Consider the reaction pathway shown below.
ΔH = -210 kJ mol⁻¹
ΔH = -50 kJ mol⁻¹
ΔH = -86 kJ mol⁻¹
According to Hess's Law, the ΔH value, in kJ mol⁻¹, for reac... show full transcript
Worked Solution & Example Answer:Consider the reaction pathway shown below - Scottish Highers Chemistry - Question 16 - 2022
Step 1
Step 1: Determine ΔH for reaction W to Z
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Answer
The reaction pathway states that the enthalpy change (ΔH) for the reaction from W to Z is -210 kJ mol⁻¹.
Step 2
Step 2: Determine ΔH for reaction W to X
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Answer
The enthalpy change for the reaction from W to X is -50 kJ mol⁻¹.
Step 3
Step 3: Determine ΔH for reaction X to Y
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Answer
The enthalpy change for the reaction from X to Y is -86 kJ mol⁻¹.
Step 4
Step 4: Apply Hess's Law to find ΔH for reaction Z to Y
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According to Hess's Law, the total enthalpy change for a reaction is the sum of the enthalpy changes of the individual steps. Thus, to find ΔH from Z to Y, we can rearrange the reactions:
Starting from W to Z, we have:
-210 kJ.
To reach Y from Z, we must go from Y to W, which can be seen as reversing the W to X (adding +50 kJ) and then adding the enthalpy change from X to Y (adding +86 kJ).
Combining these:
ΔH (Z to Y) = ΔH (W to Z) + ΔH (W to X) + ΔH (X to Y)
ΔH (Z to Y) = -210 + 50 + 86 = -74 kJ mol⁻¹.
Step 5
Step 5: Conclusion
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Answer
Thus, according to Hess's Law, the ΔH value for the reaction from Z to Y is -74 kJ mol⁻¹, corresponding to option B.
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