Unit 1: Section 7 - Rate equations and Kp MDY CDS *

Question 1

what is the reaction rate?

Answer 1

the change in amount of reactant or product per unit time

Question 2

what does a graph of amount of reactant or product by time show?

Answer 2

the gradient is the reaction rate (change in y / change in x)
if it is a curve a tangent to the curve can be drawn to find the gradient

Question 3

what do rate equations do?

Answer 3

they tell you how rate is affected by the concentrations of reactants
r=k[A]^m[B]^n

Question 4

what are the orders of reaction?

Answer 4

the orders of reaction with respect to an element or compound are the powers to which the concentrations of its reactant are raised in the rate equation
the overall order is all the powers added together

Question 5

what is the rate constant?

Answer 5

(k) a number that links the rate of the reaction to the concentrations of the reactants. the bigger the value of k, the faster the reaction

Question 6

what happens to the rate constant at certain temperatures?

Answer 6

its the same for a certain reaction at a particular temperature.
if the temperature increases so does the rate constant

Question 7

why does the rate constant increase with temperature?

Answer 7

rate of reaction increases - you’re increasing the number of collisions between reactant molecules, and the energy of each collision. concentrations and orders of reaction stay the same, so k must increase.

Question 8

what are the units of k?

Answer 8

the units of the rate (mol3dm-6s-1) divided by the units of the concentrations to their correct powers

Question 9

what is the initial rate of a reaction?

Answer 9

the rate at the start of the reaction. it can be found from a concentration-time graph by calculating the gradient of the tangent at time=0

Question 10

how can the initial rate of reaction be used to work out the rate equation?

Answer 10

repeat an experiment using different initial concentrations of reactants (keep temperature constant). only change 1 at a time
calculate initial rate for each experiment
see how initial concentrations affect initial rates and figure out order

Question 11

how can you measure the initial reaction rate?

Answer 11

some reactions have a sudden colour change when a product reaches a certain concentration. rate can be found from the time it takes for this to happen

Question 12

what happens in an iodine clock reaction?

Answer 12

H2O +2I- +2H+ –> 2H2O +I2
a small amount of sodium thiosulfate and starch are added to the reaction mixture. sodium thiosulfate reacts instantly with the iodine that is formed. once all the sodium thiosulfate is used up the excess iodine turns the starch blue-black

Question 13

how can the rate equation be found for the iodine clock reaction?

Answer 13

varying the concentration of iodide or hydrogen peroxide while keeping everything else constant will give different times for the colour change.

Question 14

what are some examples of experiments where initial rate can be measured from the time taken for product to form?

Answer 14

An Iodine clock reaction
reactions that produce a precipitate that clouds a solution - time for mark to disappear
time taken for small amounts of product to be formed in a normal reaction

Question 15

how can you measure rate by continuous monitoring?

Answer 15

follow a reaction all the way through to its end by recording the amount of product you have at regular time intervals. the results can be used to work out how rate changes over time

Question 16

what are different ways you can follow the rate of a reaction?

Answer 16

following gas volume over time
loss of mass over time
colour change over time
change in pH over time

Question 17

how can you follow the gas volume over time?

Answer 17

if gas is given off it can be collected in a gas syringe and record how much you’ve got at regular time intervals. e.g. reaction between an acid and a carbonate where CO2 is given off.
use ideal gas equation and molar ratio to find concentrations

Question 18

how can you follow the loss of mass over time?

Answer 18

if a gas is given off, the system will lose mass

you can measure this at regular intervals with a balance.

Question 19

how can you follow the colour change over time?

Answer 19

using a colorimeter to measure absorbance. the more concentrated the colour of the solution, the higher the absorbance

Question 20

what’s an example of a reaction where you can follow the colour change over time?

Answer 20

the reaction between propanone and iodine, the brown colour fades. so absorbance decreases.

Question 21

how can you measure the change in absorbance?

Answer 21

plot a calibration curve
during the experiment, take a small sample from your reaction solution at regular intervals and read the absorbance
use calibration curve to convert the absorbance at each time point into a concentration

Question 22

what is a calibration curve for a colorimeter?

Answer 22

a graph of known concentrations of the coloured solution plotted against absorbance

Question 23

how can you follow the change in pH over time?

Answer 23

if the reaction produces or uses up H+ ions, the pH of the solution will change. so you could measure the pH of the solution at regular intervals and calculate the concentration of H+

Question 24

how can you use data from a concentration-time graph to construct a rate-concentration graph?

Answer 24

find the gradient at various points on the graph. this will give you the rate at that particular concentration.
now plot each point on a new graph with the axes rate and concentration and draw a smooth line

Question 25

how can you use data from a rate-concentration graph to find the reaction order?

