Module 5 (chapter 18) orders of reaction

Question 1

rate

Answer 1

change in concentration/ change in time

measured in moldm-3s-1

Question 2

order of reaction

Answer 2

-changing the concentration often changes the rate of reaction.
-it is proportional to the concentration of a particular reactant raised to a power
rate (fish symbol to show directly proportional) [A] n
-the power is the order of reaction for that reactant
-these can be different for the same reactants in different reactions

Question 3

zero order

Answer 3

-when concentration has no effect on the rate, the reaction is zero order with respect the the reactant
rate (fish) [A] 0
-in a zero order reaction any number raised the power of zero is one and the concentration doesn’t affect rate.

Question 4

first order

Answer 4

• a reaction is first order with respect to a reactant when the rate depends on its concentration to the power of 1
• rate (fish) [A] 1
• if the concentration of A is doubled, the reaction rate increases by a factor of 2 to power of 1 (2)
• if it is tripled, the reaction rate increases by a factor of 3 to the power of 1 (3)

Question 5

second order

Answer 5

• a reaction is second order with respect to a reactant when the rate depends on its concentration raised the power of 2
• rate (fish) [A]2
• if the concentration of A is doubled, the reaction rate increases by a factor of 2 squared (4)
• if it is tripled, the reaction rate increases by a factor of 3 squared (9)

Question 6

what does the rate equation and constant tell us?

Answer 6

-gives us the mathematical relationship between the concentrations of reactants and the reaction rate

Question 7

what is the rate equation?

Answer 7

rate = K [A]m [B]n

• rate constant
• concentration of A and order of reaction with respect to A
• concentration of B and order of reaction with respect to B

Question 8

what is the rate constant?

Answer 8

k is the proportionality constant. it mathematically converts between the rate of reaction and concentration of order
-the units of the rate constant spend upon the number of terms in the rate equation

Question 9

what is the overall order

Answer 9

• the overall effect of concentration of all reactants on the rate of reaction
• it is the sum of orders with respect to each reactant
  (e. g. when rate = k [A]m [B]n, the overall order is m+n)

Question 10

calculating orders from experimental results

Answer 10

• determined experimentally
• important to always measure rate after the same time in comparisons of different reactant concentrations or rates
• measure as close to the start as possible
• the initial rate is the instantaneous rate at the beginning of the experiment when t=0

Question 11

how can you continuously monitor rate?

Answer 11

-monitoring by gas collection or mass loss
-you can also monitor with a calorimeter
in a calorimeter, the wavelength of the light passing through a coloured solution is controlled using a filter. the amount of light absorbed by the solution is measured

Question 12

half life

Answer 12

the time for the concentration of the reactant to halve its original value is constant and the first order rate constant can be determined using this value

• first order reactions have constant half lives with concentrations having every half life. this pattern is called exponential decay.
• draw tangent and if it shoes successive half lives then the reaction is first order

Question 13

how do you determine K (rate constant) for first order reactions

Answer 13

• gradient of tangent is calculated giving the rate of reaction
• rate constant us calculated by rearranging the rate equation and substituting the value of rate (gradient) and the concentration at the position the tangent was drawn

Question 14

how do you calculate the rate constant from half life

Answer 14

K = ln2 / 1/2 t

Question 15

orders from concentration time graphs

Answer 15

• gradient is the rate of reaction
• the order with respect to a reactant can also be deduced from the shape of a concentration time graph for zero and first
• the order can be only be effectively obtained if all other reactant concentrations remain constant

Question 16

zero order concentration time graphs

Answer 16

• produces a straight line with a negative gradient
• reaction rates doesn’t change through the course of the reaction
• the value of the gradient is equal to the rate constant K

Question 17

first order concentration time graphs

Answer 17

• produce a downwards curve with a decreasing gradient overtime
• as the gradient decreases with time, the reaction gradually slows down
• the tine for the concentration of the reactant to halve is constant (half life). this can be used to determine the rate constant

Question 18

second order concentration time graphs

Answer 18

• a downwards curve, steeper for the start but tailing off more slowly
• you won’t be expected to analyse these graphs.

Question 19

determination of K for a first order reaction using the rate equation

Answer 19

• you can draw a tangent to the curve on a concentration time graph at any particular concentration
• calculate the gradient
• rate constant is calculated by rearranging the rate equation and substituting the value of rate (the gradient of the tangent) and the concentration where the tangent has been drawn

Question 20

determination of K for a first order reaction from the half life

Answer 20

k = ln2 / t(1/2)

-this is much more accurate than drawing a tangent

Question 21

zero order rate concentration graphs

Answer 21

• horizontal straight line with zero gradient
• rate = k[A]0 so rate = K
• y intercept gives the rate constant K
• the reaction rate doesn’t change with increasing concentration

Question 22

first order rate concentration graphs

Answer 22

• produces a straight line graph through the origin
• rate = k[A]1 so rate = k[A]
• rate is directly proportional to concentration for a first order relationship
• the rate constant can be determined by measuring the gradient of the straight line of this graph

Question 23

second order rate concentration graphs

Answer 23

• produces an upward curve with increasing gradient
• rate = k[A]2
• as this rate concentration graph is a curve, the rate constant cannot be obtained directly from this graph
• by plotting the second graph of the rate against the concentration squared, the result is a straight line though the origin. The gradient of this straight line graph is equal to the rate constant K

Question 24

the initial rates method

Answer 24

• the initial rate is the instantaneous rate at the start of the reaction when t=0 (measure gradient using tangent to find out)
• a clock reaction is more convenient, taking a single measurement (time to create a visual change to be observed often a colour or a precipitate)
• provided that there is no significant change is rate during this time, it can be assumed that the average RoR will be the same as the initial
• the initial rate is proportional to 1/t

Question 25

iodine clocks

Answer 25

• common example relies on the formation of iodine
• aqueous is coloured orange brown
• the time from the start of the reaction and the appearance of the iodine colour can be measured
• starch is usually added since it forms a complex with iodine which is an intense dark blue black colour
• separate experiments are carried out using different concentrations of one of the reactants and all other concentrations are kept constant
• the colour change is delayed by including a small amount of another chemical (aq sodium thiosulfate) which actually removes off iodine as it forms

Question 26

how accurate are clock reactions

Answer 26

• the shorter the period of time over which an average rate is measured, the less the rate changes over that time period
• it is an approximation but it is still reasonably accurate provided that less than 15% of the reaction has taken place

Question 27

multi step reactions

Answer 27

an overall chemical reaction compares the reactants and products

• the balancing numbers give the stoichiometry, the relative amounts of the species in the reaction
• a reaction can only take place when particles collide and it is unlikely that more than two particles will collide together at the same time
• the series of steps that make up an overall reaction is called the reaction mechanism

Question 28

the rate determining step

Answer 28

is the slowest step in the sequence

steps in multi step reactions will take place at different rates

Question 29

predicting reaction mechanisms

Answer 29

the rate equation only includes reacting species involved in the rate deterring step
-the orders in the rate equation match the number of species involved in the rate determining step

Question 30

hydrolysis of haloalkanes

Answer 30

RBr + OH- –> ROH + Br-

• by hot aqueous alkali
• can be determined experimentally to determine the overall order of reaction, the rate equation and a possible mechanism for the reaction