Chapter 10: Reaction rates and equilibrium

rate = change in concentration / time Units = moldm-3s-1

A measure of change in concentration of reactant/product in a given time.

(x axis is time / s, y axis is concentration in moldm-3) • Rate of reaction is fastest at the start of the reaction as the reactants are at its highest concentration. • Rate of reaction decreases as reactants are being used up and concentration of them decreases. • Once reactants have been used up, rate of reaction is 0 and concentration is 0 (levels off).

Particles need to collied with sufficient energy (to overcome activation energy) and with correct orientation for successful collisions. Many particles hit each other in a reaction, but will not form a product if they don’t have these 2 things.

• higher concentration / gas pressure increases rate of reaction • higher temperature increases rate of reaction • higher surface area / state of division (e.g. solids) increases rate of reaction

Increased concentration / gas pressure means: - more particles per volume - particles are closer to each other / crowded - so more frequent collisions - more successful collisions per unit time (particles collide with sufficient energy and correct orientation)

• Particles have higher kinetic energy • Particles therefore move faster • Leading to more frequent collisions • More successful collisions per unit time ALSO - a higher temperature means more particles have energy which is greater than or equal to the activation energy - so more successful collisions per unit time

• more points of contact between reacting particles • more frequent collisions • more successful collisions per unit time

• a catalyst provides an alternate pathway with lower activation energy • so more particles have energy greater than or equal to activation energy • more successful collisions per unit time

• monitoring the removal (decreasing in concentration) of a reactant • following the formation (increase in concentration) of a product This can be done by measuring: - gas volume (produced) - mass of reactants/products - colour

• monitoring the volume of gas produced at regular time intervals using gas collection • monitoring the loss of mass of reactants using a balance

a) draw a tangent and calculate its gradient b) mean gradient is change in y / change in x

A substance that increases the rate of a reaction without being used up in the process; a catalyst provides an alternative route for the reaction with lower activation energy

• a catalyst is not used up • it may react with the reactant to form an intermediate, or may provide a surface on which the reaction can take place • at the end of the reaction the catalyst is regenerated

• homogenous catalyst • heterogenous catalyst

• catalysts and reactants are in the same physical state • catalyst reacts with the reactants to form an intermediate • intermediate breaks down to give the product • catalyst is regenerated

A catalyst that is of the same physical state as the reactants.

A catalyst which is of a different physical state to the reactants.

• ADSORPTION - Incoming species lands on active site and forms bonds with the catalyst. It may use some bonding electrons in the molecules to weaken them so a subsequent reaction is easier • REACTION - Adsorbed gases (meaning weakly bonded) may be held on the surface in the correct orientation for a reaction to occur - this increases the chance of favourable collisions. • DESORPTION - Rearrangement of electrons and products are released from active sites

Esterification - concentrated sulphuric acid catalyses the reaction between acids and alcohols. Ozone depletion - chlorine radicals act as a catalyst

In petrol vehicles, catalytic converters contain a ceramic honeycomb mesh coated with platinum. They have a large surface area on which the reactions can take place.

Since 1992 Cars must have catalytic converters to pass MOT test. Catalytic converters: platinum, rhodium or palladium catalyst with honeycomb mesh that provides a large surface area on which reactions take place. The reactions are: - Carbon monoxide, CO oxidised to form CO2 - Nitrogen monoxide, NO reduced to N2 - Unburnt hydrocarbons are burnt to form CO2 and H2O. Catalytic converters reduce the amount of toxic gases polluting the atmosphere.

Catalysts: - lower the temperature and energy needed - so less energy means less fossil fuels are used - which reduced carbon dioxide emission - Because catalysts increase the rate of reaction by decreasing activation energy. This makes things cheaper too: - making product faster and using less energy can cut costs and increase profitability. - Economic advantages outweigh costs of using a catalyst It is estimated that 90% of chemical materials are made using a catalyst.