Thermochemistry

Thermochemistry is the study of the heat

released (-H) or absorbed (+H) by chemical

and physical changes.

Thermochemistry

Some terminology

Temperature is defined as the degree of hotness/centigrade scale, °C; kelvin, K/

Heat is a form of energy

Heat capacity is the quantity of heat required to raise the temperature of a substance by 1°C

C = mass x specific heat

Thermochemistry

Specific heat is the heat capacity of one gram of a substance.

Specific heat of water: 4.184 J/(g°C)

Calorimeter is a device used to measure the heat transferred in chemical reactions.

Thermodynamic Systems - Definitions

Isolated System: No matteror energy cross systemboundaries. No work can bedone on the system.

Open System: Free exchangeacross system boundaries.

Closed System: Energy can beexchanged but matter cannot.

Adiabatic System: Special casewhere no heat can be exchangedbut work can be done on thesystem (e.g. PV work).

Thermodynamic Systems - Definitions

Surroundings: is the portion of the universe with which a system interacts

Exothermic reaction: is a chemical reaction in which heat is liberated (-H)

Endothermic reaction: is a chemical reaction in which heat is absorbed (+H)

Thermochemical equations

e.g.

C6H6(l) + 15,O’(g) = 6CO2(g) + 3H2O(l) -H

H = Hproducts – Hreactants

H = heat of reaction

H = enthalpy (heat content P=k)

H is given at 25°C and standard atmospheric pressure

First Law of Thermodynamics

Basic concepts:

1. Work and heat are both forms of energy

2. One form of energy can be converted into another form

3. Energy cannot be created or destroyed

First law of thermodynamics is the law of conservation of

energy

The total energy of the univers is a constant

Internal energy: E

In an isolated system E = constant

not known, cannot be calculated

E = Ef – Ei (f = final) (i= initial)

can be measured

q = + heat absorbed by the system

q = - heat evolved by the system

w = + work done by the system

w = - work done on the system

w = PV(on constant temperature)

E = q-PV

q = E + PV

E = q - w q = heat

W = work

Definition of Enthalpy

We can define a new state variable (one where the path to its current state does not affect its value) called enthalpy:

H = Ei + PV

Enthalpy = Internal Energy + PV

Heat contentq = H

Second Law of Thermodynamics One statement defining the second law is that a

spontaneous natural processes tend to even out the energy gradients in a isolated system.

Can be quantified based on the entropy of the system, S, such that S is at a maximum when energy is most uniform. Can also be viewed as a measure of disorder.

S = Sfinal - Sinitial > 0

The law states that a system will always undergo a spontaneous change in such a way as to increase the entropy

S total = S system + S surroundings

Entropy change: S is

If the reaction is exothermic:

TS – H > 0

The reaction is spontaneous if H < TS

S =q

t

Stotal = S - HT

Change in Entropy

Ssteam > Sliquid water > Sice

Relative Entropy Example:

Third Law Entropies:All crystals become increasingly orderedas absolute zero isapproached (0 K =-273.15°C) and at0 K all atoms are fixedin space so that entropyis zero.

ISOLATED SYSTEM

Spontaneous Reaction Direction

The change in enthalpy for any chemical reaction is constant, whether the reaction occurs in one step or in several steps.

Law of Hess

law of constant heat summation