A Level Chemistry OCR Practice Exam

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What does the standard enthalpy change of reaction, ∆Hrө, represent?

The heat change at constant pressure for any reaction
The enthalpy change at constant volume for any reaction
The enthalpy change under standard conditions for specified molar quantities
The heat absorbed during an exothermic reaction

The correct answer is: The enthalpy change under standard conditions for specified molar quantities

The standard enthalpy change of reaction, ∆Hrө, specifically represents the enthalpy change that occurs under standard conditions for specified molar quantities of reactants and products. Standard conditions typically refer to a pressure of 1 atmosphere and a specified temperature, usually 298 K (25 degrees Celsius). The term "standard" underscores that this measurement is control over variables to allow for consistency in comparison of reactions. This definition is essential in thermodynamics because it provides a standardized reference point for comparing the energetics of different chemical reactions. Thus, when calculating or predicting the heat exchange in a reaction, scientists reference this enthalpy change to understand how the system behaves under those controlled conditions. The other choices do not capture the specific nature of the standard enthalpy change of reaction in relation to its defined conditions and quantities.