Mod 8.3 - Chemical Synthesis and Design

Question 1

What are the 2 chemical processes which are studied in this module?

Answer 1

Haber Process and Contact Process

Question 2

What is yield?

Answer 2

Yield of a reaction refers to the amount of product produced from reactants

Question 3

How is yield maximised?

Answer 3

Conditions need to be managed so equilibrium lies to the right of the equation as far as possible. Several conditions can be adapted to maximise yield

For example, need to consider;

Removal of the product

Recycling of reactants

Conducting the reaction at an appropriate temp

Choosing a suitable pressure

Using a catalyst

Question 4

What is theoretical yield?

Answer 4

It is the amount of product that can be theoretically produced from reactants. It refers to how much can theoretically be formed from limiting reagents.

The method of calculating it, is through using mole ratios to calculate which reactant results in the production of the least products. Mass can be calculated through m = n*MM

Question 5

How is percentage yield calculated?

Answer 5

% yield = (actual mass/theoretical mass) x 100

Question 6

How is a synthesis reaction designed?

Answer 6

Identify desired product, from here the reaction pathway can be developed

Choosing the reactants: Common method is to start with desired product and work backwards (retrosynthetic analysis)

Choosing reaction pathway: Here, we have many factors to consider, including;

-Possible products
-availability and cost of reactants
-conditions required for reaction
It is desirable to pick a reaction pathway which produces a chemical which is cheap, safe and efficient.

Linear and convergent pathways: Choose which one is desired - linear has one reaction following another, whereas convergent pathways are multistage reactions, and the products from one are combined into reactants for other reactions

Question 7

Why is chemical synthesis and design important?

Answer 7

It is important as it produces necessary chemicals such as;

aspirin, artificial sweeteners, sunscreen, plastic and more

Question 8

What is the haber process?

Answer 8

It involves the synthesis of nitrogen and hydrogen gas to form ammonia (NH3)

Question 9

What are the uses and importance of ammonia? (6)

Answer 9

Solutions of ammonia in water are used domestically as cleaning agents to dissolve and remove grease and dirt from floors and windows.

Ammonia is the feedstock for a large variety of industrial chemicals

Solid and liquid fertilisers

Nitric acid

Explosives, pharmaceuticals, fibres and plastics 80% of worldwide use of ammonia is for fertiliser

Most well known for its use for fertilisers.

Question 10

How does LCP provide insights into the Haber Process?

Answer 10

N2(g) + 3H2(g) ⇌ 2NH3 (g)

There are various ways that LCP can be used to analyse the effects of changes in temp and pressure on the equilibrium reaction

Because ammonia equilibrium is exothermic, an increase in temp –> equilibrium to shift to the left to use up the added heat –> high temp will reduce yield of ammonia. Thus, LCP predicts that lower temp will favour higher yields of ammonia

Additionally, because the stoichiometric equation for Haber process shows that 4 moles of reactants are converted to 2 moles of products, thus LCP predicts that an increase in pressure in system will favour the reaction that leads to lowering gas pressure due to formation of fewer molecules –> greater yield of ammonia

Thus, LCP is used to reveal that a lower temp and higher pressure can be used to increase the amount of ammonia produced

Question 11

WHat is the Haber Process equation?

Answer 11

N2(g) + 3H2(g) ⇌ 2NH3 (g)

Forward reaction is exothermic

Question 12

What are the environmental issues involved in the Haber Process?

Answer 12

1. Remission of CO2 from burning fossil fuels –> enhanced greenhouse effect, waste products such as sulphur dioxide and fertiliser run off –> acid rain
2. Presence of fertilisers -> Eutrophication of waterways –> production of algae –> kills plants and animals in water because of restricted oxygen from increased algae
3. Chemical spills: Chemical spills can be caused by fires, explosions, leaks or damage to transport vehicles or storage tanks
4. Large energy consumption

Question 13

What are ways of addressing the environmental issues? (2)

Answer 13

Waste must now be treated and can only be disposed of when it is not harmful for the environment. I.e. liquid wastes from chemical plants must meet legislation requirements before being released in sewage systems

Green Chemistry: A relatively new area of chem that aims to reduce impact of chemistry on the environment by reducing or eliminating the use or production of hazardous substances. Requires that all aspects of chemical production be analysed to ensure that nothing potentially harmful is used or produced

Question 14

What are the social considerations involved in the Haber process?

