Endothermic Reaction Exothermic Reaction

Heat is absorbed by system

Heat is lost by the system

Temperature Decreases Increases

q (heat) Positive Negative

Work is done when a force acts through a distance.

Work involved in the expansion or compression of gases is called pressure-volume work. W = -pV

When work is done by the system on the

surroundings final energy is less than the initial energy, change is negative.

When work is done on the system by the surroundings, final energy is greater than the initial energy, change is positive.

V Work

Gas expands + -

Gas compresses - +

How much work, in joules, is involved when 0.225 mol N2at a constant temperature of 25 C is allowed to expand by 1.50 L in volume against an external pressure of 0.750 atm? [Hint: How much of this information is required?]

Energy cannot be created nor destroyed. It can only be converted from one form to another.

The energy of the universe is constant.

The total internal energy U of an isolated system is constant.

U = q + w

U-internal energy of the system q- heat w- work

Internal energy of a system can change if: Work is done by the system (decreases) Work is done on the system (increases) Energy can flow in and out of the system (an open

system)

Heat and work is only present when there is a change in the system.

They are the means by which a system exchanges energy with its surroundings.

Property that has a unique value depending on the present state of the system and NOT on the path taken.

For example: Internal energy, enthalpy and entropy are state quantities.

Work and heat are process quantities Their values depend on the path between two

equilibrium states.

U = q + w

U = q pV

q= U + pV

For a reaction taking place at constant pressure (e.g. atmospheric pressure):

qp= U + pV

H= U + pV

H= qp

Enthalpy is the heat change of a reaction at constant pressure.

H = Hfinal Hinitial H = Hproducts Hreactants

Endothermic Reaction Exothermic Reaction

Heat is absorbed by system

Heat is lost by the system

Temperature Decreases Increases

q (heat) Positive Negative

H Positive Negative

When Hproducts>Hreactants reaction is endothermic When Hproducts

Standard Enthalpy (H) Enthalpy measurement made under standard conditions (1atm, 298K, 1M)

Standard enthalpy of formation(Hf) Enthalpy measurement for formation reactions where reactants are pure elements (under standard conditions).

Standard enthalpies of formation of elements in their reference form is zero for all temperatures.

Standard enthalpy of reaction (Hrxn) Enthalpy change where reactants under standard conditions change to products under standard conditions.

Standard enthalpy of combustion (Hcomb) enthalpy change of a combustion reaction which takes place under standards conditions of temperature and pressure.

Hrxn= mHfproducts - nHfreactants

Where m & n are stoichiometric coefficients

The sum of the individual steps in a reaction is equal to the overall change in enthalpy for a reaction. (Enthalpy is a state function.)

It is an indirect way of determining enthalpy.

Hydrogen peroxide slowly decomposes into water and oxygen. The enthalpy change of reaction can be calculated using standard enthalpies of formation. Hf (hydrogen peroxide(l)) = 187.8 kJ mol

1

Hf (water(l)) = 285.8 kJ mol1

Using a Hess cycle, what is the enthalpy change of reaction for this decomposition?

2H2O2(l) 2H2O(l) + O2(g)

The amount of molecular randomness in a system is called the systems entropy (S).

Entropy has the units J/(K.mol)

The larger the value of S, the greater the molecular randomness of the particles in the system.

Gases have more randomness and higher entropy than liquids, and liquids have more randomness and higher entropy than solids.

In any spontaneous process, the total entropy of a system and its surroundings always increases.

All spontaneous processes produce an increase in the entropy of the universe.

If a process is spontaneous, the reverse process is non-spontaneous.

Both spontaneous and non-spontaneous processes are possible.

Only spontaneous processes will occur without intervention.

Non-spontaneous processes require the system to be acted on by an external agent.

Reaction H S

Spontaneous - +

Non-spontaneous + -

A Spontaneous process is favoured by a decrease in enthalpy and an increase in entropy.

A non-spontaneous process is favoured by an increased in enthalpy and a decrease in entropy.

G = H TS

H - enthalpy

T Temperature in K

S entropy

Reaction H S G

Spontaneous - + -

Non-spontaneous + - +

If G = H TS = 0, then process is at equilibrium.

The process is balanced between favourable and non-favourable.

It is not energetically favourable to go in either the forward or backward direction.