Chap 10 reaction rates and equilibrium

Question 1

Define rate of reaction using an equation

Answer 1

change in concentration / time

Question 2

Why is the rate of reaction always fastest at the start

Answer 2

the concentration of all reactants are at their maximum values for the reaction

Question 3

What are two factors required for a collision between two particles to be successdul in causing a reaction

Answer 3

• particle must collide with an energy higher than the activation energy
• rewactive areas of particles must colide with each other

Question 4

Why does increasing the concentr\tion of an aquous reactant or decreasing the volume of a gaseous reactant generally increase the rate of the reaction

Answer 4

• there are a greater number of particles in the same volumer of the same number of particles in a smaller volume
• in bothe cases there will be a greater number of collissions
• if more collisions are occuring there will be a greater number of collisions that are successful
• greater number of successful collisions per unit tie redsults in a faster rate of reaction

Question 5

Describe a method to monitor the raate of reaction using collection of gases

Answer 5

• rate of reaction can be monitored for reactions producing gases by measuring the amount of gas collected every set period of time until no more gas is collected and reaction has stopped

Question 6

How can the results of collection of gas be used to calculate the reaction rate

Answer 6

plotting a graph of volume of gas produced against time and finding the gradient of a tangent to the curve

Question 7

Describe a different method of calculating rate of reaction

Answer 7

• measuring the chnge in mass of reactants
• a reaction vesel can be places on a balance and the change of mass recorded with time
• a graph of decrease in mass against time can be plotted to calculate rte drom the gradient of tangent to the curve

Question 8

What is a catalyst

Answer 8

a catalyst is a substance that can alter the rate of a reaction without itself being used up during the reaction

Question 9

How does a catalyst affect rate of reaction

Answer 9

• catalysts provide an alternative route of reaction with a lower activation energy
• a lower activation energy will mean more collisions occur with enough energy to react thus increasing rate of reaction

Question 10

What is a homogenous catalyst

Answer 10

a catlayst that is in the same physical state as the reactants

Question 11

What is a heterogenous catalyst

Answer 11

A catalyst that is in a different physical state to the reactants

Question 12

How does a solid catalyt work in a reaction where the reactants are solutions

Answer 12

molecules of the reactants in solution absorb (from weak bonds) to the surface of the catalyst

Question 13

What is the use of catalysts considered to be good for the environment

Answer 13

• using a catlayst lowers the energy requireed for a reaction to take place
• reduces the need for fossil fuels to provide energy for the process
• less dependednce on fossil fuels reduces the associated emissions of CO2 which contributes to climate change

Question 14

What is shown on a Boltzman distribution

Answer 14

the distribution of energy of molecule or particles (in a gas)

Question 15

What can be determined by the area under the curve

Answer 15

the total number of molecules present

Question 16

Why does the curve of the boltzman distribution never touch the x axis

Answer 16

molecules do no have a maximum energy

Question 17

Using the boltzman distribution graph below to show how the distribution of energy in particles would be changes if the temp was increased

Answer 17

• The higher the temperature the line would be drawn to the right of the original line with a lower peak
• the position of activation energy for each temperature is the same
• the higher thetemprature greater area under curve to right of activation energy shwoing more particle have the enrgy required to react
• less particles have the activation energy at lower temprature as the areas under the curve is smaller

Question 18

How would a fully labelled distribution graph be altered by the use of a catalyst

Answer 18

• The graph would stay the same shape only position of the activation energy would move to the left
• shows increase number of particles with required energy to react as a catalyst lowers the activation energy of a reaction

Question 19

What is a reversible reaction

Answer 19

a reaction where products formed from reactants in the forward reaction but a reverse reaction also take place where the products can break down to form the reactants

Question 20

What has happened to a reversible reaction when it reached equilibrium

Answer 20

• The rate o the forward reaction is the same a the rate of the reverse reaction
• both reactions are occuring at the same time but there is no net chnge in concentration of the reactants of products

Question 21

What determines the extent of the reaction

Answer 21

Position of equilibrium

Question 22

What does le chatliers principle state

Answer 22

if a change is imposed on a system at equilibrium the position of equilibrium will shift to minimise the effect of the change

