C8.6 - Reversible reactions

Question 1

What happens in most chemical reactions?

Answer 1

In most chemical reactions, the reactants react completely to form the products

Question 2

In most chemical reactions, the reactants react completely to form the products.
How do you show this?

Answer 2

You show this by using an arrow pointing from the reactants to the products

Question 3

In most chemical reactions, the reactants react completely to form the products.
However, what happens in some reactions?

Answer 3

In some reactions, the products can react together to make the original reactants again

Question 4

Reversible reaction

Answer 4

A reversible reaction is a reaction in which the products can re-form the reactants

Question 5

Why are 2 ‘half-arrows’ used in the equation of a reversible reaction?

Answer 5

2 ‘half-arrows’ are used in the equation of a reversible reaction, because a reversible reaction can go in both directions

Question 6

It is very difficult to get a what?

Answer 6

It is very difficult to get a solution that is exactly neutral

Question 7

It is very difficult to get a solution that is exactly neutral.
What can you use to tell when just the right amount of acid has been added, when neutralising an alkaline solution?

Answer 7

When neutralisation an alkaline solution with acid, you can use an indicator to tell when just the right amount of acid had been added

Question 8

What does an indicator do?

Answer 8

An indicator forms compounds that are different colours in:

1. Acidic solutions
2. Alkaline solutions

Question 9

Litmus

Answer 9

Litmus is a complex molecule

Question 10

Litmus is a complex molecule.

How can litmus be represented?

Answer 10

Litmus can be represented as HLit, where H is hydrogen

Question 11

What colour is HLit?

Answer 11

HLit is red

Question 12

HLit is red.

What happens if you add alkali?

Answer 12

If you add alkali, HLit turns into the Lit- ion by losing a H+ ion

Question 13

If you add alkali, HLit turns into the Lit- ion by losing a H+ ion.
What colour is Lit-?

Answer 13

Lit- is blue

Question 14

HLit is red.
If you add alkali, HLit turns into the Lit- ion by losing a H+ ion.
What happens if you then add more acid?

Answer 14

If you then add more acid, blue Lit- changes back to red HLit

Question 15

The neutralisation of acids is a reversible reaction.

What do other reversible reactions involve?

Answer 15

Other reversible reactions involve salts

Question 16

The neutralisation of acids is a reversible reaction.
Other reversible reactions involve salts.
Example

Answer 16

For example:
Hydrated copper(II) sulfate (blue)  Anhydrous copper(II) sulfate (white) + Water

Question 17

What do indicators undergo?

Answer 17

Indicators undergo reversible reactions

Question 18

When does blue litmus paper turn red?

Answer 18

Blue litmus paper turns red in acidic solutions

Question 19

When does red litmus paper turn blue?

Answer 19

Red litmus paper turns blue in alkaline solutions

Question 20

What happens when you heat ammonium chloride?

Answer 20

When you heat ammonium chloride, a reversible reaction takes place

Question 21

An example of a thermal decomposition reaction

Answer 21

An example of a thermal decomposition reaction is when ammonium chloride forms:
1. Ammonia gas
2. Hydrogen chloride gas
after you heat it

Question 22

The word equation for the thermal decomposition of ammonium chloride

Answer 22

The word equation for the thermal decomposition of ammonium chloride is:
heat
Ammonium chloride —> Ammonia + Hydrogen chloride

Question 23

When you heat ammonium chloride, a reversible reaction takes place.
Ammonium chloride breaks down on heating to form ammonia gas and hydrogen chloride gas.
What do the 2 hot gases do?

Answer 23

The 2 hot gases rise up the test tube

Question 24

When you heat ammonium chloride, a reversible reaction takes place.
Ammonium chloride breaks down on heating to form ammonia gas and hydrogen chloride gas.
The 2 hot gases rise up the test tube.
What happens when they cool down near the mouth of the tube?

Answer 24

When the 2 hot gases cool down near the mouth of the tube, they react with each other

Question 25

In most chemical reactions, the reactants react completely to form the products.
In some reactions, the products can react together to make the original reactants again.
This is called a what reaction?

Answer 25

This is called a reversible reaction

Question 26

If you add alkali, HLit turns into the Lit- ion by losing a H+ ion.
Lit- is blue, so HLit turns from what to what if you add alkali?

Answer 26

Lit- is blue, so HLit turns from red to blue if you add alkali

Question 27

Indicators undergo reversible reactions, doing what to show you whether solutions are what?

Answer 27

Indicators undergo reversible reactions, changing colour to show you whether solutions are:
1. Acidic
Or,
2. Alkaline

Question 28

When you heat ammonium chloride, a reversible reaction takes place.
Ammonium chloride breaks down on heating to form what?

Answer 28

Ammonium chloride breaks down on heating to form:

1. Ammonia gas
2. Hydrogen chloride gas

Question 29

When you heat ammonium chloride, a reversible reaction takes place.
Ammonium chloride breaks down on heating to form ammonia gas and hydrogen chloride gas.
The 2 hot gases rise up the test tube.
When the 2 hot gases cool down near the mouth of the tube, they react with each other.
The gases do what again?

Answer 29

The gases re-form ammonium chloride again

Question 30

When you heat ammonium chloride, a reversible reaction takes place.
Ammonium chloride breaks down on heating to form ammonia gas and hydrogen chloride gas.
The 2 hot gases rise up the test tube.
When the 2 hot gases cool down near the mouth of the tube, they react with each other.
The gases re-form ammonium chloride again.
What forms on the inside of the glass?

Answer 30

The white ammonium chloride solid forms on the inside of the glass