Hess's Law and Enthalpy Cycles Quiz

Understanding Hess's Law and Enthalpy Cycles

• Hess's law states that if a reaction can be carried out by two different pathways, then the total enthalpy change for these two pathways will be the same, provided that the starting and final conditions are the same for both pathways.
• An enthalpy cycle or thermochemical cycle is a representation of Hess's law.
• Enthalpy cycles are useful when we cannot measure the enthalpy change of a reaction directly, and we need to find an alternative pathway to measure the enthalpy changes of these reactions.
• The two most common types of data used in Hess's law calculations are standard enthalpy change of formation and standard enthalpy change of combustion.
• The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions with all of the chemicals in their standard states.
• In enthalpy cycles using standard enthalpy change of formation data, the arrows point upwards.
• When using standard enthalpy change of formation data, we need to consider the direction of the arrows, and if it points in the wrong direction, we need to reverse the sign of the standard enthalpy change of formation.
• The standard enthalpy change of formation of chlorine is zero because chlorine is an element, not a compound.
• In a calculation of standard enthalpy change of reaction, we need to write down all the elements involved in the reaction, including their standard states, and use the standard enthalpy change of formation data provided to calculate the final answer.
• Standard enthalpy change of reaction is calculated by adding the standard enthalpy change of formation of the products and subtracting the standard enthalpy change of formation of the reactants.
• In the example provided, the standard enthalpy change of reaction for the reaction between methane and chlorine to make chloromethane and hydrogen chloride is -101.1 kilojoules per mole.
• Practicing with questions and examples can help in understanding and applying Hess's law and enthalpy cycles.