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Question 1

Haber process

Answer 1

Nitrogen and hydrogen gas are used to make ammonia in the Haber process. The word and symbol equations for this reaction are shown below.

Question 2

factors to consider when designing an industrial process such as the Haber process

Answer 2

• cost of extracting and refining the raw materials
• cost and availability of reactants
• Energy costs (costs associated with reaching and maintaining the conditions required for the reaction, such as temperature and pressure) also affect how profitable a reaction is.

Question 3

conditions used for the Haber process:

Answer 3

Temperature: 450 °C
Pressure: 200 atmospheres
Catalyst: Iron

Question 4

Haber process - temperature

Answer 4

The forward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way - away from ammonia and towards N₂ and H₂. So the yield of ammonia would be greater at lower temperatures.

The trouble is, lower temperatures mean a lower rate of reaction. So in industry, the temperature is increased anyway, to get a reasonable rate of reaction.

The 450 °C is a compromise between maximum yield and speed of reaction. It’s better to wait just 20 seconds for a 10% yield than to have to wait 60 seconds for a 20% yield.

Question 5

Haber process - pressure

Answer 5

Higher pressures favour the forward reaction, since there are four molecules of gas on the left-hand side for every two molecules on the right-see the equation on the previous page. This moves the equilibrium towards ammonia and away from nitrogen and hydrogen. So higher pressures increase the yield of ammonia. High pressures also increase the rate of reaction.

The pressure is set as high as possible to give the best yield, without making the plant too expensive to build (it’d be too expensive to build a plant that would stand pressures of over 1000 atmospheres, for example). Hence the 200 atmospheres operating pressure.

Question 6

Reaction vessel for haber process

Answer 6

Question 7

Haber process - catalyst

Answer 7

Catalysts are useful too. For example, in the Haber process, the iron catalyst makes the reaction go faster, which means that it reaches equilibrium faster. It doesn’t change the position of equilibrium though.

Without the catalyst, the temperature would have to be raised even further to get a quick enough reaction this would reduce the yield even further and lead to higher energy costs. So the catalyst is very important.

Question 8

Elements needed for plant growth and fertilizers

Answer 8

The three main essential elements needed by plants are nitrogen, phosphorus and potassium. Plants absorb these nutrients from the soil. If plants don’t get enough of these elements, their growth and life processes are affected. These elements may be missing from the soil if they’ve been used up by a previous crop.

Fertilisers replace these elements if they’re missing from the soil, or provide more of them. This helps to increase the crop yield, as the crops can grow faster and bigger.

Question 9

Ammonia-based fertilisers

Answer 9

Fertilisers often contain compounds that are derived from the ammonia produced in the Haber process. Ammonia-based fertilisers have some important advantages compared to traditional fertilisers, like manure. Ammonia-based fertilisers are also soluble, so all the chemicals can dissolve down into the soil to reach the plants.

Question 10

Nitrogen compounds in fertilisers

Answer 10

Reacting ammonia with oxygen and then water produces nitric acid.
Nitric acid can itself be reacted with ammonia to form an ammonium salt (a common compound found in fertilisers).
The salt ammonium nitrate is an especially good component of fertiliser as it has a high proportion of nitrogen per unit mass.

Question 11

Preparing ammonium sulfate

Answer 11

Ammonium sulfate is an important component of fertiliser that can be made in the lab on a small scale, or in industry on a much larger scale.

Question 12

Preparing ammonium sulfate - In laboratory

Answer 12

Ammonium sulfate can be made in the lab by reacting ammonia solution with dilute sulfuric acid in a titration. You can make most fertilisers using this titration method just choose the right acid (nitric, sulfuric or phosphoric) and alkali (ammonia or potassium hydroxide) to get the salt you want.

