Question 1

Calculate the standard heat of formation of carbon monoxide (CO) from given combustion data.

Accepted Answer

Introduction / Context: Hess’s law enables computation of heats of formation from tabulated reaction enthalpies. Here, we deduce the formation enthalpy of CO using two combustion reactions leading to CO2. Given Data / Assumptions: C + O2 → CO2, ΔH = −394 kJ/kg·mole. CO + 1/2 O2 → CO2, ΔH = −284.5 kJ/kg·mole. Target: C + 1/2 O2 → CO (standard heat of formation of CO). Concept / Approach: Use Hess’s law by subtracting the CO combustion step from the carbon combustion step to isolate the formation of CO from its elements. Step-by-Step Solution: Write target: C + 1/2 O2 → CO (ΔH_f,CO = ?).Subtract (CO + 1/2 O2 → CO2, −284.5) from (C + O2 → CO2, −394).Resulting reaction: C + 1/2 O2 → CO with ΔH = −394 − (−284.5) = −109.5 kJ/kg·mole. Verification / Alternative check: Alternatively, use ΔH_f(CO2) = −394 and ΔH_rxn(CO→CO2) = ΔH_f(CO2) − ΔH_f(CO) to solve for ΔH_f(CO) = −394 − (−284.5) = −109.5. Why Other Options Are Wrong: +109.5 has wrong sign; +180 and +100 are inconsistent with given data; −180 does not follow from the provided enthalpies. Common Pitfalls: Forgetting to reverse/sign-change when subtracting reactions; mixing per-mole bases (ensure kg·mole basis is consistent). Final Answer: -109.5

Question 2

In adsorption/solution thermodynamics, which “heat of solution/adsorption” depends on both temperature and adsorbate concentration (surface coverage)?

Accepted Answer

Introduction / Context: In adsorption and in solution thermodynamics, heats can be defined in two ways: differential (at an incremental addition) and integral (average over the total amount). Distinguishing their dependencies is essential for interpreting calorimetric data and modeling isotherms. Given Data / Assumptions: “Adsorbate concentration” refers to surface coverage or solution composition. Temperature affects intermolecular interactions and thus heats. Concept / Approach: The differential heat is defined as the incremental enthalpy change per incremental amount added at a specified composition/coverage. It varies with both temperature and current coverage (or concentration). The integral heat is the average from zero up to a given amount and is less sensitive as it averages the varying differential values, though it can change with composition due to averaging. Step-by-Step Solution: Define differential heat: q_diff = (∂H/∂n) at fixed T and fixed coverage state.Recognize its strong dependence on the instantaneous state variables (T and coverage).Select “differential” as the quantity explicitly dependent on adsorbate concentration and temperature. Verification / Alternative check: Typical adsorption isotherms show q_diff decreasing with increasing coverage for heterogeneous surfaces, confirming dependence on concentration/coverage. Why Other Options Are Wrong: Integral heat is an average and does not represent the instantaneous dependence in the same way; “both” overstates the equivalence; “neither” and “only at infinite dilution” are incorrect. Common Pitfalls: Confusing average (integral) with incremental (differential) measures; ignoring coverage effects in calorimetric interpretation. Final Answer: differential

Stoichiometry Questions