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Flashcards in Equilibrium Deck (39):
1

What is meant by chemical equilibrium?

State in which the rate of forward reaction is equal to rate of reverse reaction

2

Why is chemical equilibrium described as a dynamic state?

Both reactions are continuously occurring, both continue

3

State Le Châtelier’s principle

If “stress” is applied to system at equilibrium, system reacts to relieve stress applied

4

Write the equilibrium constant expression of
N2O4 ->2NO2

[NO2]^2
———
[N2O4]

5

The colour of equilibrium mixture is paler at 0 degrees celsius than at T, where T > 0 degrees celcius
Explaining your reason, deduce whether the decomposition of dinitrogen tetroxide into nidtogen dioxide is an exothermic or an endothermic reaction
N204 -> 2NO2
Colourless Dark Brown

Endothermic
At lower temperature, rxns try to minimise effect of reduced temp so equilibrium shifts in direction that produces heat, which is exothermic which is a forward reverse reaction, [more N2O4 is produced]
At higher temperature, rxns try to minimise effect of higher temperature so equilibrium shifts in direction that absorbs heat. Since forward reaction is exothermic, more NO2 is made

6

Would there be a change in the value of Kc at T If a different initial concentration is used?

No, KC constant is at given temperature
It will only change if temperature is changed

7

Use le Châtelier’s principle to predict and explain the conditions of temperature and pressure that would maximise the yield of C
A + B -> C

Low temperature -
Favours forward reaction which is exothermic, more of C is produced
High pressure -
Favours fewer molecules in forward reaction, more of C produced

8

Why is a low temperature not used industrially

Reaction rate is too slow, uneconomical at low temperature

9

State and explain the effect of the presence of the catalyst on the equilibrium yield of the product

None, as it alters the rate of forward reaction and reverse reactions equally
[causes rxns to reach equilibrium faster but does not change position of equilibrium]

10

The graph shows the relationship between temperature and kc for this equilibrium
A -> B + C
Deduce whether the dissociation or gas A is exothermic or endothermic

Endothermic
Kc increases with temperature and at high temperature reaction shifts to right to oppose stress and therefore is endothermic

11

Explain how an increase in temperature would affect the pressure of equilibrium mixture
A -> B + D

Equation will shift to right [forward reaction] which will decrease pressure of equilibrium [alter, and go to side with fewer molecules]

12

What effect would an increase in temperature and pressure have on the value of kc for this reaction
2NO -> N2 + O2 <> H = -183 KJ

Temp - If reaction is exothermic, increasing temp will reduce kc value as reaction shifts in backward direction to oppose added heat
Pressure - no effect, only temp change affects kc constant

13

When you increase the temperature what does it favour?

Endothermic

14

When you decrease the temperature what does it favour

Exothermic

15

How to determine if a reaction is exothermic or endothermic

If temperature decreases to get products of forward reaction, the forward reaction is exothermic
If temperature increases to get products of forward reaction, the forward reaction is endothermic
Talk about products increasing!!

16

Identify one industrial problem associated with the use of high pressures

Danger of leaks, danger of explosions, high costs

17

H2 + I2 -> 2HI
Colourless purple colourless
When the colour of the mixture has become pale pink, has reaction ceased? [stopped] explain

No, chemical equilibrium is at dynamic state [continuously occurring]

18

H2 + I2 -> 2HI
Colourless purple colourless
Use le châtelier’s principle to predict and explain the effect of a decrease in temperature on the yield of hydrogen iodide

Decrease in yield
Reaction tries to minimise effect of reduced temperature and equilibrium shifts in reverse

19

H2 + I2 -> 2HI
Colourless purple colourless
Colourless purple colourless
Use le châtelier’s principle to predict and explain the effect of a decrease in temperature on the intensity of colour of the equilibrium mixture

More intense, darker
Reaction tries to minimise effect of reduced temperature and equilibrium shifts in reverse

20

H2 + I2 -> 2HI
Colourless purple colourless
What change, if any, will an increase in the pressure on the equilibrium mixture have on the yield of hydrogen iodide? Explain

No change, equal number of molecules on both sides

21

What effect does an increase in pressure have?

[opposite - side with fewer molecules]
System alters to oppose stress to side with fewer molecules

22

What effect does a decrease in pressure have on an equilibrium reaction?

[think of opposite - side with more molecules]
The system alters to oppose stress, and moves to side with more molecules

23

Have to explain the temperature -

- State whether it’s exothermic or endothermic
- Oppose stress

24

A + B - C + D
What occurs when concentration of A increases?

System alters to minimise effect and equilibrium shifts to right and more of C and D produced

25

A + B - C + D
What occurs when concentration of C increases?

System alters to minimise effect, equilibrium shifts to left therefore more of A and B produced

26

What happens if we increase the volume of the container?

Causes decrease in pressure

27

What happens if we decrease the volume of the container?

Increases the presssure

28

PCl3 + Cl2 - PCl5
What would the effect on concentration of PCl5 in equilibrium mixture when you introduce more PCl3 into mixture

If more PCl3 is introduced into the container, the equilibrium will shift to the right to absorb increase. Therefore the concentration of PCl5 will increase

29

PCl3 + Cl2 - PCl5
What would the effect on concentration of PCl5 in equilibrium mixture when pressure is increased?

The equilibrium shifts to the aids with smaller amount of molecules to oppose stress, therefore is a forward reaction and PCl5 concentration increases

30

PCl3 + Cl2 - PCl5
What would the effect on concentration of PCl5 in equilibrium mixture when temperature increases and the forward reaction is exothermic

We are told the forward reaction is exothermic which implies that the reverse reaction is endothermic. If the temperature is increased, the equilibrium will be shifted in the reverse direction in order to absorb this heat. Therefore, concentration of PCl5 will decrease

31

Give an example of an industrial application of le chatelier’s principle

The Haber Process - manufacture of ammonia

32

When the colour of the mixture becomes pink has the reaction ceased?

No, dynamic balance - continuously occurring

33

What effect would a decrease in temperature have on the colour intensity of the equilibrium mixture
Colourless + purple -> colourless
Where forward reaction endothermic

More intense colour

34

Describe what is observed when a small amount of sodium carbonate is added to a sample of ethanoic acid. Write a balanced equation for this reaction

Fizzing as CO2 is produced
2CH3COOH + Na2CO3 -> 2CH3COONa + CO2 + H20

35

What occurs when the concentration is increased

Go opposite direction to use up excess of what is being increased

36

What occurs when the concentration is decreased?

The opposite side increased to make up for what is being removed

37

The addition of a catalyst which is specific to this reaction did not affect the position of the equilibrium. Why?

Catalyst alters rate of forward reaction and reverse reaction to same extent

38

How can the colour change caused by the addition of HCL to cobalt chloride be reversed?

Add water

39

I2 + I- -> i3-
State and explain the effect on the equilibrium concentration of triiodide ions of adding a substance that reacts with iodine eg, starch

concentration of triiodide ions decreases
reaction shifts to the left to combat stress