Kinetic

Question 1

What is the rate of an equation?

Answer 1

How quickly the concentrations of reactants and products are changing
mathematically described as the rate of change of [A] with respect to time: d[A]/dt

Question 2

How to determine rate of reaction at time t?

Answer 2

Gradient of tangent to the graph at time t
d[A]/dt = -kt (negative so reactant used)
d[B]/dt = kt (positive so product formed)

Question 3

What is instantaneous rate?

Answer 3

Rate at a specific moment in time

Question 4

Units of rate?

Answer 4

Rate of change of concentration of a reaction or product with time
moldm-3s-1

Question 5

Rate law?

Answer 5

Rate = k[A]a[B]b

Question 6

What is an elementary reaction?

Answer 6

A single step process that involves one or two molecules

Question 7

Two important features of elementary reactions?

Answer 7

Equation describe what is actually going on at the molecular level, for a complex reaction which is made up of elementary reactions the equation only gives the stoichiometry
For an elementary reactions it is possible to use the chemical equation to form the rate equation but this is not the case for complex reactions

Question 8

Rate equation form elementary reaction?

Answer 8

An elementary reaction is a reaction that takes place at the molecular level ie what is really happening. The stoichiometric equation represents the overall reactions in terms of initial reactants and final products but does not show how a reaction actually happens, the molecularity of an elementary reaction is the number of molecules that come together to react

Question 9

Unimolecular?

Answer 9

1 molecular involves eg isomerisation ot radioactive decay

Question 10

Bimolecular?

Answer 10

2 molecules react

Question 11

What is a reaction mechanism?

Answer 11

The sequence of elementary molecular events that lead from reactant to products, what happens at the molecular level during a chemical reaction from experimental determined kinetic data are used to postulate the mechanism for a reaction

Question 12

Using the rate law to test a proposed mechanism?

Answer 12

Rate law is experimentally determined, we want to propose a reaction mechanism that accounts for the observed rate law, to do this we must be able to formulate the rate law that corresponds to the proposed mechanism and compare this with the experimentally observed rate law

Question 13

What is the rate law?

Answer 13

Relates the concentration of reactants and the overall rate of reaction, empirically the rate of reactions is often but not always found to be related to the concentration of that reactant raised to a simple power

Question 14

Rate constant k?

Answer 14

Rate constant k relates concentrations of reactants to the overall rate of reaction

Question 15

Reaction order?

Answer 15

Power to which the concentration or pressure of a reactant is raised in the rate law, the order overall is the sum of order of reactant, until you have carried out experiments you cannot know that this is true, in general the rate law cannot be found from a chemical equation it must be found experimentally, order in an experimental observation depends on the mechanism of the reaction

Question 16

How does the rate constant affect reaction rate?

Answer 16

The larger the value of k the faster the reaction is

Question 17

Rate constant units?

Answer 17

Units for the rate constants are important and depends on the type of reaction and the reaction order 
1st order rate law k = s-1
2nd order rate law k = dm3mol-1s-1
3rd order rate law k = dm6mol-2s-1
zero order rate law k = moldm-3s-1

Question 18

Integrated rate law?

Answer 18

By going from the rate law to the integrated rate law the time information is available which can allow us to determine how much of a species has reacted at a particular time

Question 19

First and second order decay proportionality dependence?

Answer 19

First order decays proportionally to exp(kt)

Second order decays proportionally to 1/t

Question 20

Pseudo first order reactions?

Answer 20

Take one of the reactants out of the equation by making its concentration much greater than the other one, therefore its changes in concentration are negligible called the isolation technique

Question 21

What is half life?

Answer 21

The time taken for the concentration of a reagent to fall to 1/2 of its initial value

Question 22

Methods to determine order of reaction?

Answer 22

Tangents to the concentration vs time curve
Initial rates
Integrated rate equations
Half lives

Question 23

Tangents to the concentration vs time curve ?

Answer 23

Plot rates against [A] if its a straight lie tenth results are 1st order, if there is a curve its not first order, test for second order by plotting rate vs [A]2 if linear the reaction is second order. Can be complicated and take a long time so use log log plots, equation is always a straight line, if gradient is 0 then 0th order reaction, if gradient is 1 then 1st order reaction etc

Question 24

What is a reaction theory?

Answer 24

Based on a model of how the reaction occurs at a molecular levels - these may be used to help design appropriate experiments to measure rate constants

Question 25

What is collision theory?

Answer 25

Simple model in which reactant modules are assumed to behave like hard spheres, collisions between spheres may result in reaction and a calculated collision frequency is used to estimate A

Question 26

What is transition state theory?

Answer 26

A more sophisticated theory in which one assumes that reactions form a transition state which is a short lived complex at the maximum on the energy profile, the A factor is calculated base on an assumed structure for the transition state

Question 27

What is molecularity?

Answer 27

Number of molecules in the transition state - applies only to elementary reactions

Question 28

Unimolecular?

Answer 28

Transition state involves one molecules of reactant

Question 29

Bimolecular?

Answer 29

Transition state involves two reactant molecules wither coming together to form a large molecule as the product or bond break and new bonds form and two new molecules are formed (products)

Question 30

What is a catalyst?

Answer 30

A catalyst increases the rate of a chemical reaction without being consumed in the reaction - catalysts provide an alternative pathway in which the rate determining step has a lower Gibbs energy of activation than uncatalysed reactions thus speeding up the reaction