Question

The activation energies of the forward and backward reactions in the case of a chemical reaction are \(30.5\) and \(45.4 \mathrm{~kJ} / \mathrm{mol}\) respectively. The reaction is : (a) Exothermic (b) Endothermic (c) Neither exothermic nor endothermic (d) Independent of temperature

Short answer

The reaction is exothermic because the activation energy for the forward reaction is less than the activation energy for the backward reaction.

Step-by-step solution

Understand Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. For a forward reaction, it refers to the energy needed for the reactants to transform into products. For the reverse reaction, it is the energy required to revert the products back to reactants.

Compare Activation Energies

Determine whether the reaction is endothermic or exothermic by comparing the activation energies. If the activation energy of the forward reaction is lower than the backward reaction, the reaction is exothermic. Conversely, if the forward activation energy is higher, the reaction is endothermic.

Identify Reaction Type

Since the activation energy for the forward reaction (30.5 kJ/mol) is lower than that for the backward reaction (45.4 kJ/mol), the reaction releases energy to the surroundings, and is therefore exothermic.

Key concepts

Exothermic and Endothermic Reactions

Chemical reactions are often classified based on heat exchange with their surroundings. Exothermic reactions release heat, making the environment warmer. This means that the reactants have more energy, and as they transform into products, excess energy is given off, usually in the form of heat. Common examples include combustion reactions or the reaction between acids and bases.

On the contrary, Endothermic reactions absorb energy from the surroundings, which can result in a temperature drop. These reactions require energy to proceed, meaning the reactants absorb heat to form the products. Photosynthesis in plants is a classic example of an endothermic reaction.

Applying this to our exercise, by noting the lower activation energy for the forward reaction, we can confidently categorize the reaction as exothermic. This is because less energy is required to initiate the process than is released upon its completion, leading to a net release of energy.

Chemical Reaction Kinetics

Understanding the speed at which chemical reactions occur is crucial, and this field of study is known as chemical kinetics. Reaction kinetics looks at the rate of a chemical reaction and the various factors that influence this rate, which include temperature, concentration of reactants, surface area, and the presence of catalysts. The activation energy is a key concept here, serving as a barrier to the reaction; only when the reactants have sufficient energy to overcome this barrier will the reaction proceed at a perceptible rate.

In the context of our exercise, a lower activation energy for the forward reaction suggests that it's easier for the reactants to transform into products, hinting at a faster reaction rate in the forward direction. This implies that, at a given temperature, the forward, exothermic reaction tends to occur more readily than the endothermic reverse process.

Thermodynamics in Chemistry

The laws of thermodynamics govern all chemical reactions and determine the spontaneity and energy changes within a system. In chemistry, the first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transformed. This is directly applicable to exothermic and endothermic reactions, as the heat released or absorbed is merely energy changing forms.

Furthermore, the second law of thermodynamics explains that the entropy, or the degree of disorder in a system, usually increases over time, which affects how a reaction proceeds. In our exercise, the exothermic reaction likely leads to an increase in entropy as energy is dispersed in the form of heat to the surroundings, which aligns with this principle of thermodynamics.

To sum it up, the reactions' energy exchange behaviors express fundamental thermodynamic principles that allow us to predict the direction and feasibility of chemical processes.