Topic 6b- Reversible Reactions

Question 1

What is a reversible reaction?

Answer 1

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.

Question 2

How are reversible reactions represented?

Answer 2

Reversible reactions and are represented:
A + B ⇆ C + D

The double arrow means that the reaction can go in either direction.

Question 3

How can the direction of reversible reactions be changed?

Answer 3

By changing the conditions of a reaction, the overall direction can be changed and the relative amounts of products and reactants can be altered.

Question 4

Give an example of a reversible reaction.

Answer 4

ammonium chloride ⇆ ammonia + hydrogen chloride

Question 5

What is equilibrium?

Answer 5

Equilibrium meas BALANCE.

Equilibrium is when the amounts of reactants and products reach a balance- their CONCENTRATIONS stop changing.

Question 6

Describe how a reversible reaction reaches equilibrium.

Answer 6

1. As the REACTANTS react, their concentrations FALL-so the FORWARD REACTION will SLOW DOWN. But as more and more PRODUCTS are made and their concentrations RISE, the BACKWARD REACTION will SPEED UP.
2. After a while the forward reaction will be going at EXACTLY THE SAME RATE as the backward one- the system is at EQUILIBRIUM.

Question 7

What happens to the forward and backward reactions at equilibrium?

Answer 7

At equilibrium, BOTH reactions are still happening, but there’s NO OVERALL EFFECT (it’s a dynamic equilibrium). This means the CONCENTRATIONS of reactants and products have reached a balance and WON’T CHANGE.

Question 8

In what environment is equilibrium reached?

Answer 8

Equilibrium is only reached if the reversible reaction takes place in a CLOSED SYSTEM.

Question 9

What is a closed system?

Answer 9

A closed system is just means that NONE of the reactants or products can ESCAPE and nothing else can get IN.

Question 10

What conditions are necessary for equilibrium to occur?

Answer 10

When a reversible reaction occurs in a CLOSED SYSTEM and the RATE (speed) of the forward reaction equals the reverse reaction, then an equilibrium is reached.

Question 11

Has the reaction stopped at equilibrium?

Answer 11

The reaction hasn’t stopped and is still going (i.e. dynamic= moving) but the amount of reactants relative to products remains the same- it is in balance, i.e. equilibrium.

Question 12

What does it mean if the equilibrium lies to the right?

Answer 12

If the equilibrium LIES TO THE RIGHT, the concentration of PRODUCTS is greater than that of the reactants.

Question 13

What does it mean if the equilibrium lies to the right?

Answer 13

If the equilibrium LIES TO THE RIGHT, the concentration of PRODUCTS is greater than that of the reactants.

Question 14

What does it mean if the equilibrium lies to the left?

Answer 14

If the equilibrium LIES TO THE LEFT, the concentration of REACTANTS is GREATER than that of the products.

Question 15

What does the position of equilibrium depend upon?

Answer 15

The POSITION OF EQUILIBRIUM depends on the following CONDITIONS and the reaction itself:

1) The TEMPERATURE
2) The PRESSURE (this only affects equilibria involving gases).
3) The CONCENTRATION of the reactants and products

Question 16

What are the energy changes in reversible reactions?

Answer 16

If a reversible reaction is EXOTHERMIC in one direction, it is ENDOTHERMIC in the opposite direction. The energy absorbed by the endothermic reaction is equal to the energy released by the exothermic reaction.

Question 17

What is an endothermic reaction?

Answer 17

A reaction which takes IN energy from the surroundings.

Question 18

What is an exothermic reaction?

Answer 18

A reaction which transfers energy To the surroundings.

Question 19

Give an example of a reversible reaction, showing the exothermic and endothermic energy.

Answer 19

hydrated copper sulfate —>anhydrous copper sulfate +

Blue

Question 20

Explain what happens when hydrated copper sulphate reacts.

Answer 20

If we heat blue hydrated copper sulphate ( an ENDO thermic process), the water will be removed and white anhydrous copper sulphate will be produced.

If water is added (an EXO thermic process- it gets hot!), to the WHITE powder you get BLUE CRYSTALS back again as it’s been hydrated. This is EXOTHERMIC.

Question 21

What does anhydrous and hydrated mean?

Answer 21

Anhydrous just means without water.

Hydrated means with water.

Question 22

What is Le Chatelier’s principle?

Answer 22

Le Chatelier’s Principle is the idea that if you change the CONDITIONS of a reversible reaction at equilibrium, the system will try to COUNTERACT that change.

Question 23

Why is Le Chatelier’s principle useful?

Answer 23

It can be used to PREDICT the effect of any changes you make to a system.
So by altering the temp, pressure or conc of the reactants, you could alter the YIELD of the reaction- making sure you end up with more of the product you want (and less of the reactants.

Question 24

What similarity do all reactions share?

Answer 24

All reactions are EXOTHERMIC in one direction and ENDOTHERMIC in the other.

Question 25

What happens if the temperature of a reaction is increased?

Answer 25

If the temperature is RAISED, the ENDOTHERMIC reaction is favoured because it absorbs the extra heat supplied.

So the yield of the ENDOTHERMIC reaction will INCREASE and the yield of the exothermic will decrease.

Question 26

What happens if the temperature of a reaction is decreased?

Answer 26

If you DECREASE the temperature, the equilibrium will move in the EXOTHERMIC DIRECTION, to produce more heat.

This means you’ll get MORE PRODUCTS for the EXOTHERMIC DIRECTION and fewer products for the endothermic reaction.

Question 27

In summary (for exam answers):

What happens to the yield when the forward reaction is exothermic?

Answer 27

When the forward reaction is exothermic:

• if the temp is raised, yield decreases
• if the temp is lowered, yield increases

Question 28

What happens to the yield when the forward reaction is endothermic?

Answer 28

-if the temp is raised, yield increases.

However, the product can easily be reversible as the heat is transferred back so it is reversible.

Question 29

What happens to a reaction if the pressure is changed?

Answer 29

Changing the pressure only affects an equilibrium involving GASES.

Question 30

What does the pressure of a gas depend upon?

Answer 30

The pressure of a gas depends on the NUMBER OF MOLECULES.

Question 31

Describe the effect of increasing the pressure on a reversible reaction at equilibrium.

Answer 31

If you INCREASE the pressure, the equilibrium tries to REDUCE it-it moves in the direction where there are FEWER molecules of gas.

Question 32

Describe the effect of decreasing the pressure on a reversible reaction at equilibrium.

Answer 32

If you DECREASE the pressure, the equilibrium tries to INCREASE it- it moves in the direction where there are MORE molecules of gas.

Question 33

What happens if you change the concentration of either reactants or products?

Answer 33

If you change the concentration of EITHER reactants or products, the system will NO LONGER be at equilibrium.

Question 34

How does the system react to a change of concentration?

Answer 34

The system/reaction responds to bring itself BACK to equilibrium again.

Question 35

Describe the effect of increasing concentration of reactants on a reversible reaction at equilibrium.

Answer 35

If you increase the concentration of the REACTANTS the system tries to decrease it by making more PRODUCTS until equilibrium is reached again.

Question 36

Describe the effect of decreasing concentration of products on a reversible reaction at equilibrium.

Answer 36

If you decrease the concentration of PRODUCTS the system tries to increase it again by making the reactants react to form products until equilibrium is reached again.