Question 23

If the concentration of a reactant is increased for a sytem at equilibrium what effect will this have on the position of equilibrium

Answer 23

the position of equilibrium will shift to the right to minimise the change a new position of equilibrium will be established

Question 24

In a reaction the forward reaction is exothermic the position of equilibrium can be controlled by the temperature of the reaction
Hwat effect will increasing hte temperature of the reaction have on the position of equilibrium

Answer 24

The position of equilibrium will move to the left hand side to minimise the change in tempratre causing heat to be absorbed and more reactants to be formed in the reactants

Question 25

An equilibrium was established for a reaction where all reactants and products are gases if the pressure of the reaction is increased what happens to the position of equilibrium

Answer 25

The position of equilibrium will shift to the side with the fewest gaseous moles to minimise change in pressure

Question 26

Is the position of equilibrium changed by the use of the catalyst

Answer 26

A catalyst does not have an effect on the position of equilbrium a catalyst will increase the rate of forward and reverse reactions eqully but may allow the reaction to reach equilibrium faster

Question 27

In some reversible reactions a change in concentration and therfore a shift in equilibrium may be accompanied by a colour change What must you add to yellow potassium chromate to cause a colour change What colour will the solution become

Answer 27

Addition of acid will cause the colour of the potassium chromate to change from yellow to orange as the chromate react to become dichromate Cr2O72-

Question 28

How is the equilibrium constant for a reaction determined

Answer 28

The equilibrium constant for a reaction is determined from the concentrationf of all reactants and products when the reaction has reached equilibrium

Question 29

If the answer if less than one for Kc will the position of equilibrium lie to the left or right hand side

Answer 29

the position of equilbrium will lie to the left hand side because the value of Kc is less than one

Question 30

Why do some industrial processes often require a compromise between reaction rate and the equilibrium of the reaction

Answer 30

some reactions may favour conditions that would give a very slow rate of reaction which would not be economically viable this means the conditions fo not favour product formation may be required some favourable conditions may be too exensive or too hazardous to implement

Question 31

The graph shows that the rate of reaction decreases over time Explain why in terms of collision theory n (2)

Answer 31

* concentration decreasses * fewer collisions per second

Question 32

Explain how a catalyst increases the rate of reaction (2)

Answer 32

* catalyst lowers the activation energy by providing an alternative reaction pathway * a reater proportion of molecules have more energy greater thhan or equal to activaation energy

Question 33

Using the boltzman distirbution model explain how the rate of reaction is affected by temperature (1)

Answer 33

more molecules have a energy greater than activation energy

Question 34

Explain why a catlyst reduces energy demand and benefits the environment (2)

Answer 34

* lower temperatures less heat energy * les fofssil fuels * reduces CO2 emmissions

Question 35

State two ways that the use of catalysts help chemical companies to majke their processes more sustainable and less ahrmful to the environment (2)

Answer 35

* lower temperature/ lower pressures * lower energy demand / uses le fuel / reduces CO2 emmissions * greater atom aconomy / can reduce use of toxic solvents/ rreduce use of toxic reactants/ less waster * catalyts are often enzymes generating specific products

Question 36

Explain how le chatliers principle can be used to predict the conditions of tempreature and pressire for a maximum equilibrium yield of ammonia (4)

Answer 36

* pressure: * right hand side has fewer gaseous moles * higher pressure * temperature: * forwards reaction is exothermic * reaction gives out heat * low temperature

Question 37

State le chatliers principle (1)

Answer 37

The position of dynamic equilibrium shifts to minimise the effect of any change

Question 38

Explain why the actual conditions sused by the chemical industry for high pressures and low temperatures for maximum equilibrium might be different (2)

Answer 38

* low temperatures gives a slow rate * higher pressure is expensive

Question 39

State one other feature of dynamic equilibrium except it is in a closed system (1)

Answer 39

* rate of forward reaction is equal to the rate of reverse reaction * concentration do not change

Question 40

Describe and explain the effect of increasing the concentration of reactant upon the rate of reaction (2)

Answer 40

*increasing the concentration of reactants means that there are more particles available to react in a given volume therefore the frequency of successful collisions between particles with sufficient energy to react increases * thus increasing the rate of reaction

