Enthalpy (OCR A-Level Chemistry A): Quizzes

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Hess' Law and Enthalpy Cycles
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What does Hess' law state about enthalpy change?

Total enthalpy change is path-independent

What is ΔfH∘\Delta_f H^\circ for elements in their standard state?

Always zero

Which formula calculates ΔrH\Delta_r H using formation enthalpies?

∑ΔfH\sum \Delta_f H products −∑ΔfH- \sum \Delta_f H reactants

Which formula calculates ΔrH\Delta_r H using combustion enthalpies?

∑ΔcH\sum \Delta_c H reactants −∑ΔcH- \sum \Delta_c H products

In formation enthalpy cycles, which direction do arrows point?

Upward from elements to compounds

In combustion enthalpy cycles, which direction do arrows point?

Downward to combustion products

Why is the order reversed between formation and combustion formulas?

Due to opposite arrow directions in cycles

If A=+110A = +110 kJ mol−1^{-1} and B=−150B = -150 kJ mol−1^{-1}, what is CC in A+B=CA + B = C?

−40-40 kJ mol−1^{-1}

When is the combustion enthalpy method particularly useful?

For organic compounds

What forms the common reference point in combustion enthalpy cycles?

Combustion products: \ceCO2(g)\ce{CO2(g)} and \ceH2O(l)\ce{H2O(l)}

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