kinetics ii

Question 1

rate of reaction

Answer 1

rate = change in measured property/time

Question 2

rate equation

Answer 2

shows how the changes in the concentration of the REACTANTS affect the rate of a reaction

cannot be found by just looking at equation
must be derived through experimental evidence

because species that appear in stoichiometric equation sometimes are not in rate equation
substances that are not in stoichiometric equation e.g. catalysts can appear in rate equation

Question 3

orders with respect to a substance

Answer 3

power to which a concentration is raised in the rate equation [A]2 (second order)

tells you how much the CONCENTRATION of a reactant affects the rate

Question 4

the overall order

Answer 4

the sum of the orders for ALL the substances that appear in the rate equation

Question 5

half life

Answer 5

the time for the concentration of one of the reactants to fall by half

Question 6

rate determining step

Answer 6

many reactions are multi-step
each step has its own rate and rate constant
BUT the overall rate of a multi-step process is governed by the rate slowest step I.e. the step with the highest Ea
the slowest step therefore = rds

Question 7

why must the rds include the reactants in the rate equation?

Answer 7

the rds must contain at least the reactants from rate equation as they show what concentrations affect the rate

Question 8

activation energy

Answer 8

the activation energy of a reaction is the minimum energy needed in a collision between molecules if they are to react

Question 9

dilute hcl + magnesium turnings

Answer 9

measure vol of gas formed using a gas syringe

Question 10

reaction involving change in the number or nature of ions

Answer 10

use a conductivity cell and meter to measure the changes in electrical conductivity of the reaction mixture as number or nature of ions change

Question 11

following formation of a coloured product or removal of a coloured reactant

Answer 11

use a colorimeter

light source + filter + reaction mixture + light sensitive cell + meter

Question 12

following course of a reaction by measuring the change of mass

Answer 12

change of mass as the reaction gives off a dense gas that is lost from the system

Question 13

finding the reaction rate by measuring the change in concentration of the products or change in concentration of the reactant over time

Answer 13

titration method

• you can take small samples of a reaction at regular intervals
• titrate them using a standard solution

Question 14

titration method examples

Answer 14

following the course of a reaction for a change in concentration of acids present by removing measured samples of the mixture at intervals

stopping the reaction mixture ‘quenching’ by running the reaction into alkali

then determining the concentration of one reactant or product by titration

further samples are taken out at regular intervals

Question 15

continuous monitoring

Answer 15

using half lives

data from one experiment where the concentration of one of the reactants is followed throughout the experiment

Question 16

why does the concentration of the reactants not be monitored through experiment have to be in large excess?

Answer 16

so that their concentrations stay virtually constant and do not effect the rate

Question 17

initial rate methods

Answer 17

1. choose the initial concentrations of reactants
2. do your experiment recording the concentration at various time points
3. measure the initial rate of the reaction
4. you repeat the experiment changing the initial concentration of one of the reactants and keeping the other initial concentrations constant
5. record the concentration at various time points and measure the initial rate as before
6. then repeat experiment so you have values for the initial rates for different initial reactant concentrations
7. tabulate the results initial rate vs conc
8. initial rate y and conc on x

Question 18

SN1

Answer 18

Is a nucleophilic substitution reaction with a MECHANISM that involves only one molecule/ion in the rds

Question 19

SN2

Answer 19

is a nucleophilic substitution reaction with a MECHANISM that involves 2 molecules/ions in the rds

Question 20

define the term mechanism

Answer 20

a mechanism is a series of steps through which the reaction progresses

Question 21

why do you measure the initial rate?

Answer 21

measure from where the slope is steepest at the beginning of graph
t = 0

because at that point the concentration of each reactant are known
but no products are present so there cannot be any reverse reactions which will affect the measurement of the rate

Question 22

why do catalysts not appear in stoichiometric equation but do appear in rate equation?

Answer 22

do appear in rate equation as they effect the rate

do NOT appear in stoichiometric eq because they are chemically unchanged

Question 23

propanone + iodine + acid catalyst

Answer 23

measure using a colorimeter the time taken for colour to disappear