reaction.
• This is useful in calculating energy changes for reactions that are difficult to measure directly.

Understanding Bond Energies

• Chemical bonds store energy.
• The energy required to break a bond is equal to the energy released when the bond is formed.
• When a chemical reaction occurs, bonds in the reactants are broken and new bonds are formed in the products. The energy change in the reaction is the difference between the energy needed to break the bonds and the energy released when the new bonds form.

Review these points diligently and conduct practice problems to understand the principles better. This topic necessitates a solid understanding of the application and implications of energy changes in reactions.