Answer 14

Fertilisers pollute ground water and drinking water which is important for people to survive, eutrophication causes less fish as resources for communities to depend on it

However, it also allows for benefits for society who produce ammonia, as it can provide economic benefits. Essentially good for society economically

Question 15

What are the economic considerations involved in the Haber Process

Answer 15

Strong pipes and reactor vessels are expensive, the pressure for the reaction to occur can’t be too high – > lower rate of reaction – > lower yield, this is also similar for temperature (expensive to maintain equipment)

Essentially, we need to consider the costs of implementing various adjustments to machines involved in the Haber Process

For example, construction of strong pipes to ensure high pressures + maintaining a high pressure reactor vessel is expensive etc.

Question 16

How do you obtain the reagents for the Haber Process?

Answer 16

Haber Process:
N2(g) + 3H2(g) ⇌ 2NH3 (g)

You can obtain nitrogen through the fractional distillation of liquified air as nitrogen has lower BP than oxygen (expensive method)

Common form of obtaining hydrogen is extracting it from the air using chemical reactions involving natural gas or methane. Also yields hydrogen which is required in the reaction:

CH4 (g) + H2O (g) ⇌ CO(g) +3H2 (g)

2H2 (g) + O2(g) + N2(g) ⇌ 2H2O + N2 (g) (In this case, air (which contains oxygen and nitrogen) is introduced, and some of the hydrogen burns in the added oxygen to produce steam)

Carbon monoxide is removed from the gaseous mixture.
CO(g) + H2O (g) ⇌ CO2 + H2 (g)

CO2 (g) + H2O (l) + K2CO3 (aq) –> 2KHCO3 (aq)

(just know the theory)

On top of that, hydrogen can also be obtained by the electrolysis of salt water. The hydrogen that is a by product can be sold to the ammonia industry as feedstock for the Haber Process

2H2O + 2NaCl (aq) –> H2 (g) + Cl2)g_ + 2NaOH

Question 17

What factors of production need to be considered when designing the Haber Process? (in regards to LCP) (7)

Answer 17

The catalyst

The pressure

Recycling

Proportions of nitrogen and hydrogen

The temperature

Economic considerations

separating the ammonia

Question 18

What are the considerations for the catalyst in the Haber Process?

Answer 18

The catalyst is slightly more complicated than pure ion; it has potassium hydroxide added to it as a promoter - a substance that increases its efficiency

Question 19

What are the considerations for recycling in the Haber Process?

Answer 19

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

Essentially, there will be excess nitrogen and hydrogen after the reactions, which means that to minimise wastage, the ammonia gases are compressed and cooled to be reused in the process, but liquid ammonia is tapped off into a storage

Question 20

What are the considerations for the pressure used in the Haber Process?

Answer 20

N2(g) + 3H2(g) ⇌ 2NH3 (g)

Because there are 4 molecles on the left hand side, and only 2 on the right, according to LCP, increasing the pressure of the system will favour the reaction which produces fewer molecules. Thus, to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible

Also, increasing pressure would be helpful as it brings molecules together, and increase the chances of hitting and sticking to the surface of the catalyst where they can react –> better rate of reaction for gas reaction

HOWEVER, very high pressures are very expensive to produce;

You have to build extremely strong pipes and containment vessels to withstand high pressure (increased initial capital cost). Also, high pressures cost a lot to produce and maintain –> running costs of the plant are high

Thus, a comprimise has to be made between equilibrium/rate considerations with economic considerations

Question 21

What is the compromise on pressure for the Haber Process

Answer 21

The compromise is that the pressure should be ~200atm - 400 atm (but more ranging around the 200atm mark)

Question 22

What are the considerations for temperature for the Haber Process?

