3.1.4 Enthalpies

What is the first law of thermodynamics

Energy cannot be created or destroyed but can be converted into different forms

Define enthalpy

a measure of the heat content of a substance at a constant pressure

Describe an exothermic graph

Enthalpy of reactants > enthalpy of products

Describe an endothermic graph

Enthalpy of reactants < enthalpy of products

Give the equation for enthalpy change

Enthalpy of products - enthalpy of reactants

Name two examples of exothermic reactions

Combustion of fuels, respiration

Name two examples of endothermic reactions

Photosynthesis, thermal decomposition

Define enthalpy change of reaction (∆Hr)

The enthalpy change associated with a stated reaction

Define enthalpy change of combustion (∆Hc)

The enthalpy change when one mole of a substance undergoes complete combustion

Define standard enthalpy change of combustion (∆H°c)

The enthalpy change when one mole of substance undergoes complete combustion under standard conditions in standard states

Define enthalpy change of formation (∆Hf)

The enthalpy change when one mole of a compound is formed from its elements

Define standard enthalpy change of formation (∆H°f)

The enthalpy change when one mole of a substance is formed in its standard state from its elements in their standard states

Name Hess’s Law

“the enthalpy change is independent of the path taken”

For formation, which way do the Hess Cycle’s arrows face

Upwards

For combustion, which way do the Hess Cycle’s arrows face

Downwards

Define calorimetry

A practical determination of enthalpy changes

What happens in an exothermic calorimetry reactions

Water heats up

What happens in an endothermic calorimetry reaction

Water cools down

Name the calculation for finding heat energy

mass x SHC x change in temp

Give the unit for heat energy

kJ

How is ∆T calculated

with a graph

Give the equation for enthalpy change

(-) heat energy / moles

Give the unit for enthalpy change

kJ mol -1

What will the enthalpy change be in a combustion reaction (negative/positive)

Negative