Answer 25

a horizontal line means changing the concentration doesn’t change the rate so order = 0
if its a straight line through the origin the rate is proportional to [X]
order = 1
a curve means rate is proportional to [X]^2
order = 2

Question 26

what is the rate determining step?

Answer 26

the slowest step in a multi-step reaction. the overall rate is decided by the RDS

Question 27

how to find out which reactants are involved in the rate determining step?

Answer 27

if a reactant appears in the rate equation, it must affect rate.
so this reactant, or something derived from it must be in the rate determining step.
species that are involved in steps after the RDS don’t appear in the rate equation, but species up to and including the step are involved

Question 28

what does the number of molecules in the RDS show?

Answer 28

the order of a reaction with respect to a reactant shows the number of molecules of that reactant that are involved in the rate determining step

Question 29

what if the rate equation contains reactants from multiple steps?

Answer 29

the RDS is the step that contains the reactants from the equation or that are derived from products in the equation. so the RDS contains reactants made from molecules in previous steps

Question 30

how can the rate determining step help you work out the reaction mechanism?

Answer 30

all of the reactants in the equation must be involved in the step in some way. so the correct mechanism must be the one with a step that involves all of the reactants in the equation or their derivatives

Question 31

what is the partial pressure?

Answer 31

in a mixture of gases, each individual gas exerts its own pressure - this is called its partial pressure.
the total pressure of a gas mixture is the sum of all the partial pressures of the individual gases

Question 32

what is a mole fraction?

Answer 32

the proportion of a gas mixture that is made up of a particular gas

Question 33

what is the formula for the mole fraction of a gas in a mixture?

Answer 33

total number of moles of gas in the mixture

Question 34

what is the formula for the partial pressure of a gas?

Answer 34

mole fraction of gas x total pressure of the mixture

Question 35

what is Kp?

Answer 35

the equilibrium constant for a reversible reaction where all the reactants and products are gases

Question 36

what is the expression for Kp?

Answer 36

aA + bB dD + eE
Kp= P(D)^d P(E)^e / P(A)^a P(B)^b
P(X) = partial pressure of X

Question 37

why does rate slow over time?

Answer 37

most reactions start faster and slow down due to there being less reactant particle as time goes on so less frequent successful collisions between reactant particles

Question 38

what is the importance of the initial rate of reaction?

Answer 38

there are known concentrations of reactants, which is useful for working out the rate equations

Question 39

how can the the RDS be determined from energetics?

Answer 39

the RDS is the slowest because it requires the most energy, so it is the step with the highest activation energy

Question 40

what are the 2 nucleophilic substitution mechanisms for OH- into 2-bromo-2-methyl propane?

Answer 40

SN1 - has the slowest step of C-Br breaking in 2-bromo-2-methyl propane
SN2 - has the slowest step of OH- reacting with 2-bromo-2-methyl propane

Question 41

what is the units for Kp?

Answer 41

its the same as the units used for the gases (Pa)

you must work out the units by cancelling the powers on the top and bottom of the equation

Question 42

what factors can affect Kp?

Answer 42

only temperature can affect Kp
if temperature causes a shift to the right Kp increases
left, Kp decreases

Question 43

how to work out Kp?

Answer 43

write balanced equilibrium equation
find moles at equilibrium of each species
work out mole fractions
work out partial pressure
substitute into calculation

Question 44

What does the Arrhenius equation show?

Answer 44

How the rate constant (k) varies with temperature and the activation energy

Question 45

What is the Arrhenius equation?

Answer 45

k=A e^(-Ea/RT)
k=rate constant
Ea = activation energy (J)
T = temperature (K)
R = gas constant (8.31 J/K/mol)
A= Arrhenius constant

Question 46

What happens to the rate constant as the activation energy increases?

Answer 46

k decreases
Large Ea= slow rate
Not many of the reactant particles will have the energy to react, so only a few collisions will be successful, so rate is slow

Question 47

What happens to the rate constant as the temperature increases?

Answer 47

k increases
Higher temp = reactant particles move faster with more energy, so they are more likely to collide and with the right activation energy, so rate increases

Question 48

What is the logarithmic form of the Arrhenius equation?

Answer 48

ln(k)=ln(A) - Ea/RT

Question 49

How to plot the Arrhenius equation on a graph?

Answer 49

Use the log form to plot ln(k) against 1/T

this gives -Ea/R as the gradient and ln(A) as the y-intercept

Question 50

what is A, the Arrhenius constant?

Answer 50

a frequency factor that accounts for the frequency of collisions with the correct orientation

Question 51

what do graphs of rate by time look like for different orders of reaction?

Answer 51

zero order - straight horizontal line
1st order - straight linear line
2nd order - curved line

Question 52

what do graphs of concentration by time look like for different orders of reaction?

Answer 52

the gradient is the rate
zero order - straight linear line
1st order - curved line
2nd order - steeper curved line