Kinetics

Question 1

Reaction rate

Answer 1

• how quickly a reaction takes place (change in concentration of reactant or product over time)
• affected by concentration, surface area, pressure, temperature, or inclusion of a catalyst
• can be measured by change of gas volume of product over time, pH over time, mass over time, or colour

Question 2

Maxwell-Boltzmann

Answer 2

• peak of curve = most probable energy for any particle
• Area = total no. particles
• At lower temperatures, most likely energy will decrease so peak will move up and move backwards, but area will be the same

Question 3

Rate equation

Answer 3

• rate = rate constant (k)[A]^m[B]^n
• m and n are the orders of reaction for those reactants, they represent the effect its concentration has on the rate
• k only varies with temperature
• unit of rate can be calculated by cancelling the units of the products
• rate equation is determined experimentally

Question 4

Orders of reaction

Answer 4

• Zero order = [A]^0
• concentration of A has no effect on the rate, graph of rate against [A] is just a flat line
• First order = [A]^1
• concentration of A is directly proportional to Rate, graph is a straight line through origin with constant gradient
• Second order = [A]^2
• rate is proportional to concentration of A^2, graph is curved
• order of whole reaction = total of orders of all products

Question 5

Clock reactions

Answer 5

• a reaction where there is a sudden increase in the concentration of a product after a limiting reactant is used up, leading to an observable end point e.g colour change
• it must be assumed that concentrations don’t change, temperature is constant and the endpoint of the change is close to the endpoint of the reaction

Question 6

Iodine clock reaction

Answer 6

• sodium thiosulfate immediately reacts with any Iodine produced, but when it runs out the Iodine immediately turns the starch a blue-black colour
• transfer 25cm^3 of H2SO4 of known concentration to a beaker, add 20cm^3 water, and 1cm^3 starch
• measure the amount of potassium iodide of known conc. and add it to the beaker with 5cm^3 sodium thiosulfate
• add 10cm^3 of H2O2 last and simultaneously start a stop watch. Stir until it turns blue and record the time taken. Repeat with varying amounts of Potassium Iodide

Question 7

Rate determining step

Answer 7

• the step of the reaction with the slowest rate, if the mechanism involves multiple steps
• any species involved in the steps after the RDS will not be in the rate equation
• orders of reaction show how many moles of that reactant are involved in the RDS i.e ([A]^2 means 2 moles of A in RDS)
• catalysts will appear in the rate equation but not the chemical equation

Question 8

Arrhenius equation

Answer 8

• k = Ae^(-Ea/RT)
• rate constant = Arrhenius constante^(-Activation energy/Gas constanttemperature)
• ln(k) = ln(A)-Ea/RT can be plotted as a graph like y= mx + c on a graph of lnK against 1/Twith gradient of -Ea/R and y-intercept of lnA

Question 9

Gas equilibria

Answer 9

• gas equilibrium constant = Kp = same as equilibrium constant but instead of concentration partial pressure is used, used for reversible reactions in gas phase
• partial pressure = mole fraction of gas*total pressure of the mixture
• mole fraction of gas = number of moles in the gas/total number of moles of gas in the mixture
• is affected by Le Chateliers principle like normal equilibrium constant, changes in temperature affect it but not pressure or adding a catalyst