Question 13

Preparing ammonium sulfate - In laboratory - method

Answer 13

Here’s how you can make ammonium sulfate in the lab:

1. Set up your apparatus as shown. Add a few drops of methyl orange indicator to the ammonia solution it should turn yellow.
2. Slowly add the dilute sulfuric acid from the burette into the ammonia solution, until the yellow colour just changes to red. Gently swirl the flask as you add the acid. Go especially slowly when you get close to the end point. Methyl orange is yellow in alkalis, but red in acids, so this colour change means that all the ammonia has been neutralised and you’ve got ammonium sulfate solution.
3. The ammonium sulfate solution isn’t pure it’s still got methyl orange in it. To get pure ammonium sulfate crystals, you need to note exactly how much sulfuric acid it took to neutralise the ammonia, then repeat the titration using that volume of acid, but no indicator.
4. To get solid ammonium sulfate crystals, gently evaporate the solution (using a steam bath) until only a little bit is left. Leave it to crystallise then filter out the crystals and leave them to dry.

Question 14

Preparing ammonium sulfate - In industry

Answer 14

The method above isn’t used to make ammonium sulfate in industry. It’s impractical to use burettes and steam baths for large quantities of reactants, and using crystallisation to produce solid ammonium sulfate is too slow.

The industrial production of ammonium sulfate usually has several stages, as the ammonia and sulfuric acid have to be made from their raw materials first. Ammonia is made using the Haber process and sulfuric acid is produced using a process called the Contact process. One industrial method to produce ammonium sulfate uses a large reaction chamber filled with ammonia gas. Sulfuric acid is sprayed into the chamber where it reacts with the ammonia to produce ammonium sulfate powder.

Question 15

Chemical cells

Answer 15

Chemical cells are electrochemical cells that produce a voltage across the cell until one of the reactants has been used up. A fuel cell is a chemical cell that’s supplied with a fuel and oxygen (or air) and uses the reaction between them to efficiently release energy

Question 16

Hydrogen-oxygen fuel cells

Answer 16

There are a few different types of fuel cells, using different fuels and different electrolytes. One important example is the hydrogen-oxygen fuel cell. The reaction between hydrogen (the fuel) and oxygen releases energy, which is used in hydrogen-oxygen fuel cells to produce a voltage. This voltage can be used to power an electrical device (e.g. an electric car) that the fuel cell is connected to. The reaction in the fuel cell doesn’t produce any harmful pollutants the only product is water. The overall reaction in a hydrogen-oxygen fuel cell is:

Question 17

Advantages of fuel cells

Answer 17

Hydrogen fuel cells are much more etticient than power stations or batteries at producing electricity. If you use the heat produced as well, their efficiency can be greater than 80%. The electricity is generated directly from the reaction, so fuel cells don’t require any heavy machinery (such as turbines or generators). Because there aren’t a lot of stages to the process of generating electricity, there are fewer places for energy to be lost as heat.

Unlike a car engine or a fossil fuel burning power station, there are no moving parts in a fuel cell, so energy isn’t lost through friction. Fuel cell vehicles don’t produce any conventional pollutants no greenhouse gases, no nitrogen oxides, no sulfur dioxide, no carbon monoxide. The only by-products are water and heat. This would be a major advantage in cities, where air pollution from traffic is a big problem. This could mean no more smelly petrol and diesel cars, lorries and buses.

Another advantage of fuel cells is that they could be used to replace batteries, which are incredibly polluting to dispose highly toxic metal compounds.

Question 18

Disadvantages of fuel cells

Answer 18

Despite their many advantages, fuel cells are not likely to mean the end of either conventional power stations or our dependence on fossil fuels.

Hydrogen is a gas so it takes up far more space to store than liquid fuels like petrol. Hydrogen gas is also very explosive, which makes it difficult to store safely.

The hydrogen fuel is often made either from hydrocarbons (which come from fossil fuels), or by electrolysis of water, which uses electricity (which is normally generated by burning fossil fuels).

It would be expensive to develop and install the necessary technology to use fuel cells widely (e.g. safe hydrogen storage tanks would have to be installed in petrol stations across the country).