3.1.5 Kinetics

Question 1

What is the Rate Equation?

Answer 1

Rate of Reaction = Amount of Reactant Used (or Product Formed) / Time

Question 2

What is Collision Theory?

Answer 2

1) Particles in liquids and gases are always moving and colliding with each other
2) They don’t react every time they collide though, only when conditions are right

Question 3

What are the 2 conditions for a reaction to occur according to Collision Theory?

Answer 3

1) They collide in the right direction, they need to be facing each other in the right way
2) They collide with at least a certain minimum amount of Kinetic Energy

Question 4

Why don’t molecules in a gas have the same amount of energy?

Answer 4

Some don’t have as much kinetic energy and move more slowly.
Others have more kinetic energy and move more quickly/violently.
Most molecules are somewhere in between.
If you plot a graph of the number of molecules of gas with different Kinetic energies, you get a MAXWELL-BOLTZMANN DISTRIBUTION

Question 5

What does the peak of a Maxwell-Boltzmann represent?

Answer 5

Most likely energy of any single molecule

Question 6

What effect does increasing the temperature have on a Maxwell-Boltzmann graph?

Answer 6

Increased temperature = more molecules with a higher kinetic energy. Peak of graph shifts to right and is lower than original. Greater proportion of molecules that pass Activation Energy

Question 7

Why does increasing the temperature make reactions occur faster?

Answer 7

1) Particles will have more kinetic energy
2) Greater proportion of molecules will have the minimum energy for reaction to occur
3) At higher temps, as molecules moving faster, they collide more often
4) As both effects occur @ once (more collisions & more energetic collisions), small increases in temp can lead to large increases in Rate of Reaction (RoR)

Question 8

Why does increasing the concentration increase the rate of reaction?

Answer 8

1) Particles closer together

2) If they’re closer, they collide more often.

Question 9

Why does increasing the Pressure also increase the rate of reaction?

Answer 9

1) Works in the same way as increasing concentration

2) Increased pressure pushes gas particles closer together, therefore more likely to collide.

Question 10

What is a catalyst?

Answer 10

A substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. Catalyst is chemically unchanged at the end of the reaction

Question 11

How can timing how long it takes for a ppt to form measure reaction rates?

Answer 11

1) Use this method when it involves a ppt which clouds a solution
2) Watch a mark through the solution and time how long it takes to be obscured
3) If same observer uses same mark each time, you can compare rates of reaction, roughly same amount of ppt will have been formed when mark becomes obscured
4) HOWEVER, method is subjective. Different people may not agree on exact moment the mark disappears

Question 12

How can measuring a change (decrease) in mass measure reaction rates?

Answer 12

1) When one or more products is a gas, you can measure Rate of Formation using mass balance
2) As gas is given off, mass decreases
3) Method is accurate and easy to do. Does however release gas into the atmosphere, so usually done in fume cupboard

Question 13

How can measuring volume of gas produced measure reaction rates?

Answer 13

Involves using gas syringe to measure volume of gas being produced

1) Can only use this method when 1 or more products is a gas
2) Gas syringes usually give volumes to the nearest 0.1cm3, so this method is accurate

Question 14

What happens at Dynamic Equilibrium?

Answer 14

Reversible reaction balances out

Forward reaction occurring at same rate as Reverse/Backward Reaction so amount of reactants and products aren’t changing anymore

Question 15

What is an example of a Reversible Reaction?

Answer 15

H2(g) + I2(g) ⟶ 2HI(g)

Hydrogen + Iodine ⟶ Hydrogen Iodide

Question 16

In what type of a system can dynamic equilibrium only occur in?

Answer 16

CLOSED System

Question 17

What is Le Chatelier’s Principle?

Answer 17

If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change

Question 18

What is Kc?

Answer 18

The Equilibrium Constant
aA + bB ⇌ dD + eE
Kc = ((D)d x (E)e) / ((A)a x (B)b)

Question 19

Why does Temperature change Kc?

Answer 19

1) Value of Kc only valid for one particular temperature
2) If you change temp of system, you will also change equi concs of products and reactants, so Kc will change
3) If temp change = more product @ equi, Kc rises. If temp change = less product @ equi, Kc decreases
4) Changing conc of reac or product will not affect value of Kc
5) Catalysts don’t affect value of Kc either. They speed up reaction in both directions by the same amount

Question 20

If electrons are transferred, what type of reaction is it?

Answer 20

Redox (Reduction and Oxidation)
OILRIG
Oxi=Loss, Reduc=Gain
Reduction and Oxidation occur simultaneously, hence Redox
An Oxidising Agent Accepts Electrons and gets reduced
A Reducing Agent donates electrons and gets oxidised

Question 21

What 5 factors affect rate of chemical reactions?

Answer 21

1) Increasing the Temperature - increases speed of molecules, which increases energy & # of collisions
2) Increasing Conc of Solution - If more particles present in given volume, collisions more likely and RoR faster. As reaction proceeds, reacs get used up and RoR drops
3) Increasing Pressure of Gas Reaction - Same effect as increasing conc of solution. More molecules/atoms in given volume so collisions more likely
4) Increasing SA of solid reacs - Greater TOTAL SA, the more of its particles available to collide with molecules in a gas or a liquid. Breaking solid lump into smaller pieces increases RoR as more sites for reaction
5) Using a Catalyst

Question 22

What are the 4 conditions that apply to all Equilibria?

Answer 22

1) Can only be reached in a CLOSED System
2) Can be approached from either direction
3) Is a Dynamic Process - reached when rate of two opposing processes are the same
4) You know that equi has been reached when macroscopic properties of system do not change with time. E.g. density, conc, colour and pressure - properties that don’t depend on total quantity of matter