Thermodynamics

definition

Enthalpy change ΔH
- amount of heat released or absorbed
by a chemical system when a chemical
reaction occurs at constant pressure.

how to determine ΔH?

products have lesser energy than reactants

Heat released to
surroundings

-ve ΔH

exothermic reaction

products have more energy than reactants

Heat absorbed to
surroundings

+ve ΔH

endothermic reaction

Standard Enthalpy Change

standard enthalpy change ΔH°

reaction at the same specific temperature

standard enthalpy of reaction, Δrxn H°

∆rxn H° = Sum of Δf H° (product) - Sum of Δf H° (reactant)

only be used with the Δf H°
of reactants and products

unit of Δrxn H° is kJ

standard enthalpy change
of formation, Δf H°

one mole of compound is formed from
its element in their reference states

Δf H° of an element &
7 diatomic elements
(H2, N2, O2, F2, Cl2, Br2, l2)

Δf H° = 0 kJ/mol

standard bond dissociation
enthalpy

bond breaking

energy is absorbed

endothermic reaction

ΔH bond breaking is +ve

bond forming

energy released

exothermic reaction

ΔH bond forming is -ve

definitions

"disorder' of a system

possible arrangements or configurations of a system

Examples

most number of possible outcomes

gaseous state

least number of possible outcomes

solid state

how to determine ΔS?

by comparing the physical states of the
reactants and products

solid to gas

increase in the number of possible arrangements

gas to solid

decrease in the number of possible arrangements

by comparing the number of particles
of the reactants and products

increase in the number of particles

increase in the number of possible arrangements

decrease in the number of particles

decrease in the number of possible arrangements

Standard Entropy Change ΔS

∆rxnS = S (product) - S (reactant)

Spontaneous Process

occurs under a specific set of conditions

occurs without continuous input of energy

Examples

non-spontaneous

water freezing at 25 °C 1 atm

spontaneous

ball moving down a slope

require a 'pus' to get started

How to determine
spontaneity?

- both endothermic and
exothermic reaction
can be spontaneous
- sign of ∆H alone is insufficient
to predict spontaneity in every process/reaction.

Change in enthalpy ΔH

Change in entropy change ΔS

First Law

energy can be converted from one form to another
but cannot be created or destroyed.
Based on the law of conservation or energy.

Second Law

the entropy of the universe increases in spontaneous processes and remains unchanged during equilibrium processes.
(∆S universe = ∆S system + ∆S surroundings)

equilibrium process,
∆S universe = 0

spontaneous process,
∆S universe = + ve

ΔG = ∆H system - T∆S system

TΔSuniverse is denoted by ΔG