Chemistry - Reversible reactions, industrial processes and important chemicals

Question 1

What are reversible reactions?

Answer 1

In a reversible reaction, the products can react to produce the original reactants again.

Question 2

What arrow is used in a reversible reaction?

Answer 2

⇌
Two arrows are used, each with just half an arrowhead (⇌). The top one points right, and the bottom one points left.

Question 3

What is ammonia used for?

Answer 3

-Fertilisers
-explosives
-cleaning fluids

Question 4

Why is ammonia important for plants?

Answer 4

Ammonia is a vital route by which nitrogen in the air can be made available to plants to enable them to build protein molecules. Plants cannot use nitrogen directly from the air. They need nitrogen compounds, dissolved in water, which they absorb through their roots.

Question 5

How do you test for ammonia?

Answer 5

Ammonia, NH3, is an alkaline gas and so turns damp red litmus paper blue.

Question 6

How are hydrogen and Nitrogen obtained?

Answer 6

-Hydrogen is obtained by reacting natural gas (mostly methane) with steam, or from cracking oil fractions.
-Nitrogen is obtained from the air. Air is 78 per cent nitrogen and nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen leaving nitrogen behind.

Question 7

What conditions are needed for the Haber process?

Answer 7

• a high temperature – 450°C
• a high pressure –200 atmospheres
• an iron catalyst

Question 8

What equation summarises the Haber process?

Answer 8

nitrogen + hydrogen ⇌ ammonia

Question 9

What are the stages of the Haber process?

Answer 9

1)Having obtained the hydrogen and nitrogen gases (from natural gas and the air respectively), they are pumped into the compressor through pipes.
2)The gases are pressurised to about 200 atmospheres of pressure inside the compressor.
3)The pressurised gases are pumped into a tank containing beds of iron catalyst at about 450°C. In these conditions, some of the hydrogen and nitrogen will react to form ammonia.
4)The unreacted nitrogen and hydrogen, together with the ammonia, pass into a cooling tank. The cooling tank liquefies the ammonia, which can be removed into pressurised storage vessels.
5)The unreacted hydrogen and nitrogen gases are recycled by being fed back through pipes to pass through the hot iron catalyst beds again.

Question 10

What happens if you increase the pressure in the Haber process?

Answer 10

Increasing the pressure of the reaction increases the yield of ammonia. However, if the pressure is made too high, the equipment needed to safely contain the reaction becomes very expensive.

Question 11

What happens if you increase the temperature in the Haber process?

Answer 11

Increasing the temperature of the reaction actually decreases the yield of ammonia in the reaction. This means that we could get a bigger yield of ammonia with a lower temperature. However, if the temperature is too low, the rate of the reaction would be so slow that it would take too long to make the ammonia.

Question 12

What are compromise conditions?

Answer 12

Used in the Haber process - they are chosen to give a good compromise between yield, rate and cost.

Question 13

What are the advantages of ammonia in fertilisers?

Answer 13

• Increases crop growth.
• Increases crop yield.
• Better soil quality.

Question 14

What are the disadvantages of ammonia in fertilisers?

Answer 14

• Causes eutrophication.
• Causes blue baby syndrome.
• Turns the soil acidic.

Question 15

What are the raw materials needed to make sulfuric acid?

Answer 15

• sulfur
• air
• water

Question 16

How do you make sulfuric acid?

Answer 16

The contact process
1) In the first stage of the contact process, sulfur is burned in air to make sulfur dioxide. sulfur + oxygen → sulfur dioxide
2)In the second stage, sulfur dioxide reacts with more oxygen to make sulfur trioxide sulfur dioxide + oxygen ⇌ sulfur trioxide
3)In the final stage, sulfur trioxide reacts with water to make sulfuric acid sulfur trioxide + water → sulfuric acid

Question 17

What are the conditions to reverse the second stage of the contact process?

Answer 17

• a catalyst of vanadium(V) oxide, V2O5
• a temperature of around 450°C (chosen as a compromise temperature, giving a decent yield with a good rate of reaction)
• a pressure of approximately 2 atmospheres

Question 18

What are the uses of sulfuric acid?

Answer 18

• Fertilisers
• detergents
• paints
• dyes
• plastics

Question 19

How does sulfuric acid react with blue crystals of hydrated copper(II) sulfate?

Answer 19

Concentrated sulfuric acid will remove water molecules from the blue crystals of hydrated copper(II) sulfate and leave behind a white powder, which is anhydrous copper(II) sulfate.

Question 20

How does sulfuric acid react with glucose?

Answer 20

Concentrated sulfuric acid will remove water molecules from glucose molecules to leave behind pure carbon. This reaction is highly exothermic and produces a lot of steam which forces the carbon upwards in the reaction vessel. This makes the carbon look like a black snake growing out of the reaction container.