∆H⁰rxn, for the reaction of Ca(s) + 2H+(aq) -> Ca2+(aq) + H2(g) is -544 kJ/mol.
∆H⁰rxn, for the reaction of CaO(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) is -193 kJ/mol.
Based on these results, what is the ∆Hf for CaO(s) [in kJ/mol] if the heat of formation of water is -286 kJ/mol?
Can someone please help me with this. I calculated all the given information above; however, I have no idea how to connect the dots and answer this question. Thank you!
Add equation 1 to the reverse of equation 2 to equation 3. Then add the delta Hs; remember to reverse the sign if you reverse the reaction. The final reaction will be
Ca(s) + 1/2 O2 --> CaO(s) Notice that everything else cancels.
Delta H for the desired reaction will be -544 + 193 - 286 = ?? kJ/mol.
-637 kJ/mol.
okay, I understand now... thanks Dr. Bob!
thanks Dr. bob.
I don't understand why you add the water heat of formation. Can someone please explain this, it doesn't make any sense to me.
To find the ΔHf for CaO(s), we can make use of the Hess's law, which states that if a reaction can be expressed as the sum of two or more other reactions, then the ΔH for the overall reaction is equal to the sum of the ΔH values of the individual reactions.
In this case, we can break down the formation of CaO(s) into two steps:
1. The reaction Ca(s) + 1/2O2(g) → CaO(s)
2. The reaction 1/2O2(g) + H2O(l) → H2(g)
We already have the given value for ΔH⁰rxn for the second reaction (-286 kJ/mol), and we need to determine the ΔH⁰rxn for the first reaction.
Using the given thermochemical equations, we can add the two reactions together to obtain the overall reaction:
Ca(s) + 2H⁺(aq) + 1/2O2(g) → Ca2⁺(aq) + H2O(l) + H2(g)
Since the overall reaction is the sum of the two reactions, the ΔH⁰rxn for the overall reaction is the sum of the ΔH⁰rxn values for the individual reactions:
ΔH⁰rxn for overall reaction = ΔH⁰rxn for reaction 1 + ΔH⁰rxn for reaction 2
ΔH⁰rxn for overall reaction = -544 kJ/mol + (-286 kJ/mol)
Now, we can rearrange the equation to solve for ΔH⁰rxn for the first reaction:
ΔH⁰rxn for reaction 1 = ΔH⁰rxn for overall reaction - ΔH⁰rxn for reaction 2
ΔH⁰rxn for reaction 1 = (-544 kJ/mol + (-286 kJ/mol))
After calculating the value for ΔH⁰rxn for reaction 1, we can conclude that it represents the heat of formation for CaO(s) because it is the enthalpy change for the formation of one mole of CaO(s) from its elements in their standard states.
Note: Remember to check the stoichiometric coefficients of the balanced equations to ensure they are properly balanced.