Question 41

Describe and explain the effect of increasing the pressure of a gaseous reaction (3)

Answer 41

* increasing the pressure of the reacting gases effectively increases the concentration of the gases in a given volume * increasing the pressure means that the number of particles likely to collide with sufficient energy to react increases increasing the rate of reaction

Question 42

Define and explain the term catalyst (3)

Answer 42

*Catalysts speed up the rate of a chemical reaction without being used up by the reaction *They do this by offering an alternate reaction pathway *with lower activation energy

Question 43

Explain why using catalysts can lead to a reduction in pollution (2)

Answer 43

* Using a catlalyt generally lowers the temperature needed to make the reaction occur hence reducing the energy requirement for the reaction * this reduction of energy in turn leads to a reduction in the burning of fossil fuels which release carbon dioxide when they are burnt

Question 44

Explain what is meant by a heterogenous catalyst (1)

Answer 44

A catalyst that is in a different physical state to the reactants

Question 45

Explain What is meant by a homogenous catalyst (1)

Answer 45

A catalyst that is in the same physical state as the reactants

Question 46

Using an example explain why catalysts have a great deal of economic importance (3)

Answer 46

* iron is used as a catalyst in the production of ammonia in the Haber process * this reduces the temperature needed in order to make the hydrogen and nitrogen react and so speed up the reaction This reduces the time taken to reach equilibrium and hence reducing production costs

Question 47

Explain how catalysts increase the rate of reaction (3)

Answer 47

* reducing the activation energy needed to initiate the reaction *allowing reaction to take a different reaction pathway which has a lower activation energy *so increases the rate of reaction

Question 48

What would the effect be on the Boltzmann distribution curve when increasing the temperature of a reaction (3)

Answer 48

* Increasing the temperature would increase the average kinetic energy of the particles and have the effect of pushing the curve down and to the right *having the effect of increasing the number of molecules with more than the minimum energy needed for successful collisions *therefore increase the overall rate of reaction

Question 49

What would the effect upon the graph be with the addition of a catalyst - Boltzmann distribution curve (3)

Answer 49

* adding a catalyst reduces the activation energy and dotted line shifts to the left this has the effect of increasing the number of particles in the section of the graph represents the molecules with energy equal to or greater than the lowered activation energy * leads to more successful collisions and hence increase in overall rate of reaction

Question 50

Define the term dynamic equilibrium (2)

Answer 50

* The point in a reaction during which the rate of the forward reaction is the same as the rate as the backwards reaction *concentration of reactants and products stay constant for a given set of conditions and hence equilibrium is achieved

Question 51

Outline le Chatelier's principle (2)

Answer 51

*If a change is made to a system at equilibrium the position of equilibrium *will move so as to minimise the effect of the change

Question 52

What would be the effect of adding a catalyst to a reversible reaction (2)

Answer 52

* Equilibrium may be reached faster however the position of equilibrium is not changed

Question 53

Explain why the temperature above 350 degrees is not used in practice in the Haber process and state the actual temperature used and why (3)

Answer 53

* 350 degrees fewer particles have sufficient energy to react take a long time to produce ammonia * at 450 degrees the rate increases but the yield is reduced as the forward reaction is exothermic

Question 54

Explain why 400 atm is not used in practice in the haber process and state the actual pressure used and why (3)

Answer 54

* A high pressure of 400 atm would give the greatest yield of ammonia however it is extremely expensive * A lower temperature of 200 atm is used *reduces yield obtained but reduces cost

Question 55

Describe how the effect of changing temperature on the position of equilibrium can be monitored with a reaction involving cobalt chloride (3)

Answer 55

*Solution of Cobalt Chloride dissolves in water and forms a pink solution *if solution is heater position of equilibrium shift towards the endothermic direction and becomes blue *if temperature is decreases equilibrium moves in the exothermic direction and returns to pink

Question 55

If a reaction has a large value of Kc estimate the position of equilibrium for the reaction Give a reason for your answer (2)

Answer 55

Larger value of Kc the position of equilibrium will lie toward the products of the reaction