Hess's law flashcard

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 the reaction takes.

Front

Define an endothermic reaction.

Back

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings, resulting in a positive enthalpy change (∆H > 0).

Front

What is an exothermic reaction?

Back

An exothermic reaction is a chemical reaction that releases energy to its surroundings, resulting in a negative enthalpy change (∆H < 0).

Front

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

Back

To calculate the enthalpy change, sum the enthalpy changes of the individual steps of the reaction.

Front

What is the enthalpy change (∆H) for the combustion of 1 mole of CH4(g)?

Back

The enthalpy change for the combustion of 1 mole of CH4(g) is -890 kJ.

Front

If the enthalpy change for a reaction is +68 kJ, what is the ∆H for the reverse reaction?

Back

The ∆H for the reverse reaction is -68 kJ.

Front

What does a negative ∆H value indicate about a reaction?

Back

A negative ∆H value indicates that the reaction is exothermic, meaning it releases heat.

Front

What does a positive ∆H value indicate about a reaction?

Back

A positive ∆H value indicates that the reaction is endothermic, meaning it absorbs heat.

Front

How can you determine the enthalpy change for a reaction that is not directly measured?

Back

You can determine it using Hess's Law by combining the enthalpy changes of related reactions.

Front

What is the enthalpy change for the reaction: 2NO2(g) → N2(g) + 2O2(g) if ∆H = +68 kJ?

Back

The enthalpy change for the reverse reaction is -68 kJ.