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?

Energy

Kinetic energy

(EK)

Potential energy

(EP)

Energy due to motion

Energy due to position (stored 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

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

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

EXOTHERMIC & ENDOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases

heat.

Exothermic process: H < 0

Burning fossil fuels is an exothermic

reaction

Endothermic process: a change (e.g. a chemical reaction) that requires

or absorbs heat.

Endothermic process: H > 0

Photosynthesis is an endothermic reaction

(requires energy input from sun)

Forming Na+ and Cl- ions from NaCl is

an endothermic

process

Measuring Heat

reaction

reaction

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)

temperature (oC)

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 is the quantity of energy required to change the temperature of a 1g sample

of something by 1oC

Specific Heat Capacity (Cs)

Heat capacity

Mass

=

J / oC / g J / oC

g

=

Vaporization

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

Liquid 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)

Chemical Reactions

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