The equation for an enthalpy change is shown. The enthalpy change is Q.Q2C(s) + 3H 2(g) + 3.5O 2(g) 2CO 2(g) + 3H 2O(l)What is the correct expression to calculate Q?A 2 [CO 2(g)] – 3 [H 2(g)]B 3 [H 2O(g)] + 2 [CO 2(g)]C 2 [CO 2(g)] – 3 [H 2(g)]D 3 [H 2O(l)] + 2 [CO 2(g)]
Question
The equation for an enthalpy change is shown. The enthalpy change is Q.Q2C(s) + 3H 2(g) + 3.5O 2(g) 2CO 2(g) + 3H 2O(l)What is the correct expression to calculate Q?A 2 [CO 2(g)] – 3 [H 2(g)]B 3 [H 2O(g)] + 2 [CO 2(g)]C 2 [CO 2(g)] – 3 [H 2(g)]D 3 [H 2O(l)] + 2 [CO 2(g)]
Solution
The correct expression to calculate Q, based on the given chemical reaction, would be the difference between the sum of the enthalpies of the products and the sum of the enthalpies of the reactants.
In this case, the products are CO2(g) and H2O(l), and the reactants are C(s), H2(g), and O2(g).
The stoichiometric coefficients in front of each compound in the balanced chemical equation give the number of moles of each compound involved in the reaction.
Therefore, the correct expression to calculate Q would be:
Q = [2 * enthalpy(CO2(g)) + 3 * enthalpy(H2O(l))] - [enthalpy(C(s)) + 3 * enthalpy(H2(g)) + 3.5 * enthalpy(O2(g))]
This corresponds to option D: 3 * [H2O(l)] + 2 * [CO2(g)].
Please note that the enthalpy values of the compounds (in their standard states) are usually given or can be found in thermodynamic tables.
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