reaction kinetics

Question 1

rate of reaction definition

Answer 1

the rate of reaction is defined as the change in concentration of a reactant or a product per unit time

Question 2

what is the rate equation

Answer 2

rate = k [A]^m [B]^n

Question 3

what does zero order mean

Answer 3

rate of reaction is independent of the concentration of A

changes in [A] does not change the rate of reaction

Question 4

what does first order mean

Answer 4

rate of reaction is directly proportional to [A]

Question 5

what does second order mean

Answer 5

rate of reaction is directly proportional to [A]^2

Question 6

what is the Arrhenius equation

Answer 6

k=Ae^(−Ea/RT)
A = pre exponential factor

Question 7

what is the rate constant k affected by

Answer 7

temperature (increase in temperature = increase in rate constant)

activation energy (decrease in activation energy = increase in rate constant)

unaffected by changes in concentration

Question 8

how to deduce the units of k

Answer 8

units of k = (mol dm^-3 s^-2) / (mol dm^-3)^overall order of reaction

Question 9

definition of t 1/2 (half life)

Answer 9

the half life of a reaction is the time taken for the concentration of a reactant to decrease to half its initial value

Question 10

what does concentration-time graph show

Answer 10

straight line = rate is constant as concentration decreases = 0 order

curve with constant half life = first order with respect to [R]

curve with half lives not constant = order of reaction with respect to [R] is not first order. (plot a rate-concentration graph to find out)

Question 11

half life equation

Answer 11

half life = ln2 / k

Question 12

what does rate-concentration graph show

Answer 12

horizontal line = rate is independent of [R], zero order

straight line passing through the origin = rate is directly proportional to [R], first order

parabola = order is not 1, plot a straight line graph to determine (lg graphs, gradient is order of reaction)

Question 13

what does rate-time graph show

Answer 13

horizontal line = 0 order
curve = first order
curve (with decrease much faster compared to first order reaction) = second order

Question 14

what is the collision theory

Answer 14

the collision theory states that to react, the reactant particles must collide with sufficient energy and with the correct orientation

Question 15

activation energy definition

Answer 15

the activation energy of a reaction is the energy barrier the reactants must overcome in order to become the products

Question 16

draw the maxwell-boltzmann energy distribution curve

Answer 16

y axis = number of particles
x axis = kinetic energy
index (shaded area) = proportion of molecules with KE >/= activation energy

Question 17

deduction of r.d.s with rate equation

Answer 17

fast equation before slow equation: reactants must be included in the rate equation
slow equation: r.d.s
fast equation after slow equation: ignore

Question 18

deduction of rate equation with r.d.s

Answer 18

write rate equation for r.d.s (rate = k2[reactant])
substitute intermediate reactants by writing a kc equation
write:
overall rate equation is rate= k [reactant] where k = k2kc

Question 19

factors affecting rates of reaction

Answer 19

collision frequency
proportion of the particles that have sufficient energy to overcome the activation energy

Question 20

effect of concentration or pressure on rate

Answer 20

at a higher concentration, there are more reactant particles per unit volume. the reactant particles are close together and collide more frequently. frequency of effective collisions increases. hence rate of reaction increases

Question 21

effect of temperature on rate

Answer 21

when temperature is increased, reactant particles have a greater average kinetic energy. they move more quickly and particles collide more frequently. the proportion of particles that have kinetic energies greater than or equal to the activation energy increases. Frequency of effective collisions and hence rate of reaction increases

Question 22

catalyst definition

Answer 22

a catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway of lower activation energy, while remaining chemically unchanged at the end of the reaction

Question 23

effect of catalyst on rate

Answer 23

a catalyst provides an alternative reaction pathway of lower activation energy. the proportion of particles that have kinetic energies greater than or equal to the lower activation energy increases. Frequency of effective collisions increases and hence rate of reaction increases.

Question 24

what are the 2 different types of catalysis

Answer 24

homogenous catalysis
heterogeneous caatalysis

Question 25

how does the homogeneous catalysis work

Answer 25

homogeneous catalysis involves the formation of an intermediate between the catalyst and one of the reactants. the catalyst is regenerated in a later step of the reaction. the catalysed reaction may involve two or more steps, each of which has lower activation energy than the uncatalysed reaction.

Question 26

catalytic role of Fe2+ in the I-/S2O8 2- reaction

Answer 26

the uncatalysed reaction is slow as the repulsion between the two negative ions (I- and S2O8 2-) results in a high activation energy. the catalyst Fe2+ and intermediate Fe3+, being positively charged, easily attract the negative ions. Thus, the catalysed pathway has a lower activation energy than the uncatalysed reaction.

Question 27

the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide

Question 28

how does the heterogeneous catalysis work

Answer 28

reactants are adsorbed onto the active sites by formation of weak attraction forces. This brings the reactant molecules closer together, weakening the covalent bonds in the reactant molecules and lowering the activation energy. It also orientates the reactant molecules correctly for the reaction to occur. the reaction occurs where bonds are broken and new bonds are formed. The product desorbs from the catalyst surface so that active sites are exposed for further reaction.

Question 29

details of haber process (technically chemical equilibria)

Answer 29

catalyse: fine iron catalyst (cheap and can be used in a large scale) pressure = 250 atm (pressure cannot be too high as it is expensive even though high pressures would favour the forward reaction and increase the yield) temperature = 450°C (moderately high temperatures to increase rate of reaction by providing reacting particles with enough energy even though low temperatures would favour the forward exothermic reaction) the yield of NH3 is further increased by removing it as it forms to continuously shift the position of equilibrium to the right

Question 30

what are enzymes

Answer 30

enzymes are homogeneous catalysts, through the mechanism of action that resembles heterogeneous catalysis. Enzymes are highly specific and catalyses only one reaction.