Recognizing the Reverse Reaction in Thermochemistry

Recognizing the reverse of an equation

Write the reverse of the equation 2C(s) + 3H2 (g) → C2H6 (g) ΔH = − 84.5 kJ

C2H6 (g) → 2C(s) + 3H2 (g) ΔH = + 84.5 kJ

In thermochemistry, it is important to understand that chemical reactions can proceed in both the forward and reverse directions. The reverse reaction is simply the opposite of the forward reaction, where the products become the reactants, and the reactants become the products.

To recognize the reverse of an equation, we look at the reactants and products and interchange their positions. In this case, the given equation is: 2C(s) + 3H2(g) → C2H6(g) ΔH = -84.5 kJ

To write the reverse equation, we switch the positions of the reactants and products: C2H6(g) → 2C(s) + 3H2(g) ΔH = +84.5 kJ

By reversing the equation, we also reverse the sign of the heat of reaction (∆H). In the original equation, the heat of reaction is -84.5 kJ, indicating that the reaction releases 84.5 kJ of heat energy. In the reverse equation, the heat of reaction becomes +84.5 kJ, indicating that the reaction now absorbs 84.5 kJ of heat energy.

It's important to note that the reverse reaction occurs under different conditions compared to the forward reaction. While the forward reaction may be exothermic (releasing heat), the reverse reaction becomes endothermic (absorbing heat) due to the change in the sign of the heat of reaction.

Understanding the reverse of an equation is crucial in thermochemistry, as it allows us to recognize that a reaction can proceed in both directions depending on the prevailing conditions. The reverse reaction is often observed when the products of a reaction have a higher concentration or are removed from the system, causing the reaction to shift towards the reactants.

In summary, recognizing the reverse of an equation involves interchanging the positions of the reactants and products and changing the sign of the heat of reaction (∆H). In the given example, the reverse of the equation 2C(s) + 3H2(g) → C2H6(g) with a heat of reaction of -84.5 kJ is C2H6(g) → 2C(s) + 3H2(g) with a heat of reaction of +84.5 kJ. Understanding the reverse reaction is essential in thermochemistry to comprehend the bidirectional nature of chemical reactions and the associated heat changes.