Question

applying hesss law
consider the intermediate equations:
c(s) + o₂(g) → co₂(g) δh₁=-393.5kj
2co(g) + o₂(g) → 2co₂(g) δh₂=-566.0kj
2h₂o(g) → 2h₂(g) + o₂(g) δh₃ = 483.6kj
with the overall reaction:
c(s)+h₂o(g) → co(g)+h₂(g) δh₍rxn₎=?
what must be done to calculate the enthalpy of reaction? choose three correct answers.
the third equation must be reversed.
the second equation must be halved.
the third equation must be halved.

Explanation:

Step1: Analyze reactants and products

We want to get $C(s)+H_2O(g)
ightarrow CO(g) + H_2(g)$. The first - equation has $C(s)$ as a reactant which is correct. The second equation has $CO$ on the product side but in a different stoichiometry. The third equation has $H_2O$ as a reactant in the wrong stoichiometry and $H_2$ and $O_2$ as products.

Step2: Manipulate equations for correct stoichiometry

For the $CO$ part, we don't need to halve the second equation as it won't help in getting the correct overall reaction. For the $H_2O$ and $H_2$ part, we need to reverse the third equation to get $H_2O$ on the reactant side and then halve it to match the stoichiometry of the overall reaction.

Answer:

The third equation must be reversed; The third equation must be halved