dynamic equilibrium
rate of forward reaction equals rate of reverse reaction in a reversible reaction. no net change in the concentration of substances in a closed system.
Le Chatelier’s principle
Stress imposed on a system at equilibrium will cause a shift in the equilibrium position/composition to minimise the effect of the change.
Equation for Gibbs free energy from enthalpy and entropy
change in G = change in enthalpy - (temp x change in entropy)
K
Keq
Kc
Kp
K - equilibrium expression
values:
Keq - equilibrium constant
Kc - concentration
Kp - pressure
Physical equilibrium
The equilibrium that exists during a physical change when the rate of the forward process equals the rate of the reverse process
Condition required to reach equilibrium
Closed system (a system in which only energy can be exchanged between the system and the surroundings (matter cannot be exchanged))
(sealed) Reaction vessel
A (sealed) container in which reactions are carried out
Equilibrium constant (Kc)
A ratio between the concentration of products and reactants for a reversible reaction at equilibrium (indicator of the extent of the reaction).
Unit - no unit (ratio)
What does the magnitude of Kc imply?
Kc«1 : almost exclusively products
Kc < 1 : higher concentration of reactants
Kc = 1 : appreciable amounts of reactants and products
Kc>1 : higher concentration of products
Kc»1 : almost exclusively products
Changes that can be made to a system at equilibrium
- Changes in concentration by either removing or adding a reactant or product from the system at equilibrium
- Changes made to the pressure of a GASEOUS reaction by changing the volume of the reaction vessel, in accordance with Boyle’s law.
- Changes made to the temperature of a system at equilibrium by either adding or removing heat.
Catalysts do not change the position of equilibrium, it only increases the rate of reaction, reaching the equilibrium sooner. This is because the rate of forward and reverse reactions are increased by the same amount.
Effects of temperature on equilibrium constant
Reaction quotient (Q)
Ratio between the concentrations of products and reactants for a reversible reaction not in equilibrium.
Comparison with Kc determines the direction in which the reaction will proceed.
Method of calculating the equilibrium concentrations of reactants and products
ICE box
(When the value of Kc is very small, x is negligeble in the denominator –> its effect on the concentration of reactants is negligeble, making it easier to calculate x and therefore the equilibrium concentrations)
Gibbs free energy
a thermodynamic potential that determines if a chemical reaction is spontaneous.
In both a spontaneous and a non-spontaneous reaction, the Gibbs energy reaches a minimum at which point the reaction is at equilibrium (Q=Kc) and the Gibbs free energy change is zero.
The position of the minimum value of Gibbs energy in a spontaneous reaction
Lies further towards the product side (the equilibrium mixture will contain mostly products - the equilibrium constant for a spontaneous reaction is greater than 1, with its value increasing with greater spontaneity)
Gibbs free energy change > 0
Kc < 1
The position of the minimum value of Gibbs energy in a non-spontaneous reaction
Lies further towards the reactant side (the equilibrium mixture will contain mostly reactants - the equilibrium constant for a non-spontaneous reaction is less than 1, with its value decrease with less spontaneity)
Gibbs free energy change > 0
Kc < 1
What information can be used to measure the position (extent) of an equilibrium reaction?
The equilibrium constant and the Gibbs free energy change can both be used to measure the position of an equilibrium reaction.
Relationship between the Gibbs free energy change and the equilibrium position
delta G = -RTlnK
Where lnK = the natural log o
Entropy (S)
A measure of the amount of disorder of the particles in a system. / A measure of dispersal or distribution of matter and/or energy in a system.
The distribution of energy among particles in a system
i.e. In a system with higher entropy, the particles are more spread out and moving more (more ways in which energy could be distributed).
More dispersed energy = the system tends to remain in randomness
How can entropy change be predicted in a chemical reaction?
A change in the amount (mol) of gaseous reactants or products.
An increase in the number of moles of gas creates an increase in entropy and a decrease in the number of moles of gas on the products side signifies a decrease in entropy. If there are the same number of moles of gas in the reactants and products, then the entropy change will be quite small.
The standard entropy value of a substance
Entropy change from heating the substance from absolute zero (0K) to the thermodynamic standard temperature of 298K.
A perfect crystal at absolute zero (0 K) has a standard entropy of zero. Therefore, all substances have a positive standard entropy value.
Standard entropy change equation
Standard entropy change (delta S) Sum of standard entropy values of products - sum of standard entropy values of reactants
Change in Gibbs free energy
Takes into account the direct entropy change resulting from the transformation of the chemicals and the indirect entropy change of the surroundings resulting from the transfer of heat energy.
