physical chem; rate equations

Question 1

how is rate calculated from a graph?

Answer 1

rate be found from the gradient
gradient = change in y / change in x

Question 2

what are the orders of reaction?

Answer 2

an order is a power to which a concentration is raised in the rate equation - tells us how the concentration of the substance affects the rate

Question 3

what is the zero order?

Answer 3

changes in conc do not affect the rate
e.g. if [A] doubles then the rate doesn’t change

Question 4

what is the 1st order?

Answer 4

changes in conc have a proportional change on the rate
e.g. if [A] doubles then rate doubles

Question 5

what is the 2nd order?

Answer 5

changes in conc have a squared proportional change on the rate
e.g. if [A] doubles the rate quadruples

Question 6

what is the rate constant (k)?

Answer 6

a constant number that allows us to equate rate & conc

Question 7

why is the rate fixed at a certain temp?

Answer 7

as changes in temperature causes rate to change

Question 8

how is the rate calculated?

Answer 8

rate = k [A] [B]

Question 9

how is the rate constant calculated?

Answer 9

1. write the rate expression
2. rearrange to make k the subject: k = rate / [x]
3. work out the units

Question 10

what is the iodine clock experiment?

Answer 10

it’s a reaction that times how long it takes for a colour change to occur

1. add sodium thiosulfate & starch (acts as an indicator) to reaction mixture
2. sodium thiosulfate reacts immediately with I₂
3. reaction is monitored by sitting reaction vessel on paper with a cross on it - time how long it takes for the cross to disappear

Question 11

write the balanced equation for the iodine clock reaction including states

Answer 11

H₂O₂ + 2I⁻ + 2H⁺ + → I₂ + 2H₂O

Question 12

why does the +ve test for starch produce a blue/black colour?

Answer 12

when there is no more sodium thiosulfate left the I₂ reacts with starch - produces blue/black colour

Question 13

how can we calculate the order of reaction for the iodine clock experiment?

Answer 13

by varying the conc of I₂ &/or H₂O₂ & keeping everything else constant - results in time taken for the blue/black colour to appear changing - we can work out the order of reaction

Question 14

state the 3 ways in which rate can be calculated

Answer 14

1. by change in pH of a reaction
2. by calculating the amount of mass lost
3. by calculating volume of gas produced

Question 15

explain how the change in pH of a reaction allows us to calculate rate

Answer 15

1. the pH of a reaction may change over time if H⁺ ions are used up/produced
2. a pH meter measures pH of a reaction at regular intervals - can then calculate H⁺ ions conc

Question 16

explain how using the amount lost in a reaction allows us to calculate rate

Answer 16

1. for reactions that produce gas - place reaction on balance & measure the mass loss (which is gas loss)
2. mole calculations can be used to work out no.of moles of gas lost therefore amount of reactants left

Question 17

explain how using the volume of gas produced allows us to calculate rate

Answer 17

1. measure the amount of gas produced using a gas syringe & measure over a specified time
2. use the ideal gas equation to work out the no.of moles of gas produced - then use molar ratio in equation to work out reactants conc’s

Question 18

for which reactions is a colorimeter used?

Answer 18

reactions with colour changes are measured with a colorimeter

Question 19

what does a colorimeter measure?

Answer 19

measures the absorbance of light by a coloured sample - the more concentrated a sample is = darker in colour

Question 20

what is a rate-concentration graph?

Answer 20

is created by knowing the rate. the rate is found by taking the gradient at various points on the conc-time graph

Question 21

describe the rate-concentration graph for the zero order

Answer 21

straight horizontal line (as its constant)

Question 22

describe the rate-concentration graph for the 1st order

Answer 22

straight diagonal line (as changes are in equal amounts)

Question 23

describe the rate-concentration graph for the 2nd order

Answer 23

curved line (as changes are in unequal amounts)

Question 24

what is the rate determining step?

Answer 24

it’s the slowest step in a multi-step reaction

Question 25

why is the rate determining step important?

Answer 25

as the whole reaction rate depends on how quick the rate determining step is

Question 26

what affects the rate of reaction?

Answer 26

reactants that appear in the rate equation - these reactants must appear in the rate determining step & catalysts

Question 27

what is the arrhenius equation?

Answer 27

k = Ae (-Ea/RT)

k - rate constant
Ae - arrhenius constant
Ea - activation energy (J)
R - gas constant (8.31 JK-1 mol-1)
T - temperature (k)

Question 28

explain what happens to the rate constant of Ea gets smaller & why

Answer 28

Ea gets smaller = rate constant gets bigger - means that as Ea drops, the rate of reaction increases so many more particles have enough energy to react

Question 29

explain what happens to the rate constant as temperature increases & why

Answer 29

temp increases = rate constant increases - as when temp increases, particles have more KE & are more likely to collide with at least the Ea so rate of reaction increases

Question 30

rearrange the arrhenius equation to make Ea the subject

Answer 30

Ea = (InA - Ink) x RT

Question 31

rearrange the arrhenius equation to make k the subject

Answer 31

Ink = InA - (Ea/RT)