Topic 16: Kinetics II

Question 1

What is meant by zero order

Answer 1

The concentration of the reactant does not affect the rate of reaction

Question 2

What is meant by first order

Answer 2

If the concentration doubles, the rate of reaction will double rate and concentration are directly proportional to one another

Question 3

What is meant by second order

Answer 3

If the concentration of the reactant doubles, the rate of reaction will be four times faster (2)2

Question 4

What does a zero order rate vs concentration of A graph look like and explain

Answer 4

1. Horizontal line

The rate does not depend on the concentration of A in a zero order reaction. If the concentration of A stays the same , rate stays the same.

Question 5

What does a first order rate vs concentration of A graph look like and explain

Answer 5

1. Straight line through the origin

The rate is directly proportional to the concentration of A. If the concentration of A doubles the rate doubles.

Question 6

What does a first +/second order rate vs concentration of A graph look like and explain

Answer 6

1. Curved line upwards

The rate is proportional to the square of the concentration change if the concentration of A triples the rate of reaction will be 9 times faster

Question 7

What would we see in a second order rate vs concentration graph and what would we plot and what would it look like

Answer 7

1. Straight line through the origin

we plot rate vs concentration squared

Question 8

What is meant by the half-life of a reaction and state the equation

Answer 8

The half-life of a reaction t1/2 is the time taken for the concentration of the reactant to reach half its initial value

t0.5= Ln(2) /k

Question 9

What is special about the half-life for a first order reaction

Answer 9

The half life is constant

Question 10

What is special about the half life of second order

Answer 10

If half lives rapidly increase then the order is second order

Question 11

What is a heterogeneous catalyst

Answer 11

A heterogeneous catalyst is one that is in a different phase ( state of matter) from the reactants

The reaction happens on the surface of the heterogeneous catalyst so increasing the surface area of the catalyst increases the number of molecules that can react at the same time

Question 12

What is a homogeneous catalyst

Answer 12

A homogeneous catalyst are in the same physical state as reactants

During homogeneous catalysis the reactants combine with the catalyst to make an intermediate species which then reacts to form the products and reform the catalyst

Question 13

Define rate of reaction

Answer 13

The change in concentration,volume, or moles of a substance in a unit time

Question 14

What is the initial rate of reaction

Answer 14

The rate at the start of the reaction where it is the fastest

Question 15

What is special about the rate constant, K

Answer 15

It is constant at a fixed temperature and is not affected by concentration or time

Question 16

What affects the rate constant, K

Answer 16

If we increase the temperature we increase the rate constant

Question 17

Rearrange the Arrhenius equation in log form and put it into y=mx + c form

Answer 17

Ink= InA - Ea/RT

y= mx + c form

Lnk= -Ea/R x 1/T + LnA

y= LnK 
m= -Ea/R 
x= 1/T
c= LnA

Question 18

What is the relationship between the rate constant and the rate of reaction

Answer 18

The rate constant is proportional to the rate of reaction

Question 19

What is meant by the rate-determining step

Answer 19

The slowest step in a multi-step reaction

Question 20

How to know which mechanism to choose when asked which is the rate determining step

Answer 20

The rate equation shows you what is included in the slow step, the mechanism must include what is mentioned in the rate equation. If a step has been said to be slow and is not included in the rate equation we need to include the reactants that produced it. The number of moles of reactants in the slow step will be the same as the order of the reaction of each substance.
eg= 1 mole of A in the slow step will mean A is first order

Question 21

What type of halogenoalkanes tend to react via the SN2 mechanism

Answer 21

Primary halogenoalkanes

Question 22

What type of halogenoalkanes tend to react via the SN1 mechanism

Answer 22

Tertiary halogenoalkanes

Question 23

Why do tertiary halogenoalkanes undergo the SN1 mechanism

Answer 23

The tertiary carbocation is made stabilised by the electron releasing methyl groups around it and the bulky methyl groups prevent the OH- ion from attacking the halogenoalkane

Question 24

Why do primary halogenoalkanes do not go through SN1 mechanism

Answer 24

Primary halogenoalkanes do not do the SN1 mechanism as they would only form an unstable primary carbocation

Question 25

What is a clock reaction

Answer 25

An example of the initial rate method where the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants

Question 26

Describe the steps you must take to carry out the iodine clock reaction to find the order with respect to potassium iodide

Answer 26

A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution. Starch is used as an indicator and turns blue-black in the presence of iodine

The sodium thiosulphate that is added to the reaction mixture reacts instantaneously with any iodine that forms
The iodine that forms in the first reaction is used up straight away in the second reaction, once all the sodium thiosulphate is used up any more iodine that is formed will stay in solution turning starch blue black - this is the end of the clock reaction

Varying the iodide concentration of hydrogen peroxide only gives different times of the color change

The time it takes for the reaction to occur and the concentration of the reactants helps us calculate the order with respect to iodine or hydrogen peroxide.

Question 27

What are the assumptions made when carrying out a clock reaction

Answer 27

The concentration of each reactant does not change significantly over the time period of the clock reaction

The temperature stays constant

When the end point is seen the reaction has not proceeded too far