Section 1: Reaction Rates

Question 1

What are reaction rates?

Answer 1

The reaction rate is the change in the amount of reactants or products per unit time.

Question 2

What is continuous monitoring?

Answer 2

Measure how the amount of a product or reactant changes over the complete course of the reaction

Question 3

How could you measure the formation of a gas?

Answer 3

If a gas is given off, you could collect it in a gas syringe and record how much you’ve got at regular time intervals

Question 4

How do you measure the mass of the reaction vessel at regular intervals as the gas is lost.

Answer 4

Using a mass balance

Question 5

How do you measure colour change?

Answer 5

Set the colorimeter to measure the wavelength of light that you’re interested in measuring

Calibrate the colorimeter. Place a sample of distilled water in a sample tube, know as cuvette, and place it on the colorimeter. Set the absorbance to 0.

Carry out your reaction. You should take samples of the reaction mixture at regular intervals, and measure the absorbance of each one using a colorimeter.

Question 6

How do you measure a change in ph?

Answer 6

If either hydrogen or hydroxide ions are produced or used up in a reaction, then you can monitor the progress of the reaction by measuring the change in the ph

Question 7

How do you process data of an experiment?

Answer 7

Concentration-time graph

Question 8

How can you work out the gradient of a concentration time graph?

Answer 8

Gradient = change in y / change in x

Question 9

If the graph is a curve how do you find the gradient?

Answer 9

Draw a tangent

Question 10

How do you find the initial rate of the reaction through calculations?

Answer 10

Work out how many moles of product there are in the set amount you measure in each reaction.

Use the reaction equation to work out how many miles of the reactant you’re interested in will have been used up when the set amount of product has formed.

Divide this by the volume of the reaction mixture to calculate the change in concentration of your reactant

Divide the change in concentration of the reactant by the time taken for the set amount of product to form. This is the initial rate of your reaction.

Question 11

How can you calculate initial rates from a graph?

Answer 11

Draw a tangent to curve at time zero, initial rate = change in y / change in x

Question 12

What is a clock reaction?

Answer 12

The formation of a set amount of product is shown by a sudden visual change in the reaction mixture.

Add an unknown amount of substance that reacts with the product you’re interested in to the reaction vessel. The substance reacts immediately with the product as it forms. As soon as the substance is used up, the presence of unreacted product in the reaction vessel causes a sudden change, such as colour change. This is the endpoint of the reaction. The quicker the endpoint is reached, the faster the initial rate of the reaction.

Question 13

What are reaction orders?

Answer 13

The reaction order with respect to a particular reactant tells you how the reactant concentration affects the rate.

Question 14

What happens if you double the reactants concentration and the rate stays the same?

Answer 14

The reaction is zero order

Question 15

What is the reaction order if you double the reactants concentration and the rate also doubles?

Answer 15

The reaction is first order

Question 16

What is the reaction order if you double the reactants concentration and the rate quadruples?

Answer 16

The reaction is second order

Question 17

What is the overall order of a reaction?

Answer 17

The sum of all the different reactants. E.g if there are two reactants in a reaction, and one has an order of 1 and the other has an order of 2, then the overall order is 3

Question 18

What is the rate equation for A + B –> C + D?

Answer 18

Rate = k[A]m[B]n

Question 19

In the equation rate = k[A]m[B]n what do the all stand for?

Answer 19

K= rate constant
[A]= concentration of A
[B]= concentration of B
M and n = orders of reaction

Question 20

What is the rearrangement of the rate equation to find the rate constant?

Answer 20

K = rate / [A]m[B]n

Question 21

How to calculate the rate constant of a first order reaction?

Answer 21

The rate constant is equal to the gradient of the rate concentration graph of that reactant

So.. gradient = change in y / change in x

Question 22

How do you calculate rate constant from half life?

Answer 22

K = ln2 / t1/2

T1/2 = half life

Question 23

What is the half life of a reaction?

Answer 23

The half life of a reaction is the time it takes for half of the reactant to be used up

Question 24

What is the rate determining step?

Answer 24

The step with the slowest rate

Question 25

How do you know what is involved in the rate determining step?

Answer 25

If it’s involved in the rate equation, it must affect the rate. So this reactant, or something derived from it, must be in the rate determining step

Question 26

How can you predict rate equations from the rate determining step?

Answer 26

Find the slowest step of the reaction
Use the reactants as [A] [B]
M and n are how many molecules of the reactants are in the equation

Question 27

What are the rules for suggesting a mechanism from the rate determining step / an equation and rate equation?

Answer 27

The rate determining step of the reaction must fit with the rate equation

All of the equations in the mechanism must balance

The different steps of the reaction must add up to the overall equation

Question 28

What does k = Ae -Ea/Rt stand for in the Arrhenius equation?

Answer 28

K = rate constant 
Ea = activation energy (j mol-1)
T = temperature (K)
R = gas constant (8.31 J K-1 mol-1)
A= the pre exponential factor (another constant) the units are the same as the rate constant

Question 29

What happens to the rate constant in the Arrhenius equation when activation energy gets bigger?

Answer 29

K gets smaller

Question 30

What does a large activation energy mean for the Arrhenius equation?

Answer 30

Slow rate. If a reaction has a high activation energy, then not many of the reactant particles will have enough energy to react. So only a few collisions will result in the reaction actually happening, and the rate will be slow.

Question 31

What happens to the rate constant in the Arrhenius equation if temperature increases?

Answer 31

K increases - high temp mean reactant particles move around faster and with more energy, so they’re more likely to collide and more likely to collide with at least the activation energy, so reaction rate increases

Question 32

What is the rearranged Arrhenius equation?

Answer 32

Ln K = -Ea/RT + lnA

Question 33

How do you plot a graph using the Arrhenius equation?

Answer 33

Plot link against 1/T