If you need to reverse the following reaction in order for it to be an intermediate reaction in a Hess's law problem, what would be the final value for the enthalpy of reaction you use for this intermediate reaction?
$C_2 H_4+3 O_2 \rightarrow 2 CO_2+2 H_2 O, \Delta H=-1410 kJ$
A. -1410 kJ
B. -2820 kJ
C. 2820 kJ
D. 1410 kJ
Answer (1)
When reversing a chemical reaction, the sign of the enthalpy change ( Δ H ) is also reversed.
Given the reaction C 2 H 4 + 3 O 2 → 2 C O 2 + 2 H 2 O with Δ H = − 1410 kJ .
Reversing the reaction gives 2 C O 2 + 2 H 2 O → C 2 H 4 + 3 O 2 .
The new enthalpy change is 1410 kJ .
Explanation
Analyzing the Reaction The given reaction is: C 2 H 4 + 3 O 2 → 2 C O 2 + 2 H 2 O with an enthalpy change of Δ H = − 1410 kJ . We need to reverse this reaction and determine the new enthalpy change.
Reversing the Reaction When a reaction is reversed, the products become reactants and vice versa. The reversed reaction is: 2 C O 2 + 2 H 2 O → C 2 H 4 + 3 O 2 .
Calculating the New Enthalpy The enthalpy change for the reverse reaction is the negative of the enthalpy change for the forward reaction. Therefore, if the forward reaction has Δ H = − 1410 kJ , the reverse reaction has Δ H = − ( − 1410 kJ ) = 1410 kJ .
Examples
In chemistry, Hess's Law is used to calculate the enthalpy change of a reaction by breaking it down into a series of steps. If one of these steps involves reversing a reaction, you need to change the sign of the enthalpy change for that step. For example, if you are trying to determine the overall enthalpy change for the formation of methane from its elements, and one of the intermediate steps involves the combustion of methane, you would need to reverse the combustion reaction and change the sign of its enthalpy change before adding it to the other steps.
