Equilibrium

Closed system

allows energy, but not matter to be exchanged with surroundings

Open systems

Allows matter and energy to be exchanged with surroundings

What is the difference between chemical and physical change

Physical: No change to the chemical composition e.g change in state, dissolving

Chemical change: involves a rearranging of chemical bonds, reactants → products

Reversible reactions

• it does not go to completion

• reactants combine to form products, the products regenerate the reactants

• eventually the rate of forward and reverse reactions will be the same and chemical equilibrium is established

• if conditions change to favour either reaction then it will proceed at a greater rate

Activation energy

• the minimum energy that is required for a collision to result in a reaction

• the difference in enthalpy between reactants and the activated complex

• Activation energy indicates how easily the reaction will be reversed

At equilibrium in a closed system at a constant temperature there is:

• constancy of macroscopic properties - all observable properties (composition, density, colour) remain constant

• a dynamic condition - equal but opposing rates of reaction - forward and reverse reaction occur but at equal rates

dynamic equilibrium

chemical equilibrium is dynamic: a state in a closed system where the rates of the forward and reverse processes are equal, resulting in no net change in the system's properties.

the equilibrium constant (K)

a numerical value of relative concentrations of products to reactants at equilibrium

A and B are reactants and C and D are products

a,b,c and d are the coefficients in the equation

only aqueous and gaseous substances are included

PRODUCTS IN NUMERATOR AND REACTANTS IN THE DENOMINATOR

what does the size of the equilibrium constant mean?

changes in value of K

• K changes if temperature changes

• K gives no indication of RoR

• solids and liquids are NOT included in the K expression

• K does not change if you alter concentration, pressure or volume

• K only refers to the equilibrium position

Reaction quotient

the ratio between products and reactants under any conditions

Q>K therefore the concentrations of reactants must increase and products decrease to reach equilibrium

Q=K therefore system is at equilibrium

Q<K therefore the concentrations of reactants must decrease and products increase to reach equilibrium

Changes imposed to an equilibrium system

• changes to temp

• adding or removing reactant or product (changing concentrations or in gases, partial pressure)

• changing pressure by changing volume

• Diluting

how do we predict vs how to we explain changes

Explaining changes: collision theory and reaction rates

Predicting these changes: using Le Chatelier’s Principle

Factors which effect equilibrium position

• changing temperature

• changing concentration

• changing the pressure of a gas

factors which do not affect the equilibrium position

• presence of a catalyst

• adding an inert gas (or any gas not changing the partial pressure of the gases in the system)

LCP

if a chemical system at equilibrium is subjected to a change in conditions, the system will behave in such a way as to partially counteract the imposed change

Changing temperature

Predicting via LCP

• if the temperature is increased the system will favour the endothermic reaction which absorbs heat

• if the temperature of the system is lowered the exothermic reaction will be favoured which raises the temperature through creation of energy

Explaining via collision theory and reaction rates

• if temp is increased, both reactions speed up, but endothermic reaction speeds up more because the activation energy is higher in the endothermic therefore and increase will affect the percentage of particles able to react in an endothermic reaction than an exothermic

• decreasing the temp decreases the average kinetic energy of the system. Both reaction decrease: endothermic slows to a greater extent due to its larger energy requirement.

How does the enthalpy convey if the reaction is exothermic or endothermic

• (-) enthalpy = exothermic

• (+) enthalpy = endothermic

Adding or removing reactant or product (changing concerntrations)

Predicting:

• Increasing the conc of reactants favours formation of products

• decreasing the conc of reactants favours the formation of reactants

• increasing the conc of products favours formation of reactants

• decreasing the conc of products favours the formation of products

• if the partial pressure of a gaseous species increases, LCP predicts the net reaction will occur in the direction which tends to lower the partial pressure of said species

Explaining;

• increasing conc increases the frequency of collisions

sentence used to describe the effect of a product/reactant when it’s conc is increased

• partially reduced, but higher than original value

what is the rule for water in equilibrium reactions

it does not exist

Changing the pressure by changing the volume

Predicting:

• if volume of system is reduced the system will oppose this change by favouring the reaction which produces the lease number of molecules

• decreasing the pressure favours the reaction which will produce more molecules

• addition or removal of products which are solids or liquids will have no effect on equilibrium concentrations in gaseous specied

diluting

summary table

what is a homogeneous system

• a system that contains reactants and products in the same phase (e.g. all gaseous)

what is a heterogeneous system

contain reactants and products in different phases

solubility equilibrium

• if a reaction with reactants in a solid state and products in an aqueous state has a high K value we can determine that the reactant is very soluble in water. And if it has a low K value we can the determine that the reactant is insoluble in water.