18.1 Orders, Rate Equations and Rate Constants, 18.2 Concentration-time Graphs, 18.3 Rate-concentration Graphs and Initial Rates, 18.4 Rate-determining Steps

Question 1

How is rate of reaction measured? What is its units?

Answer 1

• Change in concentration/ change in time
• Its units are therefore mol dm^-3 s^-1

Question 2

How is rate of reaction linked to the concentration of reactants?

Answer 2

• The rate of reaction is directly proportional to the concentration of a reactant raised to a power

Question 3

What is this power known as?

Answer 3

• The order of reaction (for that reactant, as different reactants can have different orders)

Question 4

List 3 types of orders, and what each one means for the rate of reaction.

Answer 4

• Zero order; the concentration doesn’t affect the rate
• First order; the rate of reaction is directly proportional to the change in the change in concentration (if the concentration of the reactant doubles, the rate doubles)
• Second order; the change in the rate of reaction is directly proportional to the square of the change in concentration of the reactant (if the concentration of the reactant doubles, the rate of reaction increases by a factor of 4)

Question 5

What value is used for the rate of reaction when calculating the order of a reaction? How is it found?

Answer 5

• The initial rate (at t=0)
• From doing a reaction and plotting the change in concentration over time
• The change in concentration is found through titrations/ colorimetry

Question 6

What do the rate-concentration graphs of zero, first and second order reactions look like?

Answer 6

• Zero order: straight horizontal line
• First order: linear line, crosses at the origin
• Second order: positive quadratic, turning point at the origin

Question 7

What is the rate equation?

Answer 7

• rate = k [A]^m [B]^n
• k= rate constant
• A and B: reactants
• m: order of reaction with respect to A
• n: order of reaction with respect to B

Question 8

What is the rate constant?

Answer 8

• The number that converts between the rate of reaction and the concentrations and orders

Question 9

What is the overall order of a reaction? How would this be calculated from the rate equation?

Answer 9

• The sum of the orders in respect to each reactant
• m+n

Question 10

Explain the shape of rate-concentration graphs for zero-order reactions.

Answer 10

• Straight horizontal line since rate=k

Question 11

Explain the shape of rate-concentration graphs for first-order reactions.

Answer 11

• Linear line through the origin since rate=k[A]

Question 12

Explain the shape of rate-concentration graphs for second-order reactions.

Answer 12

• Positive quadratic through the origin since rate=k[A]^2

Question 13

What is continuous monitoring? What is it used for?

Answer 13

• Continuous measurements taken during the course of the reaction
• To plot concentration-time graphs

Question 14

What are three properties that can be monitored for the purpose of plotting a concentration-time graph?

Answer 14

• Gas produced
• Mass loss
• Colour change

Question 15

What device is used to measure colour change?

Answer 15

• A colorimeter

Question 16

How do colorimeters work?

Answer 16

• Since solutions transmit light that is the same colour as them and absorb all of the other colours, colorimeters pass light of the complementary colour (colour opposite in the colour wheel) to the solution through the solution
• The amount of light absorbed is measured, which is directly proportional to concentration

Question 17

How can a concentration-time graph be created using a colorimeter?

Answer 17

• Prepare standard solutions of the coloured chemical in the reaction
• Select the filter with the complementary colour
• Zero the calorimeter with water
• Take readings of the absorbance of the standard solutions
• Plot a calibration curve (concentration against absorbance)
• Do the reaction, taking samples at timed intervals
• Find the absorbance of these samples, and use the calibration curve to find their concentration
• Then plot a graph of concentration against time

Question 18

What are 2 useful features of a concentration-time graph?

Answer 18

• Its gradient is the rate of reaction
• Its shape tells you the order with respect to a reactant for zero and first order reactions

Question 19

What is the shape of a zero order reaction on a concentration-time graph? What does its gradient show?

Answer 19

• Straight line with a negative gradient
• The rate constant

Question 20

What is the shape of a first order reaction on a concentration-time graph?

Answer 20

• Exponential the shape of 1/x

Question 21

What is special about a first order reaction? What is this pattern known as?

Answer 21

• The time taken for the concentration of the reactant to half is constant, and is therefore known as the half-life (t1/2)
• Exponential decay

Question 22

How can the rate constant be calculated for a first order reaction?

Answer 22

• ln(2) / t1/2
• Use a concentration-time graph to find the rate of reaction
• Divide the rate by the concentration at that time

Question 23

What is a more efficient way of finding the instantaneous rate of reaction (rate of reaction when t=0)? What is beneficial about it?

Answer 23

• A clock reaction
• You only take a single measurement of when a visual change happens, but you can assume that the average rate of reaction is the same as the initial rate

Question 24

What calculation is done from the results of a clock reaction, and what does the calculation tell you?

Answer 24

• You divide 1 by the time taken for the visual change to happen
• The initial rate is proportional to this value

Question 25

How does an iodine clock reaction work?

Answer 25

• Hydrogen peroxide is reacted with iodide ions, which forms iodine, which turns the reaction mixture brown
• A small volume of thiosulfate ions and starch are added to the mixture
• Thiosulfate ions react with iodine and turn it back to iodide ions, turning the solution colourless
• When the thiosulfate ions are used up, iodine is no longer converted into iodide ions, so it reacts with starch to turn the mixture blue-black
• You time how long it takes for the reaction mixture to turn blue after adding the thiosulfate ions- this tells you how long it took for iodine to be formed, and therefore you can calculate the order in respect to hydrogen peroxide
• You repeat this and change the concentration of hydrogen peroxide

Question 26

How do you calculate the order in respect to hydrogen peroxide?

Answer 26

• You divide 1 by the time taken for the colour change to happen
• You plot this value against the concentration of hydrogen peroxide
• This gives you a rate-concentration graph

Question 27

What limitation is there to clock reactions?

Answer 27

• The rate of reaction has to be measured over a short period of time, so that the calculated value is close to the true value
• Otherwise there’s a larger change in rate

Question 28

How do reactions actually happen, and why?

Answer 28

• They happen in multiple steps
• It is unlikely that all of the correct particles will collide at the same time with the correct energy and orientation

Question 29

Which step in reactions is the most important, and why?

Answer 29

• The slowest one
• It is the rate-determining step

Question 30

How are rate equations and the rate-determining step linked?

Answer 30

• The rate equation only includes species involved in the rate determining step (so zero-order reactants aren’t in the rate-determining step)
• The orders in the rate equation are the same as the number of moles in the rate-determining step