Module 5: Chapter 18 (Reaction Rates)

concentration / pressure temperature surface area of a solid use of a catalyst

only effective collisions lead to chemical reactions: • particles must collide with the correct orientation • the particles have sufficient energy to overcome the activation energy Ea barrier of the reaction (E > Ea)

Activation energy Ea is the minimum energy required to start a reaction by the breaking of bonds.

how fast a reactant is used up OR how fast a product is formed (conc/time)

Changing the concentration often changes the rate of reaction the rate of reaction is proportional to the concentration of a particular reactant raised to a power rate α [A]n Different reactants in a chemical reaction can have different orders and hence affect the rate in different ways Common orders: Zero order (0) First order (1) Second order (2)

rate α [A]0 The concentration of the reactant has no effect on the rate any number raised to the power zero is 1: [A]0 = 1

rate α [A]1 The concentration of the reactant affects the rate of reaction: doubling [A] (x2) increases reaction rate by a factor of 2 (21 = 2) tripling [A] (x3) increases reaction rate by a factor of 3 (31 = 3)

rate α [A]2 The concentration of the reactant affects the rate of reaction: doubling [A] (x2) increases reaction rate by a factor of 4 (22 = 4) tripling [A] (x3) increases reaction rate by a factor of 9 (32 = 9)

rate = k x [A]m x [B]n (k = rate constant (is the proportionality constant)

overall order = sum of orders with respect to each reactant - gives the overall effect of the concentrations on the rate of reaction

• Concentration-time graphs can be taken from continuous measurements of a suitable property during the course of a reaction • This property must be proportional to the concentration of the reactant or product

• Monitoring by gas collection (volume of gas evolved) • using a gas syringe to collect gas produced • The gradient of a curve produced tells us the rate of the reaction

• Monitoring of mass loss • Using a balance and time intervals • graph of mass loss against time

• Monitoring by colour change • If one of the reacting substances or products has a colour, the intensity of this colour will change during the reaction. • The absorbance reading of a colorimeter provides a measure of the concentration. • The amount of light absorbed by a solution is measured

• Choose a suitable filter (complimentary colour) • Zero the colorimeter with water/solvent • Make up a range of samples with known concentrations • Take absorbance readings and plot a calibration curve • Set up your reaction, take a small sample, put in a cuvette and place in colorimeter • Take absorbance readings at set time intervals • Read concentrations of reaction mixture off curve or just use absorbance values (-The order of reaction with respect to a coloured chemical can then be determined from the concentration-time graph)

• rate of reaction is the gradient of a concentration- time graph • order with respect to a reactant can only be determined if all other reactant concentrations remain constant Zero order = straight line with negative gradient, rate of reaction does not change, value of gradient is equal to rate constant k First order = downward curve with decreasing gradient and the time for the concentration of the reactant to half (half-life) is constant Second order = downward curve, steeper at start but tailing off more slowly

• First order reactions have a constant half-life with the concentration halving every half-life (exponential decay) • t1/2 is the time taken for the concentration of a reactant to decrease to half its original value

rate = k x [A] k = ln 2 / t1/2

• produces a horizontal straight line with no gradient Rate = k[A]^0 rate = k • the interpet on the y-axis give the rate constant • reaction rate doesn’t change with increasing conc

• produces a straight-line graph through the origin rate = k[A]^1 rate = k[A] • rate is directly proportional to the conc for a first order relationship • rate constant can be determined by measuring the gradient of the straight line of this graph

• produces an upwards curve with increasing gradient rate = k[A]^2 • rate constant cannot be obtained directly from the graph • by plotting a second graph of the rate against conc squared, the result is a straight line (gradient of this line is equal to rate constant k)

The instantaneous rate at the start of a reaction when t=0 - can be found using a tangent on t=0 on a conc-time graph

Convenient way of obtaining the initial rate of a reaction by taking a single measurement • the time t from the start of a reaction to when a visual hange is observed is measured • provided that there is no signif change in rate during this time, assumed that average rate over time (1/t) is the same as initial rate (initial rate is proportional to 1/t)

The time taken for the concentration of a reactant to decrease to half of its original value