Kinetics and equilibrium

Question 1

What is meant by the term activation energy

Answer 1

The minimum energy needed for a succesful reaction to occur or a succesful collision to occur

Question 2

Why at a fixed temp does the rate of a reaction double when the conc doubles

Answer 2

As there are now twice the number of particles in a given volume so twice the number of collisions/ higher collision frequency

Question 3

Why is it possible to use a simplified formula as a measure of the rate of reaction

Answer 3

As it is a fixed value there is a fixed endpoint

Question 4

Give one reason why the reaction between hydrogen and chlorine is very slow at rtp

Answer 4

As not many particles have enough energy to react (Ea)

Question 5

Explain why a small increase in temp can lead to a large increase in ror between h and cl

Answer 5

As MANY momre particles have an energy in excess of Ea so higher succesful collisions

Question 6

Explain effect of increasing conc on..

Answer 6

More FREQUENT succesful collisions

Question 7

Explain when equilibrium is reached

Answer 7

When the concentrations are CONSTANT not the same bc their different its when they are constant and level off

Question 8

Use le chateliers principle to state whether a …

Answer 8

High as fwd reac is so using a higher temp = favour the forward reac, producing a higher yield of hydrogen at high temps the equilibrium shifts to the right to oppose the increase in temp.

Question 9

Why is high operating pressure used in industry

Answer 9

As the reaction gets to equilibrium faster so more particles in a given vol so increased collision frequency

Question 10

Explain why the catalyst is more effective when coated onto an unreactive honeycomb

Answer 10

As there is a bigger surface area so more effective collisions

Question 11

Will there be any chance of yield if there are the same number of moles on both side with pressure

Answer 11

No make sure you check

Question 12

State the effect if any on the value of kc of adding more hydrogen to the equilibrium mixture

Answer 12

No effect- only temp changes kc not conc or whatever.

Question 13

Define enthalpy change

Answer 13

The heat or energy change at constant pressure

Question 14

Define hess’ law

Answer 14

The enthalpy change is independent of the route taken

Question 15

Define the standard enthalpy change of formation

Answer 15

The enthalpy change at constant pressure when 1 mole of a compound is formed from its constituent elements with all the reactants and products in their standard states.

Question 16

Why can a value of something not be zero

Answer 16

As it is not in its standard state

Question 17

Why is the standard enthalpy of formation of barium and aluminium both zero

Answer 17

As they are both elements

Question 18

Why can an experiment have a much smaller experimental error

Answer 18

Due to a lower heat loss

Question 19

Define the term standard enthalpy of combustion

Answer 19

The enthalpy change when 1 mol of a compound is completely burnt in oxygen under standard conditions

Question 20

Define rate of reaction

Answer 20

The of change of concentration of a substance in a given unit of time

Question 21

If they ask you what is happening to w if its a straight line

Answer 21

The rate is zero as all the… eg mg has reacted or been used up

Question 22

If your working out the moles using the q/n thing what do i need to remember

Answer 22

If you have the q firstly make sure convert into kj. And if they give you a neutralisation value eg -51kj make it positive and do the kj/51

Question 23

If they want you to find out the minimum conc of something mo/vo

Answer 23

So do the moles divided by v make sure you divide by 1000 in dm3

Question 24

Suggest one possible risk to a person who uses a hand warmer containing sodium hydroxide and hcl

Answer 24

The container can split releasing corrosive chemicals, can burn hands

Question 25

(Forward reaction is endothermic)
Use le chateliers principle to state whether a high or low temp should be used to obtain the highest possible equilibrium of yield on the right

Answer 25

Increase temp- the forward reac is endothermic so at high temps the equilbrium will shift to the right ABSORBING HEAT COOLING DOWN THE REACTION to oppose the increase in temp.

Question 26

The carboxylic acid is very expensive and ethanol is inexpensive. The mole ratio is 1:10- why

Answer 26

So as we increase the conc of ethanol the equilbrium shifts position to the right side to oppose this shift in conc increasing the yield of the product.

Question 27

State le chateliers principle

Answer 27

When a system is at equilibrium and a factor is changed the position of equilibrium will shift to oppose this change. HAVE TO SAY EQUILIBRIUM CANNOT USE SYSTEM ALONE

Question 28

Give one reason why the reaction between hydrogen and chlorine is very slow at rtp

Answer 28

As very few particles have sufficient energy to react. High ea

Question 29

Suggest one reason why a solid catalyst for a gas phase reaction is often in the form of a powder

Answer 29

Increases sa

Question 30

Suggest a reason why the catalyst must be hot

Answer 30

To overcome activation energy

Question 31

Suggest why at a constant temp most of g eventually decomposes

Answer 31

As molecules gain energy due to collisions

Question 32

What does the area under the curve represent

Answer 32

The total number of molecules in the sample.

Question 33

State why this curve starts at the origin

Answer 33

As no molecules have no energy

Question 34

Explain why the rate of reac decreases w time

Answer 34

As fewer collisions as w is used up

Question 35

Explain the process that causes some molecules in this sample to have very low energies

Answer 35

Collisions as some molecules slow down losing energy

Question 36

Explain why this reaction is described as a homogenous reaction

Answer 36

All the reagents are in the same phase.

Question 37

State the property of so2 that cause pollution when it enters rivers

Answer 37

Acidity or produces h+ ions

Question 38

why must a catalyst be hot and why does it remain hot

Answer 38

To overcome the activation energy and it stays hot as the reaction is exothermic and so releases heat

Question 39

How would you change iron in the hand warmer to make sure it stays warmer for longer

Answer 39

Increase the size of iron so decrease the surface area so less of the iron reacts, reacts slower

Question 40

Pure magnesium reacts completely with an excess of dilute sulfuric acid and is very rapid initially. This reaction slows down and stops before all of the calcium has reacted. Use your knowledge of the solubilities of group 2 sulfates to explain why these reactions of mg and calcium w dilute sulfuric acid are so different

Answer 40

As mag sulfate is soluble and calcium sulfate is insoluble the calcium is coated w sulfate which prevents further reaction w the acid

Question 41

Predict the effect of kc if the reaction is carried out at a lower temp

Answer 41

Increase

Question 42

At any time, most of the molecules of G have energy less than the activation energy.
Suggest why, at a constant temperature, most of G eventually decomposes.

Answer 42

Molecules gain energy (or always some molecules have E > Ea) 1
Due to collisions

Question 43

State the effect, if any, of adding a catalyst on the time required for G to decompose,
compared with a similar sample without a catalyst. Explain in general terms how the
catalyst has this effect.
Time for decomposition ………………………………………………………………………………..
Explanation ………

Answer 43

Decreases 1
Ea lowered (1)
By alternative route (1)
So more molecules have energy > Ea

Question 44

State why this curve starts at the origin

Answer 44

No molecules have no energy

OR all molecules have some energy

Question 45

The rate of a chemical reaction may be increased by an increase in reactant
concentration, by an increase in temperature and by the addition of a catalyst.
State which, if any, of these changes involves a different activation energy. Explain
your answer.
Change(s) ………………………………………………………………………………………..
Explanation …

Answer 45

Changes: Catalyst (1)
Explanation: Alternative route (1), with a lower activation energy (1)
OR a lower activation energy (1)
so more molecules can react (1)/more molecules have this energy

Question 46

suggest why a catalyst may become less efficient if the co2 and hydrogen contain impurities

Answer 46

as impurities block active sites