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Enthalpy changes that are difficult to measure directly can often be determined using Hess' Law to construct an enthalpy cycle - CIE - A-Level Chemistry - Question 3 - 2014 - Paper 1
Question 3
Enthalpy changes that are difficult to measure directly can often be determined using Hess' Law to construct an enthalpy cycle. Which enthalpy change is indicated b... show full transcript
Worked Solution & Example Answer:Enthalpy changes that are difficult to measure directly can often be determined using Hess' Law to construct an enthalpy cycle - CIE - A-Level Chemistry - Question 3 - 2014 - Paper 1
Step 1
Identify the Enthalpy Change Indicated by X
Answer
To determine the enthalpy change indicated by X, we use Hess' Law. This law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps.
In this enthalpy cycle, we observe the formation of methane (CH4) from its constituent elements and the subsequent reaction leading to carbon dioxide (CO2) and water (H2O). We can express this cycle in terms of standard enthalpies of formation:
- The formation of CH4(g) from its elements:
- The transformation involving X, which we need to identify.
- The formation of CO2(g) and water (H2O):
To find X, we recognize that the enthalpy change must balance the cycle. Upon evaluating the choices provided, we establish that:
X corresponds to the enthalpy of formation of water. Thus, we can conclude that:
X = -2 × the enthalpy of formation of water, leading us to select:
D: +2 × the enthalpy of formation of water.