•Enthalpy (H) is used to quantify the heat flow into or out
of a system in a process that occurs at constant pressure.
•Enthalpy is defined as
H = E + PV
E = internal energy of the system
P = pressure of the system
V = volume of the system
•Enthalpy is a state function (independent of path).
For a chemical reaction, the enthalpy change is given by the
following equation:
∆H = H (products) – H (reactants)
∆H = heat given off or absorbed during a reaction at
constant pressure
Hproducts < Hreactants
∆H < 0
Hproducts > Hreactants
∆H > 0
Thermochemical Equations
Is ∆H negative or positive?
System absorbs heat
Endothermic
∆H > 0
6.01 kJ are absorbed for every 1 mole of ice that melts at
00C and 1 atm.
H2O (s)
H2O (l)
∆H = 6.01 kJ
Thermochemical Equations
Is ∆H negative or positive?
System gives off heat
Exothermic
∆H < 0
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (l)
∆H = -890.4 kJ
The science of measuring heat.
Device used experimentally to measure the heat associated
with a chemical reaction is a calorimeter.
Coffee Cup
calorimeter
Calorimetry is based on
observing the temperature
change when a body absorbs
or discharges energy as heat.
•When substances are heated they respond differently. Some
substances might require a great deal of energy to raise its
temperature, whereas another may not. A measure of this
property is heat capacity.
•Heat capacity (C) = heat absorbed/increase in temperature
heat absorbed
C=
increase in temperature
•When an element or a compound is heated, the energy
required will depend on the amount of the substance
present.
•For example, it takes twice as much energy to raise the
temperature of two grams of water by one degree than it
does to raise the temperature of water by one degree.
•The amount of a substance must be specified when
defining its heat capacity.
•If heat capacity is given per gram, it is called specific heat
capacity and has units of J/oC•g or J/K•g.
•If heat capacity is given per mole, it is called molar heat
capacity, and it has the units J/oC•mol or J/K•mol.
•Note the heat capacities
of metals are less than
that of water.
•It takes less heat to
change the temperature
of a gram of a metal by 1
oC than for a gram of
water.
•Pressure (atmospheric pressure) remains
constant.
•Used to determine the changes in
enthalpy (heats of reactions) for reactions
occurring in solution.
•Change in enthalpy equals the heat.
•Under constant volume, a bomb
calorimeter is used.
•Weighed reactants are placed inside and
ignited.
•Energy change is determined by
measuring the increase in the
temperature of the water and other
calorimeter parts.
•Change in volume is zero so no work is
done so change in energy is equal to the
heat.