THERMOCHEMISTRY

The study of heat released or required by chemical reactions
Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt)
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy

What is Energy?

Total Energy = Kinetic Energy + Potential Energy

E = EK + EP
Kinetic energy & potential energy are interchangeable
Ball thrown upwards slows & loses kinetic energy but gains potential energy
The reverse happens as it falls back to the ground

Law of Conservation of Energy

: the total energy of the universe is constant and can neither be created nor destroyed; it can only be transformed.The internal energy, U, of a sample is the sum of 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 and molecules in a sample

Systems & Surroundings

In thermodynamics, the world is divided into a system and its surroundings
A system is the part of the world we want to study (e.g. a reaction mixture in a flask)
The surroundings consist of everything else outside the system

OPEN SYSTEM

: can exchange both matter 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 sealed reaction flask
ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask)

HEAT and WORK

HEAT is the energy that transfers from one object to another when the two things are at different temperatures 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 by heat energy
i.e. heat stimulates thermal motion

Work

is the transfer of energy that takes place when an object is moved against an opposing forcei.e. a system does work when it expands against an external pressure
Car engine: petrol burns & produces gases which push out pistons in the engine and transfer energy 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 the temperature of 1g of water by 1oC
1 cal = 4.184 J

INTERNAL ENERGY (U)

Internal energy changes when energy enters or leaves a system
U = Ufinal - Uinitial
U change in the internal energy
Heat and work are 2 equivalent ways of changing the internal energy of a system

First Law of Thermodynamics:

the internal energy of an isolated system is constant
Signs (+/-) will tell you if energy is entering or leaving a system
+ indica- indicates energy leaves a system
tes energy enters a system

WORK

An important form of work is EXPANSION WORK i.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
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 heat supplied at constant volume

ENTHALPY (H)

(comes from Greek for “heat inside”)
the change in internal energy is not equal to the heat supplied when the system is free to change its volume
some of the energy can return to the surroundings as expansion work
The heat supplied is equal to the change in another thermodynamic property called enthalpy (H)
As most reactions in chemistry take place at constant pressure we can say that:

EXOTHERMIC & 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 fossil fuels is an exothermic reaction

Endothermic process:

: a change (e.g. a chemical reaction) that requires (or absorbs) heatAn input of heat corresponds to an increase in enthalpy
Photosynthesis is an endothermic reaction (requires energy input from sun)
Forming Na+ and Cl- ions from NaCl is an endothermic process

Measuring Heat

Exothermic reaction, heat given off & temperature of water rises
Endothermic reaction, heat taken in & temperature of water drops
How do we relate change in temp. to the energy transferred?
Heat capacity (J/oC) = heat supplied (J)
Heat Capacity = heat required to raise temp. of an object by 1oC
more heat is required to raise the temp. of a large sample of a substance by 1oC than is needed for a smaller sample

Specific heat capacity

capacity is the quantity of energy required to change the temperature of a 1g sample of something by 1oCSpecific Heat Capacity (Cs)

Vaporisation

Energy has to be supplied to a liquid to enable it to overcome forces 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 flow as a liquid
endothermic process (H positive)
Freezing

cLiquid 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 Enthalpies

All chemical reactions either release or absorb heat
Exothermic reactions:
Reactants products + energy as heat (H -ve)
e.g. burning fossil fuels
Endothermic reactions:
Reactants + energy as heat products (H +ve)
e.g. photosynthesis

Bond Strengths

Bond strengths measured by bond enthalpy HB (+ve values)
bond breaking requires energy (+ve H)
bond making releases energy (-ve H)
Lattice Enthalpy
A measure of the attraction between ions (the enthalpy change 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 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

Second Law of Thermodynamics:

the disorder (or entropy) of a system tends to increase
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

A spontaneous change

is a change that has a tendency to occur without been driven by an external influencee.g. the cooling of a hot metal block to the temperature of its surroundings
A non-spontaneous change is a change that occurs only when driven
e.g. forcing electric current through a metal block to heat it

THERMOCHEMISTRY