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?
[tex]H_2+0.5 O_2 \rightarrow H_2 O, \Delta H=-286 kJ[/tex]
A. 572 k
B. -572 k
C. -286 kJ
D. 286 kJ
Answer (2)
The problem provides a chemical reaction with Δ H = − 286 kJ .
We multiply the reaction by 2, which means we also multiply the enthalpy change by 2.
Calculate the new enthalpy change: Δ H n e w = 2 × ( − 286 kJ ) .
The final enthalpy of reaction is − 572 kJ .
Explanation
Understanding the Problem We are given a chemical reaction and its enthalpy change, Δ H . The reaction is: H 2 + 0.5 O 2 → H 2 O , Δ H = − 286 kJ We need to find the enthalpy change when the reaction is multiplied by 2.
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, we need to multiply the given reaction by 2. So, the new reaction becomes: 2 H 2 + O 2 → 2 H 2 O The new enthalpy change, Δ H n e w , is calculated by multiplying the original enthalpy change by 2: Δ H n e w = 2 × Δ H
Final Answer Now, we substitute the given value of Δ H = − 286 kJ into the equation: Δ H n e w = 2 × ( − 286 kJ ) = − 572 kJ Therefore, the final enthalpy of reaction is -572 kJ.
Examples
Hess's Law is often used to calculate the enthalpy change for reactions that are difficult or impossible to measure directly. For example, if you want to find the enthalpy change for the formation of methane ( C H 4 ) from its elements, you can use a series of combustion reactions and apply Hess's Law. By manipulating these reactions (multiplying by coefficients, reversing them, etc.) and summing their enthalpy changes, you can determine the enthalpy of formation for methane. This is useful in many industrial processes, such as designing efficient combustion engines or optimizing chemical reactions for maximum energy output.
When the reaction is multiplied by 2, the enthalpy change also doubles. The new enthalpy of reaction is -572 kJ. Therefore, the answer is option B: -572 kJ.
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