Equilibrium

Question 1

What are reversible reactions?

Answer 1

Reactions that can go either way, in the forward direction or the backwards direction.

Question 2

When has a reaction reached equilibrium?

Answer 2

A system has reached equilibrium when when no further change appears to occur - all concentrations remain constant.

Question 3

What is dynamic equilibrium?

Answer 3

In dynamic equilibrium, macroscopic properties are constant (concentrations of all reactants and products remains constant) and the rate of the forward reaction is equal to the rate of the reverse reaction.

Question 4

What are the characteristics of the equilibrium state?

Answer 4

• Macroscopic properties are constant at equilibrium, at equilibrium the concentrations of all reactants and products remain constant.
• At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.
• Equilibrium can be attained only in a closed system - if it is in an open container than carbon dioxide escapes and cannot react back for example.
• All species in the chemical equation are present in the equilibrium mixture.
• Equilibrium can be attained from either direction.

Question 5

What is the difference between chemical equilibria and physical equilibria?

Answer 5

Chemical equilibria is established as a result of chemical reactions. Physical equilibria involves a change of state, for example going between a vapour and a liquid. Physical equilibria can also be referred to as a phase equilibrium.

Question 6

What does it mean when it says ‘equilibrium is a long way to the right’?

Answer 6

If the equation is A + B = C

Then it means it is making a lot more C.

Question 7

What is Le Chatelier’s principle?

Answer 7

If a system at equilibrium is subjected to a change, the position of equilibrium will shift in order to minimise the effect of the change.

Question 8

What is the effect of temperature on equilibrium?

Answer 8

INCREASE IN TEMPERATURE - It will shift to make the reaction mixture colder, and therefore will shift to the endothermic direction, to take in heat added.

Question 9

What is the effect of pressure on equilibrium?

Answer 9

INCREASE IN PRESSURE - If there is an increase in pressure equilibrium will shift in the direction of less molecules. This is mainly only for gases, solids and liquids change very little.

Question 10

What is the effect of concentration on equilibrium?

Answer 10

In general, if the concentration of one species in an equilibrium mixture is increased, the position of equilibrium shifts to reduce the concentration of this species.

Question 11

What is the equilibrium constant?

Answer 11

The equilibrium constant is the ratio of found from putting products over reactants. This calculates the equilibrium constant.
For this reaction this is how you calculate it:

2A + B = 3C

(C)^3 / (A)^2 X (B)^1
You put the number of molecules in the power.
This makes Kc, which is the equilibrium constant.

Question 12

What does Kc do?

Answer 12

An equilibrium constant provides information about how far a reaction proceeds at a particular temperature.
If Kc > 1 then the reaction makes more products than reactants and proceeds almost totally towards the products.
If Kc < 1 then the reaction hardly proceeds at all towards the products.
ONLY VALID AT EQUILIBRIUM

Question 13

What is the difference between Q and Kc?

Answer 13

Q is the reaction quotient, and is the same equation but Kc is the reaction quotient at equilibrium. So when the system is at equilibrium, the concentrations are all equilibrium concentrations and the value of Q is equal to the value of Kc at that temperature.
If the value of Q is not equal to the value of Kc then we know instantly that the system is not at equilibrium.

Question 14

What does it mean if Q is smaller than Kc?

Answer 14

It means that first of all the reaction is not at equilibrium and secondly that the reaction is proceeding to the right to make more products, because products are on top of the fraction so to proceed to equilibrium we need to increase Q as Kc is bigger.

Question 15

What does it mean if Q is bigger than Kc?

Answer 15

Well first of all it means that the reaction is not at equilibrium and that it is making more products than reactants at the moment as it is big so the number on top of the fraction must be big and the reaction must move to the left in order to reach equilibrium.

Question 16

What do you have to be careful of with Kc?

Answer 16

Because the reaction is at equilibrium this means that the concentrations are the same and neither is products or reactants kinda. Basically because it is a reversible reaction and the equation can be written either way and therefore the fraction be either way round there can be two values of Kc so it is only useful to know if you know which way round the equation was written in the first place.

Question 17

What effects the equilibrium constant?

Answer 17

The equilibrium constant is only effected by a change in temperature. This is because if you increase pressure or concentration then you will be shifting the value and Q will not longer be equal to Kc because it is not at equilibrium. The reaction will change things in order to bring it back to Kc.

Question 18

What is the effect on Kc of increasing the temperature of an exothermic reaction?

Answer 18

The reaction is exothermic so increasing the temperature of the reaction will cause the reaction to favour the backwards direction making more reactants. This will mean that there is a larger concentration of reactants than there is of products which will increase the bottom of the fraction and therefore DECREASE THE VALUE OF KC. FOR AN EXOTHERMIC REACTION, THE VALUE OF THE EQUILIBRIUM CONSTANT DECREASES AS THE TEMPERATURE IS INCREASED.

Question 19

Why does temperature change the value of Kc but pressure does not?

Answer 19

HAVE EMAILED MR MCDAID V CONFUSED

Question 20

What is the effect on Kc of increasing the temperature of an endothermic reaction?

Answer 20

This will increase the concentration of products, therefore increase the number on the top of the fraction and INCREASE KC.

Question 21

What is the effect of catalysts on Kc?

Answer 21

A catalyst increases the rate of forward and reverse reactions equally. So the presence of a catalyst does not effect the position of equilibrium or the value of the equilibrium constant; it only reduces the time taken to reach equilibrium.

Question 22

What does the equilibrium constant (Kc) tell us?

