Enthalpy, Molar Enthalpy, Intro. to Calorimetry

Enthalpy

H

The total internal (potential) energy of a substance. There is no instrument to measure this energy and quantify it.

The enthalpy change

ΔH

Quantity of energy changes during a cehmical or physical change

A measurement of the system we cannot make directly

Measuring Enthalpy Change

We measure temperature changes, ΔT, of the surroundings and use this value to calculate the heat lost or gained in the surroundings.

Since heat lost by the surroundings is absorbed into a system, or heat lost by a system is absorbed into the surroundings, we can then reason that ΔH_system= -q_surroundings

q

quantity of heat (J)

m

mass (g)

ΔT

change in temperatuer (celsius)

c

specific heat capacity (J/gdegreescelsius)

Molar Enthalpy

The enthalpy change per 1 mole of a substance undergoing a change

Symbol: ΔH_{x (indicates the type of change)}

Unit: kj/mol

eg. ΔH_vap

Combustion of hydrogen gas

Write the skeleton equation

Add the heat term given ΔH_comb= -241.8 kj/mol

• The sign of ΔH_comb is negative, so the reaction is exoterhmic and the energy value must be written on the RHS to indicate that heat is released. Notice the unit (/mol is taken away when placed into the equation)

When balancing, H₂ must remain at one mole because the heat term is for the molar enthalpy of combustion for only one mole of hydrogen.

Since molar enthalpies describe energy changes for 1 mole of a specified substance, for amounts other than 1 mole we use the equation:

ΔH

Enthalpy Change (kj)

n

#moles (mol)

ΔH_x

Molar Enthalpy (kj/mol)