WS- Hess' Law

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What is Hess's Law?

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Hess's Law states that the total enthalpy change for a chemical 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 enthalpy (ΔH)?

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Enthalpy (ΔH) is a measure of the total energy of a thermodynamic system, including internal energy and the energy required to make room for it by displacing its environment.

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How do you calculate the enthalpy change using Hess's Law?

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To calculate the enthalpy change using Hess's Law, you can add the enthalpy changes of individual steps of a reaction that lead to the same overall reaction.

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What is the enthalpy change for the reaction: CuO(s) + H2(g) → Cu(s) + H2O(g)?

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The enthalpy change for this reaction is ΔH = -85 kJ.

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What is the enthalpy change for the reaction: 2Cu(s) + Cl2(g) → 2CuCl(s)?

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The enthalpy change for this reaction is ΔH = -274 kJ.

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What is the enthalpy change for the reaction: H2(g) + Cl2(g) → 2HCl(g)?

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The enthalpy change for this reaction is ΔH = -184 kJ.

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How do you combine reactions to find the overall enthalpy change?

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You can reverse reactions (changing the sign of ΔH) and multiply reactions by coefficients (multiplying ΔH by the same coefficient) to align them with the target reaction.

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What is the significance of a negative ΔH value?

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

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What is the significance of a positive ΔH value?

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

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What is the standard enthalpy of formation?

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The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.