Rates of Reaction & Equillibrium

Rate of reaction

change in concentration of a reactant or product per unit time

For a reaction to occur…

• collision must occur so existing bonds within reactants are broken

• successful collisions require energy >= activation energy

  • + correct orientation

Transition State

occurs at the maximum potential energy stage (EA)

activation energy

minimum amount of energy required for a reaction to take place

Increasing ROR

increase frequency of successful collisions

increase proportion os particles with energy higher than EA

Increasing ROR Methods

• increasing surface area of solid

• increasing reactant conc. or pressure

increasing surface area of solid

only surface particles participate in the reaction, more exposed solid reactant particles, more particles available to react freq. of collisions increases, freq. of successful collisions increases

increasing reactant conc. or pressure

conc = no of particles in given volume when sol conc. increases, more reactant prticles moving in a given v(solution). freq. of collisions. freq. successful collisions also increases. ROR increases

Increase proportion of particles with energy higher than EA methods

increasing temp (Kinetic E)

catalyst (lowers EA)

increasing temp (Kinetic E)

as temp increases, average kinetic energy of particles increases. Greater proportion of particles have E >= EA, proportion of successful collisions increases.

Note: Increase in ROR due to increase in energy of collisions

increase in ROR due to increase in freq. of collisions

increase freq. of collisions (more energy)

catalyst (lowers EA)

lowers the activation energy by providing an alternate pathway with lower energy, because EA is lowered, the proportion of particles with energy >= EA , no of successful collisions, ROR

Finding limiting reactant

• Find moles of each

• Divide each moles by coeff.

Dynamic Equilibrium

when rate of forward reaction = rate of backwards reaction

dynamic equilibrium conditions

1. amounts and concs. of chemicl substances remain constant

2. total gas pressure is constant (if gases involved)

3. temperature remains constant

4. reaction is incomplete (all substances are present in reaction mixture)

Reaction Quotient (Qc)

reaction quotient is used to compare the concentration of the products and reactants at any point in a reaction

Equilibrium constant (KC)

the equilibrium constant is expression when reaction is at equilibrium

What does equilibrium constant mean?

tells us the extent of the reaction

large KC value present

means mostly products

small KC value means

mostly reactnts present

Qc < Kc

system needs to form more products. forward reaction favoured

Qc > Kc

system needs to form more reactants. backward reaction favoured

Open Systems

matter and energy can be exchanged with the surrounding

Closed systems

only energy is exchanged with the surroundings

Equilibrium constant changes

• should always be stated at given temp → only affected by temp

• not affected by actions such as adding reactants or products, changes in pressure, use of catalysts

Le Cnatelier’s principle

equilibrium position (relative amounts of reactants + products) of a reaction can be changed by

• adding or removing a reactant or product

• changing pressure by changing volume (gases)

• dilution (for solution)

• changing temperature

Rate Vs Yield factors (industrial)

• Rate of reaction (high)

• % yield (% of products produced)