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Aqa a level chemistry > Topic 6: Equilibrium > Flashcards

Topic 6: Equilibrium Flashcards

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1
Q

True or false - for any given reaction , the proportions of reactants and products will be the same

A

true

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2
Q

dynamic

A

forwards and reverse reactions are continuing but at the same rate in the opposite direction

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3
Q

equilibrium

A

overall composition of the mixture undergoes no net change

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4
Q

true or false - during equilibrium the pressure of the system will remain constant

A

true

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5
Q

state two features/characteristics of a system which has reached dynamic equilibrium

A

-concentrations of products and reactants are constant
-rate of forward and backwards reaction are equal

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6
Q

If the reactants were pink and products were blue, how could you make the reaction more pink (the forward reaction is endothermic)

A

Decrease the temperature so the exothermic reaction is favoured causing equilibrium position to shift to the left making reactants yield increase

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7
Q

How does the equilbrium yield change of temp is increased at constant pressure or pressure increased at constant temp

A

temp increase - decrease yield (favour endothermic)
pressure increase - increase yield (favour least moles)

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8
Q

what is a closed system

A

no chemicals can get in or out

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9
Q

define dynamic equilibrium

A

-The rates of the forward and backwards reactions are the same, concentrations of the reactants and products must remain constant

-Reaction must take place in a closed system

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10
Q

can equilibrium be reached from either side of the reaction

A

yes

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11
Q

Equilibrium graph

A

-For the reactants over time, the forward reaction would decrease than plateaus

-For the products over time the backwards reaction would increase than plateaus

-The rates of forward and backward reactions are equal

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12
Q

Le Chatiler’s principle

A

-Use to determine the effect a change will have on a mixture at equilibrium

-Changes –> temperature, pressure, concentration

-This principle states that if a factor affecting the position of equilibrium is altered, the position of the equilibrium shifts to oppose the effect of the change

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13
Q

Increase temperature

A

-The forward reaction is exothermic so backwards reaction must be endothermic

-If we increase temperature the position of equilibrium would favor the endothermic reaction to oppose the increase in temperature. This means that the yield of reactants increase but yield of products decrease

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14
Q

Decrease temperature

A

-If we decrease temperature the exothermic reaction is favored and the system shifts to the right in order to oppose the change. The yield of products increases and reactants decreases

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15
Q

increase reactant concentration

A

If the concentration of one reactant is increased, equilibrium will shift to the right to reduce amount of reactant

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16
Q

decrease reactant concentration

A

-If the concentration of one reactant is reduced, the position will shift left to increase amount of reactant

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17
Q

pressure

A

-Pressure only affects reactions involving gases

-the reaction with a greater pressure will have more moles of gaseous reactants

-If pressure of the system is increased the position of equilibrium would shift right as it favors the side of the reaction with the fewest moles of gaseous molecules.

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18
Q

When bismuth chloride is added to water, a reaction occurs and a white precipitate forms. What would be the effect on the amount of precipitate if sodium hydroxide solution is added to the mixture

A

-NaOH reacts with HCl and removes it
-equilibrium shifts right to replace lost HCl
-more BiOCl white precipitate formed

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19
Q

to increase the rate of the reaction, a vanadium pentoxide catalyst is used. Explain what effect this has on the overall percentage yield o sulfur trioxide

A

-there is no effect on the percentage yield of sulfur trioxide
-forward and backwards reaction are increase by the same rate

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20
Q

A chemist recommends lowering the temperature to 800K to save energy. Explain two disadvanatges of making this change

A

-decrease in temp causes a decrease in rate of reaction due to fewer frequent and successful collisions (less energy than Ea)
-decrease in temperature causes a shift in equilibrium position so yield decreases

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21
Q

Explain the main disadvantage of making a slight increase in pressure

A

increase in pressure causes shift to left so yield decreases bc of fewer moles on reactant side

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22
Q

effect of catalysts on equilibrium yield

A

-When a catalyst is added to a reversible reaction, it will increase the rate of both the forward and reverse reactions equally

-catalysts have no change to the position of equilibrium and therefore do not affect equilibrium yield

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23
Q

reasons why catalysts are beneficial in industry

A

-cheaper (reduced cost of production)

-increased rate of reaction

-reusable

-temperatures and pressures don’t need to be raised so high in order to increase rate

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24
Q

Industrially, ethanol is manufactured by hydration of ethene, which is a product of cracking crude oil. The forward reaction is exothermic and is catalyzed by phosphoric acid. State the best reaction conditions for this process

A

-Low temp

-high pressure

-excess ethene

-remove product as formed

25
Q

temp, catalyst, pressure

A

-high temperatures increase rate of reaction but can reduce equilibrium yield

-catalysts increase rate of reaction but have no effect on equilibrium

-high pressure increases equilibrium yield but also costs more money

26
Q

what does the position of equilibrium indicate

A

The position of equilibrium indicates the ratio of products to reactants

If the graph shows an equilibrium position that lies to the right (products more than the reactants) then the concentration of products is greater than the concentration of reactants. This means there is a high equilibrium yield

If the graph shows an equilibrium yield that lies to the left (reactants greater than products) then the concentration of reactants is always higher than the products)

27
Q

when a reversible reaction is at equilibrium

A

When a reversible reaction is at equilibrium then the ratio of concentrations of products to reactants is constant

Kc = ratio of products to reactants = position of equilibrium

28
Q

what happens if Kc is greater than 1

A

If Kc is greater than 1, then the concentration of products is greater than the concentration of reactants. This means that the equilibrium lies to the right.

