Topic 6b- Reversible Reactions Flashcards Preview

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Flashcards in Topic 6b- Reversible Reactions Deck (36)
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1
Q

What is a reversible reaction?

A

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

2
Q

How are reversible reactions represented?

A

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

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

3
Q

How can the direction of reversible reactions be changed?

A

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

4
Q

Give an example of a reversible reaction.

A

ammonium chloride ⇆ ammonia + hydrogen chloride

5
Q

What is equilibrium?

A

Equilibrium meas BALANCE.

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

6
Q

Describe how a reversible reaction reaches equilibrium.

A
  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.
7
Q

What happens to the forward and backward reactions at equilibrium?

A

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.

8
Q

In what environment is equilibrium reached?

A

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

9
Q

What is a closed system?

A

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

10
Q

What conditions are necessary for equilibrium to occur?

A

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.

11
Q

Has the reaction stopped at equilibrium?

A

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.

12
Q

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

A

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

13
Q

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

A

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

14
Q

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

A

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

15
Q

What does the position of equilibrium depend upon?

A

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

16
Q

What are the energy changes in reversible reactions?

A

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.

17
Q

What is an endothermic reaction?

A

A reaction which takes IN energy from the surroundings.

18
Q

What is an exothermic reaction?

A

A reaction which transfers energy To the surroundings.

19
Q

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

A

hydrated copper sulfate —>anhydrous copper sulfate +

Blue

20
Q

Explain what happens when hydrated copper sulphate reacts.

A

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.

21
Q

What does anhydrous and hydrated mean?

A

Anhydrous just means without water.

Hydrated means with water.

22
Q

What is Le Chatelier’s principle?

A

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.

23
Q

Why is Le Chatelier’s principle useful?

A

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.

24
Q

What similarity do all reactions share?

A

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

25
Q

What happens if the temperature of a reaction is increased?

A

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.

26
Q

What happens if the temperature of a reaction is decreased?

A

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.

27
Q

In summary (for exam answers):

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

A

When the forward reaction is exothermic:

  • if the temp is raised, yield decreases
  • if the temp is lowered, yield increases
28
Q

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

A

-if the temp is raised, yield increases.

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

29
Q

What happens to a reaction if the pressure is changed?

A

Changing the pressure only affects an equilibrium involving GASES.

30
Q

What does the pressure of a gas depend upon?

A

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

31
Q

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

A

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

32
Q

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

A

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

33
Q

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

A

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

34
Q

How does the system react to a change of concentration?

A

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

35
Q

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

A

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

36
Q

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

A

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.