If you need to multiply the following reaction by 2 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?
$H_2+0.5 O_2 \rightarrow H_2 O, \Delta H=-286 kJ$
A. 286 kJ
B. -572 kJ
C. -286 kJ
D. 572 kJ
Answer (1)
Multiply the given enthalpy change by 2.
Calculate the new enthalpy: Δ H n e w = 2 × ( − 286 kJ ) .
The new enthalpy change is − 572 kJ .
Explanation
Understanding the Problem We are given a chemical reaction and its enthalpy change, Δ H = − 286 kJ . The problem states that this reaction needs to be multiplied by 2 to be used as an intermediate step in a Hess's law problem. We need to find the new enthalpy change for the reaction.
Calculating the New Enthalpy When a chemical reaction is multiplied by a factor, the enthalpy change is also multiplied by the same factor. In this case, the reaction is multiplied by 2, so the new enthalpy change will be: Δ H n e w = 2 × Δ H o l d Δ H n e w = 2 × ( − 286 kJ )
Final Calculation Calculating the new enthalpy change: Δ H n e w = − 572 kJ
Examples
Hess's Law is a powerful tool in chemistry that allows us to calculate the enthalpy change of a reaction by using the enthalpy changes of other reactions. For example, imagine you want to find the enthalpy change for the formation of carbon dioxide from carbon and oxygen, but you can't measure it directly. You can use Hess's Law with intermediate reactions like the formation of carbon monoxide and its subsequent oxidation to carbon dioxide. By manipulating these intermediate reactions and their enthalpy changes, you can determine the overall enthalpy change for the formation of carbon dioxide. This is useful in industrial processes to optimize reaction conditions and predict energy requirements.
