Honors Chem Review Hess' Law

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What is Hess' Law?

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Hess' Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps the reaction takes. It is based on the principle of conservation of energy.

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What is the formula for calculating enthalpy change (∆H)?

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∆H = H(products) - H(reactants), where H is the enthalpy of the substances involved.

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What does a negative ∆H value indicate?

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A negative ∆H value indicates that the reaction is exothermic, meaning it releases heat to the surroundings.

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What does a positive ∆H value indicate?

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A positive ∆H value indicates that the reaction is endothermic, meaning it absorbs heat from the surroundings.

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How can Hess' Law be applied to calculate ∆H for a reaction?

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By using known enthalpy changes of related reactions, you can add or subtract these values to find the ∆H for the desired reaction.

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What is the enthalpy change for the formation of water (H2O) from hydrogen and oxygen?

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The enthalpy change (∆H) for the reaction H2(g) + 1/2 O2(g) → H2O(g) is -242 kJ.

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What is the enthalpy change for the combustion of magnesium?

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The enthalpy change (∆H) for the reaction Mg(s) + 1/2 O2(g) → MgO(s) is -602 kJ.

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What is the significance of the law of conservation of energy in Hess' Law?

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It allows us to conclude that energy cannot be created or destroyed, only transformed, which is fundamental in calculating enthalpy changes.

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How do you determine the ∆H for a reaction that is the reverse of a known reaction?

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The ∆H for the reverse reaction is equal in magnitude but opposite in sign to the ∆H of the forward reaction.

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What is the enthalpy change for the reaction 2CuO(s) → Cu2O(s) + 1/2 O2(g)?

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The enthalpy change (∆H) for this reaction is +142 kJ.