THERMOCHEMISTRYTHERMOCHEMISTRY
The study of heat released or required bychemical reactions
Fuel is burnt to produce energy - combustion (e.g. whenfossil fuels are burnt)
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy
What is Energy?
EnergyEnergy
Kineticenergy(EK)
Potentialenergy(EP)
Energy dueto motion
Energy due toposition (storedenergy)
Total Energy = Kinetic Energy + Potential Energy
E = EK + EP
Kinetic energy & potential energy are interchangeable
Ball thrown upwardsslows & loses kineticenergy but gainspotential energy
The reverse happensas it falls back tothe ground
Law of Conservation of EnergyLaw of Conservation of Energy: the total energyof the universe is constant and can neither becreated nor destroyed; it can only betransformed.
internal energyThe internal energy, U, of a sample is the sumof all the kinetic and potential energies of allthe atoms and molecules in a sample
i.e. it is the total energy of all the atoms andmolecules in a sample
Systems & Surroundings
In thermodynamics, the world is divided into a system and itssurroundings
A system is the part of the world we want to study (e.g. areaction mixture in a flask)
The surroundings consist of everything else outside thesystem
SYSTEM
CLOSED
OPEN
ISOLATED
OPEN SYSTEM: can exchange bothmatter and energy with thesurroundings (e.g. open reaction flask,rocket engine)
CLOSED SYSTEM: can exchangeonly energy with the surroundings(matter remains fixed) e.g. a sealedreaction flask
ISOLATED SYSTEM: can exchangeneither energy nor matter with itssurroundings (e.g. a thermos flask)
HEAT and WORK
HEAT is the energy that transfers from one object toanother when the two things are at differenttemperatures and in some kind of contact
e.g. kettle heats on a gas flame
cup of tea cools down (loses energy as heat)
Thermal motion (random molecular motion) is increased byheat energy
i.e. heat stimulates thermal motion
Work is the transfer of energy that takes place when anobject is moved against an opposing force
i.e. a system does work when it expands against anexternal pressure
Car engine: petrol burns &produces gases which push outpistons in the engine and transferenergy to the wheels of car
•Work stimulates uniform motion
• Heat and work can be considered as energy in transit
UNITS OF ENERGY
S.I. unit of energy is the joule (J)
Heat and work ( energy in transit) also measured in joules
1 kJ (kilojoule) = 103 J
Calorie (cal): 1 cal is the energy needed to raise thetemperature of 1g of water by 1oC
1 cal = 4.184 J
INTERNAL ENERGY (U)INTERNAL ENERGY (U)
Internal energy changes when energy enters or leaves asystem
U = Ufinal - Uinitial
U change in the internal energy
Heat and work are 2 equivalent ways of changing theinternal energy of a system
+
=
Change ininternalenergy
Energysupplied tosystem asheat
Energysupplied tosystem aswork
U = q (heat) + w (work)
q
w
q
w
U
U like reserves of abank: bank acceptsdeposits orwithdrawals in twocurrencies (q & w)but stores them ascommon fund, U.
First Law of Thermodynamics:
the internal energy of an isolated system isconstant
Signs (+/-) will tell you if energy is entering orleaving a system
+ indicates energy enters a system
- indicates energy leaves a system
EXPANSION WORK•An important form of work is EXPANSION WORKi.e. the work done when a system changes size andpushes against an external force
e.g. the work done by hot gases in an engine as theypush back the pistons
WORK
HEAT
In a system that can’t expand, no work is done (w = 0)
U = q + w
when w = 0, U = q (at constant volume)
•A change in internal energy can be identified with the heatsupplied at constant volume
ENTHALPY (H)
(comes from Greek for “heat inside”)
• the change in internal energy is not equal to the heatsupplied when the system is free to change its volume
• some of the energy can return to the surroundings asexpansion work
U < q
The heat supplied is equal to the change in anotherthermodynamic property called enthalpy (H)
i.e. H = q
• this relation is only valid at constant pressure
As most reactions in chemistry take place atconstant pressure we can say that:
A change in enthalpy = heat supplied
EXOTHERMIC & ENDOTHERMIC REACTIONSEXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction)that releases heat.
A release of heat corresponds to a decrease in enthalpy
Exothermic process: H < 0 (at constant pressure)
Burning fossilfuels is anexothermicreaction
Endothermic process: a change (e.g. a chemicalreaction) that requires (or absorbs) heat.
