Reversible Reactions and Equilibrium

Question 1

What is a reversible reaction?

Answer 1

A chemical reaction that can go forwards as well as backwards.

Question 2

Which reaction does the enthalpy change refer to in a reversible reaction?

Answer 2

The forwards reaction.

Question 3

What will the enthalpy change be for the backwards reaction in a reversible reaction?

Answer 3

The same number as the forward reaction, but with the opposite sign.

Question 4

Give an example of a reversible reaction involving dehydration/hydration:

What is its chemical equation?

Answer 4

The dehydration/hydration of copper sulfate crystals. You heat hydrated copper sulfate crystals to lose their water.

CuSO4 . 5H2O ⇌ CuSO4 + 5H2O

Question 5

What are the observations of the dehydration of copper sulfate crystals?

Answer 5

The crystals turn from blue to white as they lose all their water.

Question 6

Give an example of a reversible reaction involving thermal decomposition:

What is its chemical equation?

Answer 6

The thermal decomposition of ammonium chloride.

NH4Cl (s) ⇌ NH3 (g) + HCl (g)

Question 7

What are the observations of the decomposition of ammonium chloride

Answer 7

Nothing appears to change, because NH3 (g) and HCl (g) are both colourless.

Question 8

How can you demonstrate the decomposition of ammonium chloride?

Answer 8

By gently heating NH4Cl in a test tube. The NH4Cl at the bottom of the tube decomposes to form invisible NH3 and HCl, the invisible NH3 and HCl cool as they move up the tube.

Question 9

What is dynamic equilibrium?

Answer 9

When the forwards and backwards reactions are still happening at the same rate, and the concentrations of the reactants and products remain unchanged.

Question 10

What conditions can equilibrium only be reached in?

Answer 10

A closed system, in which none of the reactants or products can escape.

Question 11

What are the 3 factors of equilibrium?

Answer 11

-Both reactions are still happening
-Both reactions effectively cancel each other out
-There is no overall change in concentrations

Question 12

What is the equilibrium position?

Answer 12

The resulting balance of products and reactants in an equilibrium mixture.

Question 13

What does it mean if equilibrium is ‘further’ to one side?

Answer 13

There is a greater imbalance in reactants and products

Question 14

In which reaction can can you tell where the equilibrium lies?

Answer 14

In the decomposition of N2O4, the more brown the mixture is, the more NO2 must be present and the further the equilibrium is to the right.

Question 15

What 3 factors can shift the equilibrium?

Answer 15

-Amounts of reactants or products
-Gas pressure
-Temperature

Question 16

What does Le Chatelier’s principle state?

Answer 16

“The equilibrium shifts to counteract any change imposed on it.”

Question 17

How does equilibrium shift when adding a substance?

Answer 17

It shifts in whichever direction uses it up.

Question 18

How does equilibrium shift when removing a substance?

Answer 18

It shifts in whichever direction creates it.

Question 19

How does equilibrium shift when increasing gas pressure?

Answer 19

It shifts to whichever side has fewest moles of gas.

Question 20

How does equilibrium shift when decreasing gas pressure?

Answer 20

It shifts to whichever side has most moles of gas.

Question 21

How does equilibrium shift when increasing temperature?

Answer 21

It shifts in whichever direction is endothermic.

Question 22

How does equilibrium shift when decreasing temperature?

Answer 22

It shifts in whichever direction is exothermic.

Question 23

What 2 things should you do when explaining an equilibrium shift?

Answer 23

1. State how the change needs to be counteracted
2. Explain how the forwards or backwards reaction achieves that

Question 24

What is the easiest way to change the concentration of a substance?

Answer 24

To add or remove some of it from the mixture:
- If you add something, the equilibrium will shift to remove the extra you just added
-If you remove something, the equilibrium will shift to create more of it

Question 25

In the example of N2 + 3H2 ⇌ 2NH3, what happens if extra H2 is added?

Answer 25

-We need to remove the extra H2
-The equilibrium will shift right
-Because the forwards reaction uses up H2

Question 26

In the example of N2 + 3H2 ⇌ 2NH3, what happens if some NH3 is added?

Answer 26

-We need to replace the lost NH3
-The equilibrium will shift right
-Because the forwards reaction creates NH3