Hess's Law and Enthalpy

Front

What is Hess's Law?

Back

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

Front

Define enthalpy of formation.

Back

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

Front

What is the enthalpy change for an exothermic reaction?

Back

The enthalpy change (ΔH) for an exothermic reaction is negative, indicating that energy is released to the surroundings.

Front

What is the enthalpy change for an endothermic reaction?

Back

The enthalpy change (ΔH) for an endothermic reaction is positive, indicating that energy is absorbed from the surroundings.

Front

How do you calculate the standard enthalpy change of formation for a compound?

Back

The standard enthalpy change of formation can be calculated using the enthalpy changes of combustion of the elements involved, applying Hess's Law.

Front

What is the standard enthalpy of combustion for methane (CH4)?

Back

The standard enthalpy of combustion for methane is approximately -890 kJ/mol.

Front

What does a positive enthalpy change indicate about a reaction?

Back

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

Front

What is the significance of the enthalpy change in a chemical reaction?

Back

The enthalpy change indicates whether a reaction is exothermic or endothermic, which helps predict the energy changes associated with the reaction.

Front

What is the enthalpy change for the combustion of hydrogen gas (H2)?

Back

The standard enthalpy change of combustion for hydrogen gas is approximately -286 kJ/mol.

Front

What is the relationship between enthalpy and temperature?

Back

Enthalpy is a function of temperature; as temperature increases, the enthalpy of a substance generally increases due to increased kinetic energy.