1.09 Rate Equations

Question 1

Define the term rate of reaction

Answer 1

The change in concentration (of any reactant or product) per unit of time

Question 2

At a given instant, how could you calculate 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 (2 methods)

Answer 3

-Use a colorimeter at suitable intervals if there is a colour change
-If gas is produced, use a gas syringe to collect volume of gas produced in a set time, or calculate the change in mass of the reaction mixture.

Question 4

How would you measure reaction rate for really fast reactions ?

Answer 4

-Use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species
-Eg used for ClO2 -> ClO. + O.

Question 5

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

Answer 5

Only experimentally

Question 6

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

Answer 6

Only temperature

Question 7

Write a generic rate expression and define the terms

Answer 7

*Rate = k[X]ᵃ[Y]ᵇ
*k=rate constant for the reaction
*[X] and [Y] are concentrations of the species
*ᵃ and ᵇ are the orders of the reactants in the reaction.

Question 8

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

Answer 8

No, species in the chemical equation may be excluded and species not in the chemical equation such as catalysts may be included

Question 9

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

Answer 9

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

Question 10

Define the term overall rate

Answer 10

-The sum of the orders of all species in the rate expression
-For k[X]ᵃ[Y]ᵇ, the total order is a+b.

Question 11

How would you calculate the units of rate constant ?

Answer 11

-Units if rate are moldm⁻³s⁻¹ and units of concentration are mol dm⁻³
-Rearrange rate equation to get k=
-Sub in units and cancel out.

Question 12

How would you draw a rate concentration graph ?

Answer 12

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

Question 13

Draw / describe the concentration graph for a zero order reaction ?

Answer 13

Rate stays constant across all concentrations

Question 14

Draw / describe the concentration graph for a first order reaction ?

Answer 14

Rate increases in linear manner as concentration incerases

Question 15

Draw / describe the concentration graph for a second order reaction

Answer 15

Curved increases of rate against concentration
second order and above are indistinguishable

Question 16

How can you confirm that a curved increase rate against concentration graph was second order ?

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

• Known moles of sodium thiosulfate and starch; reacts with I2 in a 1:2 ratio
• I2 = 2S2O3 2- -> S4O6 2- + 2I-

Question 19

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

Answer 19

When all of the NaS2O3 has been used up and so I2 is being produced, which reacts with starch and leads to a blue-black colour.

Question 20

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

Answer 20

Record time taken for colour change to occur, use rate = 1/t
This gives initial rate.

Question 21

What is the effect of a 10K temperature increase on rate of reaction (approx.)

Answer 21

Doubles rate

Question 22

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

Answer 22

Half life is constant

Question 23

What is the Arrhenius equation ? What does each term mean ?

Answer 23

*k = Ae to the power of (-Ea / RT)
*k = rate constant for reaction
*A = pre-exponential factor (number of collisions between reactants)
*e = e
*R = gas constant
T = temperature in kelvin
*Ea = activation energy in Joules

Question 24

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

Answer 24

*In k = -Ea/RT + In A (form of y = mx + c)
*Graph of In k against 1/T is a straight line
*Gradient = -Ea/R and y intercept is In A.

Question 25

What is the rate determining step ?

Answer 25

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

Question 26

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

Answer 26

-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.