29
Q

units for Kc

A

Units for concentration = mol dm-3

-To find the units for equilibrium constant expand and then cancel e.g

30
Q

heterogenous Kc

A

-Kc can only be calculated for gases and aqueous substances and all liquids except H2O
-no water (l) and no solid

-For a liquid in a sealed onctainer = [H2O (g)] / [H2O (l)]

-[H2O (l)] –> constant depending on density and mr

31
Q

State whether the forward reaction in this equillibrium results in an increase, decrease or no change in the amount of moles of gas

A

increase –> lower yield at higher pressure so shifts left favouring backwards reaction

32
Q

At 745K 0.150dm3 of an equilibrium mixture contains 0.0285mol of hydrogen and 0.0870 mol of nitrogen. Calculate the amount of moles of ammonia present

A

1) Write equation and substitute values
2) rearrange equation to make ammonia the subject
2) Calculate (mol x vol)
3) square root answer to get units of mol/dm3
4) multiply answer by concentration to get moles

33
Q

What is the equation for equilibrium constant

A

Kc = [c] [d] / [a] [b]

34
Q

constant concentrations

A

solids (s) and water (l) have constant concentrations so cannot be used and are ignored when calculating equilibrium constant

35
Q

what is the effect of increasing pressure on the value of Kc

A

no effect on Kc

36
Q

deduce in terms of X the amount in moles of R and calculate its concentration

A

mol R = 2x

3.6 = (2x)2 / (1-X)2

37
Q

what factors affect Kc

A

temperature e.g if equilibrium shifts right, temp will increase

concentration and catalyst have no effect on value of Kc

38
Q

Explain in terms of equilibrium yield and cost why a temperature of 800 degrees is used, pressure of 300pKa and a nickel catalyst

A

-reaction is endothermic so high temperature cause equilibrium position to shift right and favor forward reaction thus increasing equilibrium yield. However high temp is more costly
-More moles of gas on RHS so equilibrium shifts right to increase yield due to low pressure. Lower pressure increases yield and has a lower cost
-catalyst has not effect on yield but lowers cost

39
Q

Describe the stages of this alternative route

A

-reactant absorbed onto platinum surface
-bond making occurs on the surface
-desorption of the product

40
Q

the pool manager maintains the water at a pH slightly greater than 7.0 (alkali)
Explain how this affects the equillibrium established when chlorine is added to water

A

-alkali ions react with acid
-equilibrium position shifts right

41
Q

suggest a reason why the catalyst must be hot

A

to overcome the activiation energy

42
Q

suggest a reason why the catalyst remains hot during the reaction

A

the reaction is exothermic

43
Q

dynamic

A

forward and backward reactions occur simultaneously

44
Q

Kc

A

[c] [d] / [a] [b]

45
Q

calculating moles at equillibrium

A

initial moles - moles reacted –> reactants

initial moles + moles formed –> products

46
Q

effect of changing conditions of value of Kc

A

-larger Kc = greater number of products (favour products)

-Kc only changes with temperature

-pressure, concentration and catalyst has no effect on Kc value

e.g if temperature increases and forward reaction is exothermic then Kc would be smaller as reactants favoured

47
Q

when one mole of ammonia is heated to a give temperature, 50% dissociates and the following equilibrium is etablished, what is tht eotal amont of gases in moles

A

1.5 mol

48
Q

which change leads to a higher concentration og SO3 in this equilibrium mixture

A

higher concentration of O2

49
Q

define Kc

A

-shows the relationship between reactants and products of a reaction at dynamic equillibrium
-calculated in homogenous solutions (aq and g)

-larger Kc = favours products

50
Q

1 litre =

A

1 dm 3

51
Q

true or false when calculating units you square if the power is 2

A

false = multiply by 2

52
Q

how do you calculate units of Kc

A

input moldm-3 into the Kc expression you have calculated e.g [moldm-3]^3

53
Q

Kc =

A

products / reactants

54
Q

Kc definition

A

Kc = at equilibrium the ratio of the concentrations of products to reactants is constant. Kc is the ratio of products to reactants.

-Kc is always the same for a particular reaction for a given temp, pressure, concentration

55
Q

why are H2O (l) and solids not included

A

-H2O (l) and solids are removed from equilibrium constant is bc their concentrations are constant

56
Q

Kc = greater than 1

A

-If the concentration of products is higher than the concentration of reactants, Kc is greater than 1. This means the position of equilibrium lies to the right

57
Q

steps for ICE table

A

1) draw ICE table and fill in from question

2) deduce concentrations at equilibrium (mol / volume)

3) write out Kc expression (always check if it is homogenous or heterogeous). If homogenous then liquid water can still be included in equation

4) work out units and subsitute

58
Q

True or false -> you must include mol to mol ratio when calculating equilibrium moles

A

true

Make sure to include mol to mol ratio e.g if 2 moles of NO2 =2.5 then one mol of O2 is 1.25

59
Q

state and explain the effect of a decrease in overall pressure on yield of products

A

-yield decreases
-equilibrium shifts the left side (more moles) to increase pressure

Aqa a level chemistry (18 decks)

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