An input of heat corresponds to an increase in enthalpy
Endothermic process: H > 0 (at constant pressure)
Photosynthesis is anendothermic reaction(requires energy inputfrom sun)
Forming Na+and Cl- ionsfrom NaCl is anendothermicprocess
Measuring HeatMeasuring Heat
reaction
reaction
Exothermic reaction, heatgiven off & temperature ofwater risesExothermic reaction, heatgiven off & temperature ofwater rises
Endothermic reaction, heattaken in & temperature ofwater dropsEndothermic reaction, heattaken in & temperature ofwater drops
How do we relate change in temp. to the energytransferred?
Heat capacity (J/oC) = heat supplied (J)
temperature (oC)
Heat Capacity = heat required to raise temp. of an objectby 1oC
• more heat is required to raise the temp. of a largesample of a substance by 1oC than is needed for asmaller sample
Specific heat capacity is the quantity of energyrequired to change the temperature of a 1g sample ofsomething by 1oC
Specific HeatCapacity (Cs)
Heat capacity
Mass
=
J / oC / g
J / oC
g
=
VaporisationVaporisation
Energy has to be supplied to a liquid to enable it to overcomeforces that hold molecules together
• endothermic process (H positive)
Melting
Energy is supplied to a solid to enable it to vibrate morevigorously until molecules can move past each other and flowas a liquid
• endothermic process (H positive)
Freezing
Liquid releases energy and allows molecules to settle into alower energy state and form a solid
• exothermic process (H negative)
(we remove heat from water when making ice in freezer)
Reaction EnthalpiesReaction Enthalpies
All chemical reactions either release or absorb heat
Exothermic reactions:
Reactants products + energy as heat (H -ve)
Endothermic reactions:
Reactants + energy as heat products (H +ve)
e.g. burning fossil fuels
e.g. photosynthesis
Bond StrengthsBond Strengths
Bond strengths measured by bond enthalpy HB (+ve values)
• bond breaking requires energy (+ve H)
• bond making releases energy (-ve H)
Lattice EnthalpyLattice Enthalpy
A measure of the attraction between ions (the enthalpychange when a solid is broken up into a gas of its ions)
• all lattice enthalpies are positive
• I.e. energy is required o break up solids
Enthalpy of hydration HhydEnthalpy of hydration Hhyd
• the enthalpy change accompanying the hydration of gas-phase ions
•Na+ (g) + Cl- (g) Na+ (aq) + Cl- (aq)
• -ve H values (favourable interaction)
WHY DO THINGS DISSOLVE?
• If dissolves and solution heats up : exothermic
•If dissolves and solution cools down: endothermic
Breaking solidinto ions
Ionsassociatingwith water
Dissolving
+
=
LatticeEnthalpy
+
Enthalpy ofHydration
=
Enthalpy ofSolution
Substances dissolve because energy and matter tend todisperse (spread out in disorder)
2nd law of Thermodynamics
Second Law of Thermodynamics:
the disorder (or entropy) of a system tends toincrease
ENTROPY (S)
•Entropy is a measure of disorder
• Low entropy (S) = low disorder
•High entropy (S) = greater disorder
• hot metal block tends to cool
• gas spreads out as much as possible
Total entropychange
entropy changeof system
entropy changeof surroundings
+
=
Dissolving
disorder ofsolution
disorder ofsurroundings
• must be an overall increase in disorder for dissolvingto occur
1. If we freeze water, disorder of the watermolecules decreases , entropy decreases
( -ve S , -ve H)
2. If we boil water, disorder of the water moleculesincreases , entropy increases (vapour is highlydisordered state)
( +ve S , +ve H)
A spontaneous change is a change that has atendency to occur without been driven by anexternal influence
e.g. the cooling of a hot metal block to thetemperature of its surroundings
A non-spontaneous change is a change that occursonly when driven
e.g. forcing electric current through a metal blockto heat it
•A chemical reaction is spontaneous if it is accompanied byan increase in the total entropy of the system and thesurroundings
• Spontaneous exothermic reactions are common (e.g. hotmetal block spontaneously cooling) because they releaseheat that increases the entropy of the surroundings.
•Endothermic reactions are spontaneous only when theentropy of the system increases enough to overcome thedecrease in entropy of the surroundings
System in Dynamic EquilibriumSystem in Dynamic Equilibrium
A + B C + D
Dynamic (coming and going), equilibrium (no net change)
• no overall change in disorder
S 0 (zero entropy change)