Match each thermodynamic symbol with its correct definition. Drag statements on the right to match the left AH” The enthalpy change of rcaction measured at thc standard state The enthalpy change when mole of compound forms from Its elements In thelr standard states AH
The Correct Answer and Explanation is:
Here’s how to correctly match each thermodynamic symbol with its definition:
ΔH°: The enthalpy change of a reaction measured at the standard state.
ΔHf°: The enthalpy change when 1 mole of a compound forms from its elements in their standard states.
Explanation
In thermodynamics, enthalpy change (ΔH) represents the heat absorbed or released during a chemical reaction at constant pressure. Enthalpy is a state function, meaning it depends only on the initial and final states of the system, not on the path taken.
1. ΔH° – Standard Enthalpy Change of Reaction
The symbol ΔH° specifically refers to the enthalpy change of a reaction under standard conditions. Standard conditions mean:
- Temperature = 298 K (25°C)
- Pressure = 1 atm
- All reactants and products in their standard states (pure solids, pure liquids, or 1 M solutions)
This value helps chemists understand how much heat is released or absorbed in a reaction when all substances are in a defined state, making reactions easier to compare. For example, in a combustion reaction, ΔH° tells us how much heat is released when a substance burns in oxygen.
2. ΔHf° – Standard Enthalpy of Formation
The symbol ΔHf° stands for the standard enthalpy of formation, which is the enthalpy change when 1 mole of a compound is formed from its elements in their most stable forms under standard conditions. For example, the formation of water from hydrogen and oxygen: H2(g)+12O2(g)→H2O(l)ΔHf∘=−285.8 kJ/mol\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \quad \Delta H_f^\circ = -285.8\ \text{kJ/mol}
This value is foundational for calculating the enthalpy changes of more complex reactions using Hess’s Law.
In summary, while both ΔH° and ΔHf° deal with energy changes, ΔH° refers to the overall reaction, and ΔHf° focuses on forming compounds from elements. Understanding the difference is crucial in thermochemistry.
