3 Enthalpy

Units SI unit = joule

1KJ = 1000J = 239.0 cal

1st law of Thermodynamics

The total energy of the universe is constant

i.e energy cannot be created or destroyed but can be changed from one form to another.

Heat (or thermal energy) q is the energy transferred between a system and its surroundings as a result of a difference in temperature only.

All other forms of energy transfer (mechanical, electrical etc) involve some type of work, w, the energy transferred when an object is moved by a force.

State functions A state function is a property dependant on

the current state of the system (e.g. its composition, volume, temp etc)

It is independent of the path the system took to reach that state.

Energy of a system is a state function

So E is a constant for any given change but q and w may vary

(q and w are not state functions)

and E = q + w

Energy can be converted from one form to anothere.g. from mechanical heat electrical light etc.

Energy is released when bonds are madeEnergy is used when bonds are broken

Chemical reactions involve

Bond breakingBond forming

Energy changes for an exothermic reaction – one where heat is released to the surroundings

Energy changes for an endothermic reaction – one where heat is absorbed from the surroundings

Examples of exothermic reactions neutralisation burning hydrocarbons respiration

Examples of endothermic reactions photosynthesis dissolving ammonium nitrate in water

The chemical energy which a system possesses is its enthalpy. Enthalpy is the change in energy at constant pressure

symbol H

And H = H products – H reactants

if energy is absorbed by a system H is positive if energy is released by a system H is negative

Energy changes for an endothermic reaction

A—B + C—D

Reactants

A—C + B—D

Products

A B C D

Bond breaking

Bond forming

Overall energy change

Products have more energy than reactants

E

A—B + C—D

A—C + B—D

Products

Bond formingBond breaking

Overall energy change

EA B C D

Energy changes for an exothermic reaction

Products have less energy than reactants

Reactants

Energy Diagrams

E

Activation energy

H

Reactants

Products

An exothermic reaction

Time

Activation energy

H

Products

An endothermic reaction

E

Reactants

Reactants

Time

Standard enthalpy of formation is the heat absorbed when 1 mole of a substance is formed from its elements in their standard state.

The standard state of a substance is 1 mole of the substance in a specified state (solid, liquid, gas) at 1 atmosphere of pressure. The value of an enthalpy change is quoted for standard conditions: gases at 1 atm, solutions at unit concentration and substances in their normal states at a specified temperature. (usually 273K or 00C)

All elements in their standard state are assigned an enthalpy of formation of 0

Standard enthalpy of reaction is the heat absorbed in a reaction at constant pressure between the number of moles of reactant shown in the equation for the reaction.

Standard enthalpy of combustion is the heat absorbed when 1 mole of a substance is completely burned in oxygen at constant pressure.

Standard enthalpy of solution is the heat absorbed when 1 mole of a substance is dissolved at constant pressure in a stated amount of solvent. This may be 100g or 100ml or an ‘infinite’ amount, i.e. a volume so large that on further dilution there is no further heat change.

HrӨ depends only on the difference between the

standard enthalpy of the reactants and the standard enthalpy of the products, not on the route by which the reaction occurs.

This is Hess’s Law – If a reaction can proceed by more than one route the overall enthalpy is the same regardless of which route is followed.

Find the enthalpy change for the reaction CH2= CH2(g + HCl(g) C2H5Cl(g)

Given the following data

HfӨ = standard enthalpy change of formation

HfӨ CH2CH2 = +52.3 KJ mol-1

HfӨ HCl = - 92.3 KJ mol-1

HfӨ C2H5Cl = -105 KJ mol-1

H = HfӨ products - Hf

Ө reactants

HrӨ = -105 – (+52.3) + (-92.3)

= -65KJ mol-1

Note that the formula H = HfӨ products - HfӨ reactants

Applies when the data given is the enthalpy of formation.

If the data given is the enthalpy of combustion the following formula is used H = HcӨ reactants - HcӨ products

It doesn’t matter what the reaction is called. It may be called a combustion reaction but if the data given is the enthalpy of formation use the formula H = HfӨ products - HfӨ reactants !

Calculate the standard enthalpy of reaction for the following

2C + 2H2(g) + O2 CH3CO2H(l)

Given the following enthalpies of combustion

C -394 KJ/molH2(g) -286 KJ/mol

CH3CO2H(l) -876 KJ/mol

r = (2x -394) + (2x-286) – (-876) KJ/mol = -484 KJ/mol