Thermochemistry

ThermochemistryThermochemistry is concerned with the

heat changes that occur during chemical reactions.

Can deal with gaining or losing heat

EnergyThe capacity for doing work or supplying

heat.- Energy is only detected because of its effects

Energy stored within the structural units of chemical substances is called Chemical Potential Energy (Gasoline)

Heat (q)Energy that transfers from one object to

another because of a temperature difference between them

Heat is not detectable, only changes caused by heat

Heat always flows from a warmer object to a cooler object

UniverseThe system and its surroundings make up

the universeSystem – part of the universe you focus onSurroundings – everything else in the

universe

EndothermicA process that absorbs heat from the

surroundingsHeat flowing into a system from its

surroundings is defined as positive q

heat

system

ExothermicA process that releases heat to its

surroundingsHeat flowing out of the system to its

surroundings is defined as negative q

heat

system

Calorie vs. JouleCalorie – the quantity of heat needed to

raise the temperature of 1g of water 1oC1 Calorie = 1kilocalorie = 1000 calories

Joule – the quantity of heat needed to raise the temperature of 1g of water 0.2390oC1 J = 0.2390 cal 4.184J = 1 cal

Heat CapacityThe amount of heat needed to increase the

temperature of an object exactly 1oCThe greater the mass of the object, the

greater its heat capacity.Which has more heat capacity, a drop of

water, or an entire pool?

Specific HeatThe amount of heat it takes to raise the

temperature of 1g of the substance 1oCThe higher the specific heat, the longer it

takes to heat up.

q

m x ΔTC = =

heat

mass (g) x change in temp

CalorimetryThe accurate and precise measurement of

heat change for chemical and physical processes.

The heat released by the system is equal to the heat absorbed by its surroundings

Calorimeters

Enthalpy (H)For systems at constant pressure, the heat

content is the same as a property called enthalpy of the system

If a reaction occurs at constant pressure, enthalpy can be used interchangeably with heat.

q = ΔH

ΔH = m x C x ΔT

Thermochemical EquationAn equation that includes the heat of

change

CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ

Heat of ReactionThe heat change for the equation exactly

as written.

Heats of reaction at constant pressure (1atm @ 25o C) reported as ΔH

Cont.Exothermic Endothermic

CaO(s) + H2O(l)

Ca(OH)2(s)

ΔH = -65.2kJ ΔH + 129kJ

Na2CO3(s) + H2O(g) + CO2(g)

2NaHCO3(s)

Exothermic CaCl2(s) Ca2+(aq) + 2Cl-(aq)

ΔHsoln = -82.8kJ/mol

H2O(l)

Endothermic

NH4NO3(s) NH4+(aq) + NO3-(aq)

ΔHsoln = 25.7 kJ/mol

H2O(l)

Heat of CombustionThe heat of reaction for the complete

burning of one mole of a substance.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + 890kJ

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

OR

ΔH = - 890kJ

Molar Heat of Fusion (ΔHfus)The heat absorbed by one mol of a

substance in melting from a solid to a liquid at a constant temperature.

H2O(s) H2O(l) ΔHfus = 6.01kJ/mol

Molar Heat of Solidification(ΔHsolid)The heat lost when one mole of a liquid

solidifies at a constant temperature

H2O(l) H2O(s) ΔHsolid = -6.01kJ/mol

Molar Heat of Vaporization (ΔHvap)The amount of heat necessary to vaporize

one mole of a given liquid

H2O(l) H2O(g) ΔHvap = 40.7kJ/mol

Molar Heat of Condensation (ΔHcond) The amount of heat released when 1 mol of

vapor condenses

H2O(g) H2O(l) ΔHcond = -40.7kJ/mol

Molar Heat of Solution (ΔHsoln) The heat change caused by dissolution of

one mole of substance

NaOH(s) Na+(aq) + OH-(aq)

ΔHsoln = -445.1 kJ/mol

H2O(l)

Hess’s LawIndirect method to measure heat reaction

If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction

Cont.Diamond to Graphite

a. C(s, graphite) + O2(g) CO2(g) ΔH= -393.5kJ

b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ

Write equation a in reverse to cancel out unwanted information

c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ

Cont.Now, add equation b and c

b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ

c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ

Cont.

b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ

c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ

• Now, add equation b and c

C (s, diamond) C (s, graphite) ΔH= -1.9kJ

Standard Heat of Formation (ΔHf

o)The change in enthalpy that accompanies

the formation of one mole of a compound from its elements with all substances in their standard states at 25oC

ΔHo = ΔHfo (products) – ΔHf

o (reactants)