how fast?

Question 1

define the rate of reaction

Answer 1

a change in concentration of a reactant or product per unit of time

Question 2

at a given instant, how could you calculate the rate of reaction?

Answer 2

change in concentration of reactant or product / time

Question 3

if the order is zero with respect to a reactant, what does that mean?

Answer 3

changing the concentration of the reactant has no effect on the rate

Question 4

what is the effect on rate in a first-order reaction?

Answer 4

rate is directionally proportional to the concentration

Question 5

what is the effect on rate in a second-order reaction?

Answer 5

change in rate = change in concentration squared

Question 6

what is a generic rate equation

Answer 6

Rate of reaction = k[A]^m[B]

Question 7

what do square brackets mean in chemistry?

Answer 7

concentration

Question 8

do zero-order reactants appear on the rate equation? why?

Answer 8

no, because they don’t have an effect on the rate

Question 9

how is overall order of a reaction calculated?

Answer 9

sum of individual orders

Question 10

how would you calculate the units of the rate constant?

Answer 10

units of rate are mol dm^-3 s^-1
units of concentration are mol dm^-3

k = initial rate / initial conc

sub in units and cancel them out

Question 11

when the overall order is 3, what would be the units for the rate constant?

Answer 11

dm^6 mol^-2 s^-1

Question 12

what are the different ways you could measure the rate of reaction experimentally?

Answer 12

• use a colourimeter at suitable intervals if there is a colour change
• if gas is evolved, use a gas syringe to collect the volume of gas evolved
• measure the change in mass of the reaction mixture or the mass of precipitate formed
• measure change in pH

Question 13

define the rate-determining step

Answer 13

the slowest multi-step in a mechanism that determines what order a reaction is

Question 14

what does half-life mean?

Answer 14

the time taken for concentration of a reactant to decrease by half

Question 15

what is the symbol for half life?

Answer 15

t 1/2

Question 16

what is the relationship between first-order reactions and half-life?

Answer 16

first-order reactants have constant half lives

Question 17

what is the equation that is used to determine rate constant using half-life in a first-order reaction?

Answer 17

k = ln2 / t 1/2

Question 18

how would you draw a rate concentration graph?

Answer 18

• plot [A] against time
• draw tangents at different values

Question 19

what is the relationship between rate and time?

Answer 19

rate is proportional to 1/t

Question 20

how do you determine the rate constant from a rate concentration graph of first order?

Answer 20

k = rate / concentration

Question 21

how does the rate-determining step link to the species involved in the rate equation

Answer 21

any species involved in the rds appear in the rate equation
species only involved after rds do not appear in rate equation

Question 22

for a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate-determining step?

Answer 22

the order of the reactant

Question 23

if the rate equation shows that [A] is second order, how many molecules of A will be present in the rate-determining equation?

Answer 23

2

Question 24

what affects the value of the rate constant for a given reaction?

Answer 24

temperature, nothing else

Question 25

what is the effect of a 10C temperature increase on the rate of reaction, roughly?

Answer 25

doubles the rate of reaction

Question 26

what is the Arrhenius equation? what does each term mean?

Answer 26

k = Ae^ - Ea/RT

k - rate constant
A - pre-exponential factor (number of collisions between reactant molecules)
e - mathematical quantity
R - gas constant
T - the temperature in Kelvin
Ea - activation energy for reaction in Joules

Question 27

how can you convert the Arrhenius equation into a useful form for plotting a graph?

Answer 27

ln k = -Ea/RT + ln A