At 25°C, the following heats of reaction are known: At the same temperature, calculate ΔH for the following reaction:

Question

At 25°C, the following heats of reaction are known:  At the same temperature, calculate ΔH for the following reaction:

Quizplus · Accepted Answer

To solve the problem, we need to use Hess's Law, which states that the enthalpy change of a reaction is independent of the pathway between the initial and final states. In other words, if a reaction can be expressed as a sum of two or more reactions, then the enthalpy change of the overall reaction is the sum of the enthalpy changes of the individual reactions.

In this problem, we need to find the enthalpy change for the reaction:

CO(g) + 1/2O2(g) → CO2(g)

We can express this reaction as a sum of two known reactions:

CO(g) + 1/2O2(g) → CO2(g)        ΔH = -283.0 kJ/mol   (Given)
C(graphite) + O2(g) → CO2(g)   ΔH = -393.5 kJ/mol   (Given)

To obtain the desired reaction, we need to reverse the direction of the first reaction and multiply its enthalpy change by -1:

CO2(g) → CO(g) + 1/2O2(g)        ΔH = +283.0 kJ/mol

Now, we can add the two reactions to obtain the overall reaction and its enthalpy change:

C(graphite) + O2(g) → CO2(g)      ΔH = -393.5 kJ/mol  
CO2(g) → CO(g) + 1/2O2(g)         ΔH = +283.0 kJ/mol
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C(graphite) + 1/2O2(g) → CO(g)    ΔH = -110.5 kJ/mol

Therefore, the answer is B) -108.7 kJ/mol, which is the enthalpy change per mole of CO formed.

At 25°C, the Following Heats of Reaction Are Known