Answer 22

The equilibrium constant gives us information about how far a reaction goes towards completion (that is, the extent of the reaction). It gives us absolutely no information about how quickly the reaction occurs. Kinetic data, such as the rate constant, indicates how quickly equilibrium is attained but provide no information whatsoever about the position of equilibrium and how far the reaction proceeds.

Question 23

How do you find the value of Kc when you have not ben given all of the concentrations?

Answer 23

5. 00 mol H2 and 3.00 mol I2 are mixed together in a vessel of volume 10.0dm3 and allowed to come to equilibrium at 1100k. At equilibrium there were 0.43 mol I2 present in the reaction mixture. Calculate the value of the equilibrium constant.
   1) First draw a table, with three rows: moles at start, moles used, and moles at equilibrium. You can work out from the question these things.
6. 00 3.00
   - -
   - 0.43

So if 0.43 are left then 2.57 must have been used up.

5. 00 3.00
   - 2.57
   - 0.43

Because hydrogen and iodine react in a 1 to 1 ratio, then 2.57 moles of hydrogen must have reacted too.

5. 00 3.00
6. 57 2.57
   - 0.43

So 5 - 2.57 = 2.43

5. 00 3.00
6. 57 2.57
7. 43 0.43

Then you need to work out the products, so if 2.57 moles of both have reacted then there must be 2x2.57 moles of the products.

5. 00 3.00
6. 57 2.57
7. 43 0.43 5.14

These are all in moles so you need to use
con = mol/vol
to work out the concentrations

2. 43/10
3. 43/10

5.14/10

and then literally just put these values into the equilibrium constant equation (don’t forget to square 0.514 because there are 2HI).

Question 24

How do you work out the end concentrations of a reaction if you are given the starting ones and Kc?

Answer 24

Do a table like if you were given the end concentrations and + or - X instead. Then make an algebraic equation with Kc as the answer.

Question 25

What happens to the equilibrium constant if you increase the concentration?

Answer 25

Nothing. The value of the equilibrium constant is not affected by a change in concentration.

This is because, if, for example, you added water and diluted a solution at equilibrium, the concentration of the reactants and products would decrease slightly and the concentration of water would increase, this would change the value of the reaction quotient, say making it 8 instead of 4. Because Q is no longer equal to Kc the system is not at equilibrium, the reaction will shift with time adjusting to the increase in volume and slowly Q will become equal to Kc again. So it will shift back and at equilibrium the Kc will still be 4 so although the concentration of reactants have increased the value of Kc is still the same.

Question 26

What is the equation for gibbs free energy?

Answer 26

^G = -RTlnK

G is gibbs free energy
R is the gas constant
T is the temperature in kelvin
and K is the equilibrium constant

Question 27

What is the relationship between equilibrium constants and Gibbs free energy?

Answer 27

The position of equilibrium corresponds to the mixture of reactants and products that produces the minimum value of the Gibbs free energy and the maximum value of entropy.
As discussed earlier depending on the way round you write the equation there are two values of Kc, Gibbs free energy helps to calculate which.

Question 28

What does Gibbs free energy calculate?

Answer 28

Enthalpy and entropy in a single value

Question 29

When do you use Kc and when do you use Kp?

Answer 29

For reactions involving gases, the equilibrium constant obtained is Kp, which is the equilibrium constant in terms of partial pressures. Partial pressure refers to the pressure exerted by a particular gas in a mixture of gases. If the gases behave ideally the partial pressure is the same as the pressure that the same amount of a particular gas would exert if it was in the container by itself. If the pressure exerted by a mixture of 80% nitrogen and 20% hydrogen is 100kPa then the partial pressure of nitrogen is 80kPa and the partial pressure of hydrogen is 20kPa.

Question 30

What is partial pressure?

Answer 30

Partial pressure refers to the pressure exerted by a particular gas in a mixture of gases. If the gases behave ideally the partial pressure is the same as the pressure that the same amount of a particular gas would exert if it was in the container by itself. If the pressure exerted by a mixture of 80% nitrogen and 20% hydrogen is 100kPa then the partial pressure of nitrogen is 80kPa and the partial pressure of hydrogen is 20kPa.

Question 31

How do you calculate partial pressures?

Answer 31

PV=(n/v)xRT
Instead of using concentration to work out the equilibrium constant of gases you have to use partial pressure which can be calculated by looking at the equation above.

Question 32

How do you calculate the Kc from partial pressures?

Answer 32

Kp= Kc x (RT)^(number of molecules on the right - number on the left)

Question 33

When Kc is less than 1 what is gibbs free energy?

Answer 33

When Kc is less than 1 the value of lnK is negative and therefore gibbs free energy is positive because:
gibbs = -RTlnK

Question 34

When Kc is more than 1 what is gibbs free energy?

Answer 34

When Kc is more than 1 the value of lnK is positive so gibbs free energy is negative because:
gibbs = -RTlnK

Question 35

What are the units of gibbs free energy?

Answer 35

-RTlnK

T = temperature in kelvin
R = 8.31
gibbs free energy is in J

Question 36

Where is the position of equilibrium if gibbs free energy is negative?

Answer 36

If gibbs free energy is negative then lnK is positive so k is bigger than 1 and therefore there are more products than reactants.

Question 37

Which affects the equilibrium vapour pressure of a liquid in a sealed container assuming there is always some liquid present?
A: The temperature of the liquid
B: The surface area of the liquid
C: The volume of the liquid
D: The volume of the container

Answer 37

A: The temperature of the liquid

The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.

As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.