Kinetics and equilibria

3 parts of the collision theory

• For particles to react they must collide

• When particles react, chemical bonds must be broken in the reactants( This happens when particles collide with enough energy)

• Particles must collide in the correct orientation

What is the rate of reaction proportional to?

The number of effective collisions per second.

What happens when you increase the concentration of reactants?

• At a higher concentration, there will be more reactant particles in the same volume.

• Because reactant particles are closer together, likelihood of collisions increase

• Thus increasing the rate of reaction

What happens when you increase gas pressure?

• Increasing the pressure makes the particles closer together

• Increasing the frequency of collisions

• Resulting in an increased rate of reaction

How can you measure the rate of reaction?

• Measuring how quickly reactants are used up

• Measuring how quickly products are formed

How can you measure the rate of reaction on a graph?

Drawing a tangent and working out the gradient

Change in Y/ Change in X

Define activation energy (Ea)

The minimum amount of energy required by particles, in order to start a chemical reaction.

What is the transition state?

The process where existing bonds are breaking and new bonds are forming products are formed as result of this stage.

(The definition of Ea can also be the enthalpy difference between the reactants and the transition state)

How do catalyst increase the rate of reaction?

Provide an alternative pathway for the reaction to take place with a lower activation energy

Reaction profile when catalyst is used.

Important idea to remember about catalysts

• They are not used up in the reaction

• And are not permanently changed so they can be re-used

Advantages of using catalysts

• Speed up reactions even at low temperatures.

• Reduces amount of energy required

• Reducing the amount of fossil fuels burnt to provide this energy

• Saving money

• Less CO2 emissions

The Maxwell-Boltzmann distribution curve

• Shows the spread of energies that molecules of a gas, liquid or solution have at a particular temperature.

• As you go up the Y-axis the number of molecules is higher

• As you go across the X-axis the energy (molecules posses) increases

• Very few molecules have high energy.

Key points about the curve

• The curve starts at 0 because no molecule has no energy

• The curve does not touch the x-axis because there is no maximum energy that a molecule can have

• Area under the curve is the total number of molecules in the system( reaction)

Parts of the curve

• Most probable energy is the energy most molecules have

• The section right of the activation energy shows the amount to molecules that have energy higher than the Ea. Only the molecules in that section are able to collide

What happens to the curve when a catalyst is used in the reaction?

• Activation energy is lowered

• So more molecules are able to collide

What happens to the curve when the temperature is increased?

• When you increase the temperature the curve shifts right

• At higher temperatures, there are molecules with high energies

• The most probable energy (Emp) increases. But the number of particles with the most probable energy falls

• Area under the curve will not change

• The number of molecules with the activation energy has increased

define rate of reaction and its units

• Change in concentration of a substance in unit time

• mol dm-3 s-1

what happens to the curve when the concentration is increased?

• Shape of the curve will stay the same

• But the curve will be higher and the area under the curve will be greater

• Because when you increase the concentration, there will be more particles per unit volume.

• Causing a greater frequency of collisions

Define a reversible reaction

• Products can react to from reactants

What is a closed system?

• No atoms can enter or leave the system

When does a reaction reach dynamic equilibrium?

• The forward and backward reactions are taking place simultaneously

What are two features of dynamic equilibrium?

• Forward and reverse reactions are occurring at equal rates

• The concentration of the reactants and products stay the same (does not mean that the concentration of the products and reactants are the same)

Equilibrium can be approached from either side

What does dynamic equilibrium tell you?

• The position of equilibrium

• In this example the position of equilibrium lies to the left as there is a greater concentration of reactants

Le Chateliers Principle

• When an external change is applied to a system at equilibrium, the equilibrium moves in the direction that reduces the effect of that change.

What happens to equilibrium when concentration is increased?

• The equilibrium will shift in the direction which will reduce the effect of this change

• So if the concentration of the reactants is increased

• Equilibrium will shift to the right

• The yield of the product will increase

What happens to equilibrium when pressure is increased ?

• Equilibrium will shift to the direction with fewer moles

What happens to equilibrium when pressure is reduced?

• Equilibrium will shift to the direction with the higher moles

• In order to counteract the change and increase the pressure

When is the equilibrium position not effected by a change in pressure?

• When the moles of the reactants and products are the same

What does it mean when the enthalpy change for a reaction is negative?

• The forward reaction is exothermic

What happens to equilibrium when temperature is increased or decreased?

• Equilibrium will shift to oppose the change

• If temperature is increased it will shift towards endo

• If temperature is decreased it will shift towards exo

Effect of catalyst on equilibrium position

• No effect

• Used to speed up the reaction only (reaction will just reach equilibrium faster)

• It will increase the rate of the forward and reverse reaction by the same amount

Equation for the equilibrium constant (Kc)

• Concentration of products/ Concentration of reactants

• Square brackets indicate concentration (moldm-3) is being used

• The power that the concentration is raised to is the moles of that substance

Define homogenous equilibrium

• When reactants and products are all the same state of matter

What does the equilibrium constant tell you?

• Kc < 1 = Equilibrium lies towards the reactants ( The concentration of reactants is greater at equilibrium)

• Kc = 1 Equilibrium lies midway between reactants and products ( The concentration of reactants and products are equal at equilibrium)

• Kc > 1 = Equilibrium lies towards the products. ( The concertation of products is greater at equilibrium)

What are the units of Kc?

• Changes depending on the equation

• Sometimes there are no units

What are equilibrium moles ?

• Number of moles of reactants and products present in a reaction when equilibrium is achieved

How do you calculate moles at equilibrium?

• Moles of reactant at equilibrium = Initial moles- moles reacted

• Moles of products at equilibrium = Initial moles + moles formed