Module 3: Chapter 10 (Reaction rates and equilibrium)

How fast a reactant is used up OR how fast a product is formed

The rate is the fastest at the start of a reaction as each reactant is at its highest concentration

• Concentration (or pressure if reactants are gases) • Temperature • Use of a catalyst • Surface area of solid reactants • Radiation

For a reaction to occur reactant particles must come together and collide with: • the correct orientation • energy greater than the activation energy for the reaction

Low concentration - less particles in a given volume so less frequent collisions and therefore a slower rate of reaction Higher concentration - more particles in a given volume so more frequent collisions and therefore a faster rate of reaction

By monitoring the removal of a reactant or monitoring the formation of a product, for example: • monitoring volume of gas produced at regular time intervals • monitoring the loss of mass using a balance

• A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself · not used up itself · may react with the reactant to form an intermediate or may provide a surface for the reaction to happen on · at the end of the reaction it is always regenerated • A catalyst provides an alternative reaction pathway with a lower activation enthalpy

A homogenous catalyst has the same physical state as the reactants E.g. ozone depletion Cl· radicals from CFC’s catalyse the breakdown 2O3(g) <=Cl·(g)=> 3O2(g)

A heterogenous catalyst has a different physical state from the reactants

• The reactant molecules form weak bonds with the catalyst surface - this is called adsorption • Bonds with the reactant molecules break • New bonds form • Product molecules leave the catalyst surface - this is known as desorption

• Catalysts increase the rate of many industrial chemical reactions by providing an alternative reaction pathway with a lower activation enthalpy • This lowers the temperature needed for the reaction • Lower temperatures means less electricity from combustion of fossil fuels used • Less combustions leads to a reduction of CO2 emissions and other atmospheric pollutants • Examples: Rhodium, Platinum and Palladium

Shows the spread of molecular energies in a sample of gas molecules

• No molecules have zero energy, so the curve starts at the origin • The area under the curve equals the total number of molecules • There is no maximum energy for a molecule so the curve never meets the x-axis • A line of Ea and shaded area shows the proportion of molecules with an energy greater than the activation energy

The peak is lowered and shifted to the right A greater of proportion of molecules have an energy greater than the activation energy

• more molecules have an energy greater than/equal to the activation energy • a greater proportion of collisions will lead to a reaction, increasing the reaction rate • collisions will be more frequent as the molecules are moving faster, which also increases the reaction rate

• A greater proportion of molecules exceeds the new lower activation energy • more molecules collide with an energy greater than the activation energy • more effective collisions, therefore faster rate of reaction

• the rate of the forwards reaction is equal to the rate of the reverse reaction • the concentrations of the reactants and products remain constant • system must remained closed

When a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effects of that · the position of equilibrium indicates how far the reaction has progressed when equilibrium is established

A + B ⇌ C + D · An increase in conc of A or B, or a decrease in conc in C or D causes the equilibrium position to move to the right (e.g. Add an acid, concentration of H+ ions increases so equilibrium position moves to the right/left) · An increase in conc of C or D, or a decrease in conc of A or B the equilibrium concentration moves to the left

only affects gases into gases 2NO2(g) ⇌ N2O4(g) brown colourless · an increase in pressure equilibrium position move to the side with fewer gaseous molecules (here it would move to the right, colourless colour) · a decrease in pressure equilibrium position moves to side with more gaseous molecules

• a decrease in temperature, equilibrium favours the exothermic process • an increase in temperature, equilibrium will favour the endothermic process

• doesn’t change the position of equilibrium • it speeds up the rates of the forwards and reverse reaction equally • it will increase the rate at which equilibrium is established

[C]^c [D]^ [products] Kc = —————- = —————– [A]^a [B]^b [reactants]

the magnitude of Kc tells us the relative proportions of reactants and products in the equilibrium system