Answer 22

Note that the Haber process is an exothermic (forward) reaction

Thus, according to LCP, the forward rection is favoured if the temperature is lowered, as the system will respond by moving the position of equilibrium to counteract this (increasing heat), thereby favouring the forward reaction

In order to get as much ammonia as possible in the equilibrium mixture, you need as low a temp as possible

However, the lower the temp used, the slower the reaction becomes (according to collision theory). Thus, if attempting to produce as much ammonia as possible, there is no point if the temp is so low that it takes several years for the reaction to go to completion

Question 23

What is the compromise on temperatures for the Haber Process?

Answer 23

The compromise temp is 400-450 Celsius, as a reasonably high proportion of ammonia (~15%) in the equilibrium will still be produced, AND it will be produced in a very short time

Question 24

What are the considerations for separating the ammonia in the Haber process?

Answer 24

When the gases leave the reactor, they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isnt too hot, and so the temp of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled

Question 25

What are the considerations for the catalyst in the Haber Process?

Answer 25

The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn’t produce any greater percentage of ammonia in the equilibrium mixture. Its only function is to speed up the reaction.

In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.

Question 26

What are the considerations for the proportions of nitrogen and hydrogen in the Haber Process?

Answer 26

The mixture of nitrogen and hydrogen going into the reactor is in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen.

Avogadro’s Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. That means that the gases are going into the reactor in the ratio of 1 molecule of nitrogen to 3 of hydrogen.

In some reactions you might choose to use an excess of one of the reactants. You would do this if it is particularly important to use up as much as possible of the other reactant - if, for example, it was much more expensive. That doesn’t apply in this case.

There is always a down-side to using anything other than the equation proportions. If you have an excess of one reactant there will be molecules passing through the reactor which can’t possibly react because there isn’t anything for them to react with. This wastes reactor space - particularly space on the surface of the catalyst.

Question 27

What is the catalyst used for the Haber process?

Answer 27

Magnetite (Fe3O4)

Question 28

What is the compromise H2:N2 ratio?

Answer 28

3:1, to ensure no wastage of reactants

Question 29

What are the overall compromise conditions for the Haber process?

Answer 29

Pressure: 200-400 atm (typically 200-250 atm)

Temperature: 400-500 celsius (typically 400-450)

H2:N2 ratio: 3:1

Catalyst: magnetite (Fe3O4)

Question 30

WHat are the factors favouring a rapid rate of production of ammonia?

Answer 30

Temp: High temp

Gas pressure of reactants: high pressures

Catalyst: use of an effective catalyst

Question 31

What are the factors favouring a high reaction yield of ammonia?

Answer 31

Temp: Low temp

Gas pressure: High pressure

Catalyst: No effects

Question 32

What are the factors favouring minimised costs in the yield of ammonia?

Answer 32

Temp: Low temp (low temp as maintenance of high temp is expensive)

Pressure: Low pressure, as construction and operation of plants capable of sustaining high pressures is expensive

Catalyst: Catalysts are usually expensive

Question 33

What are the safety considerations for the haber process? (or most chemical synthesis)

Answer 33

Safety of gas under pressure

Check for good gas flow over catalyst (clean equipment)

Do yield and purity checks

Monitoring of reactants; the ratio of the incoming gases has to be monitored so that it is maintained at a constant 1:3 (N2 to H2). This is because a build up of any gas could increase pressure

Contaminants; The concentration of contaminants must be monitored. O2 must be completely removed as it could increase the risk of an explsoive reaction. Sulfur compounds must be at low levels as they can poison the catalyst. Levels of argon and methane must also be kept at low concentrations as they lower the efficiency of the reaction

Question 34

What sorts of monitoring do industrial chemists and chemical engineers need to do?

