5.1.1 - how fast?

Question 1

for reactions involving gases, what are the 2 methods you could use to continuously monitor the reaction progress?

Answer 1

1. can measure the change in volume of gas produced using a gas syringe
2. measure the change in mass using a balance

Question 2

for reactions involving acids and bases, how can you continuously monitor the reaction progress?

Answer 2

measure the change in pH at regular time intervals using a pH meter.

Question 3

for reactions involving visual changes, what 2 methods could you use to continuously monitor the reaction progress?

Answer 3

use a colourimeter for changes in intensity of a colour. it measures the absorbance of light through the solution.
you can time how long it takes for the formation of a precipitate to obscure a mark using a stopwatch.

Question 4

what is the definition of rate?

Answer 4

rate = (change in concentration of reactant or product)/(time taken for the change to occur)

Question 5

what are the units of rate?

Answer 5

moles per decimetre per second

Question 6

at what point in the reaction is rate the highest? why?

Answer 6

fastest at the start.
at the start the concentration of reactants is the highest, therefore at this point collisions are most frequent and more particles exceed the activation energy.

Question 7

how do you find the rate at a given point on a concentration time graph?

Answer 7

draw a tangent at that point, gradient of the tangent = rate.

Question 8

what does the order with respect to a reactant tell you?

Answer 8

how the rate of reaction will be affected if you change that reactant’s concentration

Question 9

how can you find the order of a reactant using the equation for the reaction?

Answer 9

you cant. they can only be found experimentally, it is not related to the balancing numbers in the equation like Kc is.

Question 10

define zero order with respect to a reactant.

Answer 10

the rate is unaffected by changes in concentration. for example, if the concentration of a reactant is doubled, the rate does not change.

Question 11

define 1st order with respect to a reactant.

Answer 11

the change in rate is equal to the change in concentration. for example, if the concentration of a reactant is doubled, the rate also doubles.

Question 12

define 2nd order with respect to a reactant.

Answer 12

the rate changes by the square of the change in concentration. for example, if the concentration of a reactant is doubled, the rate is timesed by 4

Question 13

define overall order

Answer 13

the sum of the individual orders of all the reactants.

Question 14

what are the 3 methods to determine the order wrt a reactant?

Answer 14

1. shape of concentration-time graph
2. shape of a rate concentration graph
3. from initial rates data

Question 15

explain how you can produce a concentration time graph to determine the order of a reactant.

Answer 15

use continuous monitoring to see how the concentration of the reactants change. any reactants not under investigation should be in excess so their concentration is effectively zero. use the shape of the graph to decide whether the reactant is 0th, 1st or 2nd order.

Question 16

describe the shape of a zero order concentration-time graph.

Answer 16

linear graph, negative gradient.
changing concentration does not affect rate, rate is constant so gradient is constant.

Question 17

describe the shape of a first order concentration-time graph.

Answer 17

downwards sloping curve (exponential decay)
[X] has a constant half life
looks similar to 2nd order, but has a constant half life whereas 2nd order does not.

Question 18

describe the shape of a second order concentration-time graph.

Answer 18

downwards sloping curve
[X] doesn’t have a constant half life
looks similar to 1st order, but doesn’t have a constant half life whereas 1st order does.

Question 19

for a first order concentration time graph, what is the equation linking the rate constant and the half life?

Answer 19

half life = t
k=ln(2)/t

Question 20

what is the half life of a reactant?

Answer 20

time taken for the concentration of a reactant to half.
in 1st order concentration time graphs, the half life is constant.

Question 21

explain how you could produce a rate-concentration graph in order to determine the order of a reactant.

Answer 21

use continuous monitoring to see how the concentration of reactants change with respect to time. any reactants not under investigation should be in excess.
plot a concentration-time graph.
find the rate at different concentrations by finding the gradient at different point.
use this to plot a rate-concentration graph.
use the shape of the rate-concentration graph to determine the order.

Question 22

describe the shape of a zero-order rate-concentration graph

Answer 22

horizontal line, with gradient = 0.
change in concentration does not affect the rate.

Question 23

describe the shape of a 1st order rate-concentration graph

Answer 23

linear graph, positive gradient, passing through the origin
rate is proportional to gradient

Question 24

describe the shape of a 2nd order rate-concentration graph

Answer 24

upwards sloping curve, starting at the origin
rate is proportional to the square of the concentration, therefore is a quadratic.

Question 25

explain how you would obtain the data to be able to determine the order of a reactant.

Answer 25

start by mixing equal amounts of the reactants use continuous monitoring and plot a concentration time graph to determine the initial rate of reaction. repeat the experiment, but with a more dilute solution of one of the reactants. dilute each solution in turn using distilled water, only alter the concentration of reactant at a time. keep the total volume the same for each experiment. in a table, show the concentration of each reactant and the initial rate for each experiment. compare how changing the concentration of one reactant affected the initial rate to determine the order.

Question 26

how would you word an initial rates question where you're showing order is 2 wrt [A] and 0 wrt [B]

Answer 26

from experiments 1 and 2, [A] doubles, whilst [B] is constant, and rate x4. 2nd order wrt [A] from experiments 2 and 3, [B] doubles whilst [A] is constant, and the rate does not change. zero order wrt [B] the overall concentration of the reaction is 2 + 0 = 2

Question 27

what is the rate equation?

Answer 27

shows the relationship between the rate and the concentration of the reactants, raised to their orders

Question 28

for the reaction: A + B + C ----> products where A is 0 order, B is 1st order and C is 2nd order what is the rate equation?

Answer 28

rate = k[B][C]^2

Question 29

how do you convert between the exponential form of the Arrhenius equation and the linear form?

Answer 29

take natural logs of both sides

Question 30

as temperature increases, how is rate affected? why?

Answer 30

particles have more kinetic energy successful collisions occur more frequently more particles exceed the activation energy so rate of reaction increases.

Question 31

in its linear form, what does the gradient of the graph of the Arrhenius equation represent?

Answer 31

gradient = - R x Ea

Question 32

in its linear form, what does the y-intercept represent?

Answer 32

ln(A) where A = the pre-exponential factor.

Question 33

what is the rate determining step?

Answer 33

the slowest step in the reaction mechanism

Question 34

how do you determine the rate determining step from the rate equation?

Answer 34

the components of the rate equation, with their orders being their molar equivalents.