Rate of reaction Module 5

Question 1

What is the reaction rate?

Answer 1

The change in the amount of reactants or products per unit time
Units are mol dm^-3 s^-1

Question 2

Ways to follow the rate of reaction?

Answer 2

Measure the volume of gas produced
Measure the loss in mass as a gas is produced
Use colorimetry to measure colour change of a reaction
Measure the pH change of a reaction

Question 3

How do you work out reaction rate from a concentration- Time graph?

Answer 3

Find the point of time where you want to know the reaction rate, then draw a tangent from that point on the curve

Work out gradient of that tangent using (change in y)/(change in x)

Must be positive value

Question 4

How do you find the initial rate and what can you use it for?

Answer 4

Draw the tangent from were time=0

Can be used by changing concentrations to work out what order chemicals are

Question 5

What does a clock reaction do?

Answer 5

Measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants.

There will be an observable endpoint when desired amount of product has formed, so the quicker the end point reached the faster the initial rate of reaction

Question 6

What assumptions do you need to make when doing a clock reaction?

Answer 6

The concentration of each reactant doesn’t change significantly over time period of clock reaction
Temperature stays constant
When the endpoint is seen, that the reaction hasn’t proceeded too far

Question 7

Explain the iodine clock reaction?

Answer 7

H2O2 (aq) + 2I- (aq) + 2H+ (aq) = 2H2O (l) + I2(g)

Small amount of Sodium Thiosulfate solution and starch are added to an excess of H2O2 (Hydrogen peroxide), and Iodide ions in acid solution (H+)

Starch is an indicator and turns blue/black in presence of iodine

Sodium thiosulfate instantly reacts with any Iodine that forms making it go back to iodide ions

So once the set amount of Sodium thiosulfate is used up solution will go blue/black, so can vary iodide or H2O2 concs to give you different times

Question 8

What does the order of reaction tell you?

Answer 8

How the reactants concentration affects the rate

Question 9

How do you know what order a reactant is?

Answer 9

0 order if you double reactants conc and rate stays the same
1st order if you double reactants conc and the rate doubles
2nd order if you double reactants conc and the rate quadruples (power of 2)

Question 10

What’s the overall order of a reaction?

Answer 10

Sum of all the orders of all different reactants

Question 11

What’s the only way to find orders of reaction?

Answer 11

Experimentally

Question 12

What must you make sure to do when trying to find order how each reactant affects the rate?

Answer 12

Measure one by one, so make sure the other is constant and in excess

Question 13

What’s the shape of a Concentration-Time graph of a 0 order reaction?

Answer 13

Straight diagonal line going down from y axis

Question 14

What’s the shape of a Concentration-Time graph of a 1st order reaction?

Answer 14

Curved line with diagonal gradient from y axis, reaching an asymptote near x axis

Question 15

What’s the shape of a Rate-Concentration graph for a 0 order reaction?

Answer 15

Horizontal line from Y axis, as changing concentration doesn’t affect rate

Question 16

What’s the shape of a Rate-Concentration graph for a 1st order reaction?

Answer 16

Positive gradient for y=x, through the origin

Rate is proportional to concentration

Question 17

What’s the shape of a Rate-Concentration graph for a 2nd order reaction?

Answer 17

Positive curve going through the origin, Rate is proportional to concentration^2

Question 18

How do you draw a Concentration-Time graph?

Answer 18

Concentration on Y axis

Time on X axis

Question 19

How do you draw a Rate-Concentration graph?

Answer 19

Rate on Y axis

Concentration on X axis

Question 20

How do work out what order reactants are from a table which gives you their concentrations in the experiment and the initial rate of reaction?

Answer 20

Find 2 experiments where everything is the same expect the concentration of one reactant, and see how the rate changes between experiments to work out what order that reactant is

Will have to use the fact that you know other reactants order of reaction to work out some of them aswell (normally order 0)

Question 21

How would you write the rate equation if A + B = C + D?

Answer 21

Rate (mol dm^-3 s^-1) = k x (A)^m x (B)^n 
k= rate constant
m= order of reaction of A
n = order of reaction of B
(A)= concentration of A
(B)= concentration of B

Question 22

What does a bigger rate constant mean?

Answer 22

The faster the reaction

Question 23

Do you include a reactant in the rate equation if it’s order 0?

Answer 23

No because anything to order 0 is 1

Question 24

How do you work out the rate constant?

Answer 24

Insert all your values into rate equation, and rearrange to find k

Question 25

What’s an easy way to find the rate constant of a first order reaction?

Answer 25

Draw the Rate-concentration graph

The rate constant is equal to the gradient

Question 26

What is half life?

Answer 26

The time taken for half the reactant to be used up

Question 27

What’s the half life for a first order reactant?

Answer 27

Constant, will always be the same amount of time for reactant to half

Meaning it’s half life can be read off it’s Conc-Time graph

Question 28

How do you work out the rate constant of a first order reaction if you know it’s half life?

Answer 28

k = ln2 / half life
Units = s^-1

Question 29

What’s the half life for a second order reaction?

Answer 29

Not constant

Question 30

What’s the rate determining step?

Answer 30

Is the slowest step in a multi step reaction, which determines the overall rate

Question 31

Rules for determining if a reactant is involved in the rate determining step?

Answer 31

If it’s in the rate equation it must affect the rate, so it must be in the rate determining step
If a reactant isn’t in the rate equation then it can’t be in the rate determining step, neither can anything derived from it

Question 32

What does the order of reaction in the rate equation tell you about the rate determining step?

Answer 32

1st order means 1 molecule
2nd order means 2 molecules

Can be used other way round, so look at rate determining step and the amount of molecules to work out what order of reaction it is

Question 33

What does the rate equation normally tell you?

Answer 33

Which chemicals and how many molecules are reacting in the first step

Question 34

For a reaction to happen, what needs to occur?

Answer 34

Particles need to:
Collide with each other
Have enough energy to react ( at least the activation energy)
Have the right orientation

Question 35

What does increasing the temperature do the reaction rate?

Answer 35

Particles have more energy, speed up, collide more per unit time
More reactant particles have required activation energy so greater proportion of the collisions will result in reaction per unit time
Therefore temperature increases rate of reaction

Question 36

What’s the Arrhenius equation?

Answer 36

k = A x e^((-Ea)/RT))

k = rate constant
Ea= activation energy (j mol^-1)
T= temperature in kelvin
R = Gas constant = 8.31
A = the pre exponential factor, another constant

Question 37

What’s the Arrhenius equation in ln form?

Answer 37

ln k = -Ea/Rt + ln A

Question 38

How do you use ln form of equation to create a Arrhenius plot and find activation energy and the pre exponential factor?

Answer 38

Plotting ln K on y axis so ln K is substituted for y
Plotting 1/T on x axis so 1/T is substituted for x
y = mx + c
LnK = -Ea/R x + lnA
Creating a graph with a gradient of -Ea/R and a y intercept of lnA

Calculate the gradient and make it equal to -Ea/R then solve for Ea

Find the y intercept and make it equal to ln A and solve for A, or just put Ea into equation

(Make sure graph is well proportioned, doesn’t need to show Y intercept)