Chemical Reactions Thermo

8/8/2019 Chemical Reactions Thermo

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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(E

K

)

Potentialenergy(E

P

)

Energy dueto motion

Energy due toposition (stored

energy)



Total Energy = Kinetic Energy + Potential Energy

E = EK + EP

Kinetic energy & potential energy are interchangeable

Ball thrown upwards

slows & loses kineticenergy but gainspotential energy

The reverse happens

as it falls back tothe ground



Law of Conservation of EnergyLaw of Conservation of Energy: the total energy

of the universe is constant and can neither becreated nor destroyed; it can only betransformed.

The internal energyinternal energy, U, of a sample is the sumof all the kinetic and potential energies of all

the 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 the

surroundings (e.g. open reaction flask,rocket engine)

CLOSED SYSTEM: can exchange

only 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 out

pistons 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=U

final -U

initial

(U change in the internal energy

Heat and work are 2 equivalent ways of changing theinternal energy of a system



+=

Change ininternal

energy

Energysupplied to

system asheat

Energysupplied to

system aswork

(U = q (heat) + w (work)

q

w

q

wU

U like reserves of abank: bank accepts

deposits 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



An important form of work is EXPANSION WORKEXPANSION WORKi.e. the work done when a system changes size and

pushes against an external force

e.g. the work done by hot gases in an engine as they

push 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 insideµ)

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 anexothermic

reaction



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 input

from sun)

Forming Na+

and Cl-

ionsfrom NaCl is anendothermic

process



Measuring HeatMeasuring Heat

reaction

reaction

Exothermic reaction, heatExothermic reaction, heatgiven off & temperature ofgiven off & temperature of

water riseswater rises

Endothermic reaction, heatEndothermic reaction, heattaken in & temperature oftaken in & temperature of

water dropswater 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 a

smaller sample



Specific heat capacity is the quantity of energyrequired to change the temperature of a 1g sample of

something 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 more

vigorously until molecules can move past each other and flowas a liquid

endothermic process ((H positive)

FreezingLiquid releases energy and allows molecules to settle into a

lower 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 hydrationEnthalpy of hydration ((HHhydhyd

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 : exothermicIf dissolves and solution cools down: endothermic



Breaking solid

into ions

Ionsassociating

with water

Dissolving+ =

Lattice

Enthalpy +

Enthalpy of

Hydration= Enthalpy of

Solution

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 water

molecules decreases , entropy decreases( -ve (S , -ve (H)

2. If we boil water, disorder of the water moleculesincreases , entropy increases (vapour is highly

disordered state)

( +ve (S , +ve (H)



A spontaneous change is a change that has atendency to occur without been driven by an

external influence

e.g. the cooling of a hot metal block to thetemperature of its surroundings

A non-spontaneous change is a change that occurs

only when drivene.g. forcing electric current through a metal block

to heat it



A chemical reaction is spontaneous if it is accompanied byan increase in the total entropy of the system and the

surroundings

Spontaneous exothermic reactions are common (e.g. hot

metal 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)

Chemical Reactions Thermo

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