Answer 34

Continuous monitoring of the hgih pressure reaction vessel.

Temperature monitoring. Make sure temp remains in ideal range

Monitoring the furnaces that produce the hydrogen and nitrogen feeedstocks

Monitoring the activity of the catalyst, which has a lifetime of 5-10 years. they must also have a high surface area

Monitoring the ammonia liquefaction process to ensure optimal yield of ammonia

Question 35

How is percentage yield calculated?

Answer 35

% yield = (actual mass/theoretical mass) x 100

Question 36

How is percentage yield calculated?

Answer 36

% yield = (actual mass/theoretical mass) x 100

Question 37

How is percentage yield calculated?

Answer 37

% yield = (actual mass/theoretical mass) x 100

Question 38

What is the contact process?

Answer 38

Its purpose is to create sulfuric acid.

In its aim to create sulfuric acid, it makes sulfur dioxide, and converts sulfur dioxide into sulfur trioxide, before converting that into sulfuric acid

Question 39

What is the contact process reaction (s) ?

Answer 39

Making sulfur dioxide:

S (s) + O2(g) –> SO2 (g)
OR
4FeS2 (s)+ 11O2 (g) –> 2 Fe2O3 (s) + 8SO2 (g)

Converting sulfur dioxide into sulfur trioxide:

2SO2 (g) + O2 (g) ⇌ 2SO3 (g) (exothermic reaction)

Converting sulfur trioxide into sulfuric acid:

Step 1) H2SO4 + SO3 (g) -> H2S2O7 (l)

Step 2) H2S2O7(l) + H2O (l) –> 2 H2SO4 (l)

Question 40

What is sulfuric acid used for?

Answer 40

Cleaning products such as detergents, bleaches, oven cleaners etc.

Used in the production of phosphate fertilisers

Manufacture explosives, dyes, glue etc.

Question 41

What are the reaction conditions to consider in the Contact Process?

Answer 41

Proportions of sulfur dioxide and oxygen

Temperature

Pressure

Catalyst

Question 42

What are the considerations for the proportions of sulfur dioxide and oxygen in the contact process?

Answer 42

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

According to Le Chatelier’s Principle, Increasing the concentration of oxygen in the mixture causes the position of equilibrium to shift towards the right. Since the oxygen comes from the air, this is a very cheap way of increasing the conversion of sulphur dioxide into sulphur trioxide.

However, by increasing the proportion of oxygen you can increase the percentage of the sulphur dioxide converted, but at the same time decrease the total amount of sulphur trioxide made each day. The 1 : 1 mixture turns out to give you the best possible overall yield of sulphur trioxide.

Question 43

What are the considerations for the temperature in the contact process? What temp is used?

Answer 43

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

According to LCP, the forward reaction will be favoured if the temp is lowered, thus you need as low as a temperature as possible to favour the amount of sulfur trioxide produced

However, the lower the temp used, the slower the reaction ( by collision theory). Thus, a balance needs to be achieved

Temp used: 400 - 450 degrees Celsius

Question 44

What are the considerations for the pressure in the contact process? What pressure is used?

Answer 44

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

Notice that there are 3 molecules on the left-hand side of the equation, but only 2 on the right.

According to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again.

In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as high a pressure as possible. High pressures also increase the rate of the reaction. However, the reaction is done at pressures close to atmospheric pressure!

However, even at relatively low pressures, there is a 99.5% conversion of sulfur dioxide into sulfur trioxide. The small improvement which could be achieved by increasing pressure isnt worth the expense of producing them

Thus, a pressure of 1-2 atm is used

Question 45

What are the considerations for the catalyst in the contact process? WHat catalyst is used?

Answer 45

The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn’t produce any greater percentage of sulphur trioxide in the equilibrium mixture. Its only function is to speed up the reaction.

In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.

Catalyst used: Vanadium pentoxide (V2O5)