Unit 1: Section 7 - Rate Equations and Kp

Question 1

What is the definition of rate of reaction?

Answer 1

• Change in concentration (of any reactant or product) per unit time
• State what is being monitored (usually production of a product)

Question 2

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

Answer 2

Rate of reaction = change in concentration / change in time

Question 3

How could you measure the rate of reaction experimentally (different methods)?

Answer 3

1. Use a colorimeter if there is a colour change
2. If a gas is evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture

Question 4

How can you determine the rate constant and rate expression for a reaction?

Answer 4

Only experimentally

Question 5

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

Answer 5

Temperature, nothing else

Question 6

Write a generic expression and state what each term means?

Answer 6

Rate = k[A]^x[B]^y
k - rate constant for the reaction
[A] and [B] are concentrations of species A and B respectively
x and y are the orders of reaction with respect to A and B

Question 7

Do species need to be in the chemical equation to be in the rate expression?

Answer 7

No - species in the chemical equation may be excluded and species not in the chemical equation, e.g. catalysts, may be included

Question 8

Define the term order of a reaction with respect to a given product

Answer 8

The power to which a species’ concentration is raised in the rate equation

Question 9

How do you find the overall order of reaction?

Answer 9

The sum of the orders of reaction of all species in the rate expression

Question 10

How would you calculate the units of the rate constant?

Answer 10

• Units of rate are moldm^-3s^-1 and units of concentration are moldm^-3
• Rearrange rate equation to get k=
• Sub in units and cancel them out

Question 11

How would you draw a rate concentration graph?

Answer 11

• Plot [A] against time, draw tangents at different values
• Draw a secondary graph of rate against [A]

Question 12

What does a rate concentration graph for a zero order reactant look like?

Answer 12

A straight horizontal line

Question 13

What does a rate concentration graph look like for a first order reactant?

Answer 13

A straight line graph y=x

Question 14

What does a rate concentration graph from a second order plus reactant look like?

Answer 14

A curve starting at (0,0) and increasing exponentially

Question 15

How could you confirm that a reactant was second order using graphs?

Answer 15

Graph of rate against concentration squared would be a straight line y=x through the origin

Question 16

How could you find the rate expression using the initial rate method?

Answer 16

• Do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes over time
• Plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of reaction
• Compare rates and concentrations between each experiment to find order reactants and overall rate equation

Question 17

What must you add to react with the I2 as it is produced for an iodine clock reaction?

Answer 17

Known moles of sodium thiosulfate and a little starch
Reacts with I2 in 1:2 ratio
I2 + 2S2O3 ^-2 -> S4O6 ^-2 + 2I-

Question 18

When does the starch turn a blue-black colour in an iodine clock reaction and why?

Answer 18

When all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch, leading to a blue-black colour

Question 19

How can you calculate the rate of reaction from the data from an iodine clock reaction?

Answer 19

Record time taken for colour change to occur
Use rate = 1/t
This is effectively the initial rate

Question 20

What is the effect of a 10K temperature increase on the rate of reaction, roughly?

Answer 20

Doubles rate of reaction

Question 21

What is true of the half life of a first order reactant (concentration against time graph)?

Answer 21

Half life is constant

Question 22

What does each term in the Arrhenius equation mean?

Answer 22

k - rate constant for reaction (same units as A)
A - Arrhenius constant (same units as k)
e - mathematical quantity
R - gas constant
T - temperature in Kelvin
Ea - activation energy for reaction in Joules

Question 23

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

Answer 23

• Graph of lnk against 1/T is a straight line
• gradient - Ea/R
• y intercept - lnA

Question 24

What is the rate determining step?

Answer 24

The slowest step in a reaction mechanism, which determines the overall rate of reaction

Question 25

How does the rate determining step link to the species involved in the rate expression?

Answer 25

* Any species involved in the rate determining step appear in the rate expression * Species only involved after the rate determining step do not appear in the rate expression

Question 26

What is the definition of the rate constant?

Answer 26

The constant of proportionality in a rate equation, the value of which is dependent on temperature and activation energy of the reaction

Question 27

How does the temperature affect k?

Answer 27

As temperature increases, concentration stays the same, so k increases

Question 28

Why does doubling the temperature have a greater effect on the rate of reaction than doubling the concentration?

Answer 28

* Reaction occurs when molecules have E≥Ea * Doubling temperature causes many more molecules to have this Ea * Whereas doubling the concentration of a species only doubles the frequency of the collisions

Question 29

What 2 things mean you can ignore the order of a reactant, why?

Answer 29

* If they have a significantly larger concentration than the other reactant - concentration change becomes negligible so has no effect on the rate * The concentration remains almost the same

Question 30

What is the definition of the overall order of a reaction?

Answer 30

The sum of powers/indices (to which the concentrations are raised in the rate equation)

Question 31

What is partial pressure?

Answer 31

Each gas's contribution to the total pressure

Question 32

How would you calculate the partial pressure of a gas?

Answer 32

Partial pressure p = mole fraction x total pressure

Question 33

What is the mole fraction?

Answer 33

Mole fraction of gas X = number of moles of gas X in the mixture / total number of moles of gas in the mixture

Question 34

A reaction is represented by aA(g) + bB(g) ⇌ cC(g) + dD(g), what is Kp for the system?

Answer 34

Kp = pC^cpD^d/pA^apB^b where pA = partial pressure of A and a = number of moles of A

Question 35

How do you calculate the units for Kp?

Answer 35

Write out the units for the partial pressures in the same arrangement as the Kp equation and cancel out/multiply together Usually in Pa, kPa, atm etc DO NOT CHANGE UNITS

Question 36

What is the effect of increasing temperature on Kp for an endothermic reaction?

Answer 36

Equilibrium shifts to the right, so partial pressures of products increase, so Kp increases

Question 37

What is the effect of increasing the overall pressure on Kp for this reaction?

Answer 37

Pressure doesn't affect Kp as, if moles are not the same on each side, either top or bottom of Kp expression will have a total pressure term that does not cancel

Question 38

What will the kinetic effect of increasing the temperature and pressure for any reaction?

Answer 38

Increasing both will increase the rate of reaction as: Temperature - many more particles habe energy greater than or equal to the activation enegry -> more successful collisions per second Pressure - more particles in the same volume -> more successful collisions per second