Consider the generic reaction:
A + 2 B → C + 3 D ΔH = 155 kJ
Determine the value of ΔH for each related reaction.
a. 3 A + 6 B → 3 C + 9 D
b. C + 3 D → A + 2 B
c. 1/2 C + 3/2 D → 1/2 A + B

Verified step by step guidance

1. insert step 1> Identify the given reaction and its enthalpy change: A + 2B \rightarrow C + 3D with \Delta H = 155 \text{ kJ}.
2. insert step 2> For reaction (a), 3A + 6B \rightarrow 3C + 9D, notice that the entire reaction is multiplied by 3. Therefore, multiply the \Delta H of the original reaction by 3.
3. insert step 3> For reaction (b), C + 3D \rightarrow A + 2B, observe that this is the reverse of the original reaction. To find \Delta H for the reverse reaction, change the sign of \Delta H from positive to negative.
4. insert step 4> For reaction (c), \frac{1}{2}C + \frac{3}{2}D \rightarrow \frac{1}{2}A + B, note that the original reaction is halved. Therefore, divide the \Delta H of the original reaction by 2.
5. insert step 5> Apply these transformations to the \Delta H value of the original reaction to find the \Delta H for each related reaction.

Key Concepts

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). In this question, the given ΔH of 155 kJ represents the energy change for the reaction as written, which is crucial for calculating ΔH for related reactions.

Stoichiometry

Stoichiometry involves the quantitative relationships between the reactants and products in a chemical reaction. It allows us to determine how changes in the coefficients of a balanced equation affect the overall enthalpy change. For example, if the coefficients are multiplied by a factor, the ΔH value must also be multiplied by the same factor to maintain the relationship between the amounts of substances involved.

Reverse Reactions

The enthalpy change for a reverse reaction is equal in magnitude but opposite in sign to that of the forward reaction. This principle is essential when calculating ΔH for reactions that are the reverse of the original reaction. Understanding this concept helps in determining the enthalpy changes for reactions that involve the same substances but in different arrangements or directions.