10 Reaction rates and equilibrium

RoR def

how fast a reactant is used up/how fast a product is being formed

formula for RoR

RoR =

t=seconds

units of RoR

moldm^-3s^-1

some things about RoR

• RoR fastest at start of reaction as each reactant is in its highest concentration

• the RoR slows down as reaction proceeds because reactants are being used up so concs decrease

• once one of the reactants has been completely used up, the concs stop changing and the RoR is 0

factors that change RoR

• conc (or pressure when reactants are gases)

• temperature

• use of a catalyst

• SA of solid reactants

collision theory

two reacting particles must collide for a reaction to occur

what are the 2 conditions to an effective collision

• particles collide w correct orientation

• particles have sufficient energy to overcome the Ea barrier of the reaction

how does increasing conc affect RoR

• increase in conc increases no. particles in same volume

• particle r closer together and collide more frequently

• in a given period of time there will therefore be more effective collisions (correct orientation and sufficient energy)

• an increased RoR

how does increasing pressure of a gas affect RoR

• increasing pressure compresses a gas into a smaller volume

• conc of gas molecules increases as the same no. gas molecules occupy a smaller volume

• gas molecules are closer together so collide more frequently

• leading to more effective collisions in same time

methods to follow the progress of a reaction

• monitoring the removal (decrease in conc) of a reactant

• following the formation (increase in conc) of a product

methods to determine the RoR of a reaction that produces a gas

• monitoring the volume of gas produced at regular time intervals using gas collection “continuous monitoring method”

• monitoring the loss of mass of reactants using a balance

catalyst def

a substance that increases the rate of a chemical reaction without being used up in the process

provides an alternative route for the reaction with lower activation energy

what r the 2 types of catalysts called

homogeneous and heterogeneous

describe a homogeneous catalyst

• a homogeneous catalyst has the same physical state as the reactants

• it reacts with the reactants to form an intermediate

• the intermediate then breaks down to give the product and regenerates the catalyst

examples of homogeneous catalysts

1. esterification

2. ozone depletion

3. Fe2+ catalysts

what is the catalyst in esterification

sulfuric acid

how is a catalyst used in esterification

• sulfuric acid

• the H+ ions form the intermediate

• all reactants are aq

what is the catalyst in ozone depletion

the Cl• radicals

how is a catalyst used in ozone depletion

Catalyst generation (initiation):

Cl₂ -> 2Cl•

 

Propagation:

O₃ + Cl• -> ClO• (intermediate) + O₂

ClO• + O₃ -> Cl• + 2O₂

• the reactant (O₃) and the catalyst (Cl•) are both gases

• overal rxn: 2O₃ → 3O₂

heterogeneous catalysts

a heterogeneous catalyst has a different physical state from the reactants

• usually solids

how do heterogeneous catalysts work

• reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst where the reaction takes place

• after reaction, the product molecules leave the surface of the catalyst by desorption

industrial processes that use heterogeneous catalysis

• making ammonia (Haber process)

• reforming

• hydrogenation of alkenes

• making sulfur trioxide for sulfuric acid (contact process)

how is a catalyst used in hydrogenation of alkenes

• nickel

• the H2 absorbs to the Ni surface

catalyst for making ammonia/Haber process

Fe(s)

catalyst and equation for reforming

catalyst for hydrogenation of alkenes

Nickel

catalyst and equation for making sulfur trioxide for sulfuric acid

why is using catalysts more sustainable

• catalysts increase the RoR of many industrial processes by lowering the Ea

• this then reduces the temp needed for the process and the energy requirements

• therefore less electricity/fossil fuels used

• product made faster w less energy - cuts costs

what do catalytic convertors contain

a catalyst made of platinum, rhodium and palladium

why are catalytic convertors used

the hot exhaust gases that pass over the catalyst ensures harmful gases are converted into less harmful products

what is the Boltzmann distribution

the spread of molecular energies in gases

features of the Boltzmann distribution

• no molecules have zero energy - the curve starts at the origin

• the area under the curve is equal to the total number of molecules

• there is no maximum energy for a molecule - the curve never meets the x-axis (energy)

what is the y and x axis for a boltzmann distribution

y axis = number of molecules w a given energy

x axis = energy

effect of temp on boltzmann distribution (think of graph)

• as temp increases the avg energy of molecules also increases

• a small proportion of molecules will still have low energy, but more molecules have higher energy

• graph is now stretched over a greater range of energy values

• the peak of the graph is lower on the y-axis and further along the x-axis: is at a higher energy

• number of molecules is the same therefore area under curve is same

(relation to boltzmann distribution) at higher temp:…

• more molecules have an energy greater than or equal to the Ea

• therefore a greater proportion of collision will lead to a reaction, increasing the RoR

• collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions

boltzmann distribution w catalyst graph

what does the position of equilibrium indicate

• the position of equilibrium indicates the extent of the reaction

• in a reversible reaction, if the temperature, pressure (gases), of concentration of the reactants or products is changed, then the position of equilibrium may change

le Chatelier’s principle

states that when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change

effect of [] change on equilibrium

• will change the rate of the forward or reverse reactions

• position of equilibrium will change

  • increase in [] = shifts to RHS

  • decrease in [] = shifts to LHS

effect of temp change on equilibrium

• direction equilibrium shifts to depends on sign of ΔH

  • increase in T = shifts to endothermic direction (ΔH is positive)

  • decrease in T = shifts to exothermic direction (ΔH is negative)

effect of pressure change on equilibrium

• shifts to side that will reduce the pressure of the system

  • increase in pressure = shifts to side w fewer moles of gas

  • decrease in pressure = shifts to side w most moles of gas

effect of catalyst on equilibrium

• doesn’t change position

• only speeds up the rate of forward and reverse rxns equally

• will increase the rate at which an equilibrium is established

equilibrium law

equilibrium law defines the equilibrium constant Kc in terms of concentrations

requirements for a dynamic equilibrium to be established

• closed system

• concentrations are constant

• rate of forward reaction = rate of reverse reaction

diagram of boltzmann distribution when using a catalyst + explanation

• catalyst allows rxn to proceed via a different route with a lower Ea

• therefore there are more molecules with energy above Ea with a catalyst

effect of a higher temp on rxn - boltzmann distribution

if given the value of the Ea and ΔH for the forward reactions, what is the Ea for the reverse reaction?

ΔH + Ea

be careful of signs