F335- Agriculture and Industry:

Definitions:

Batch Process- manufacturing process where a fixed quantity of reactants are added to the reaction vessel, allowed to react and then the products are removed.

Continuous Process- manufacturing process where the reactants are constantly fed in and the products are taken out.

By-Products- the unintended products made during the manufacture of some other substance. Formed from an unwanted reaction.

Capital Costs- costs needed to establish the chemical plant. Catalyst- a catalyst is a substance that alters the rate of a reaction by providing an alternate

pathway with lower activation energy, without being used up in the process itself. A heterogeneous catalyst is in a different state to the reactants. A homogeneous catalyst is in the same state as the reactants.

Co-products- the other useful products formed during a reaction, other than the intended one.

Feedstocks- the reactants that go into a chemical process.The feedstocks are the reactants needed for the chemical process. These are prepared from raw materials that are present in the natural environment, for example, natural gas, crude oil, coal, limestone etc.The raw materials usually have to be treated to ensure that they are sufficiently pure and present in the correct proportions.It is important that the feedstock is in a form that is easy to handle.Transferring gases and liquids is relatively easy, as they can be transported by pipes within the chemical plant or across the country. However, the cost of pumping the gas or liquid can be costly, and so the length of piping and number of pumps is kept to a minimum.Solids are expensive to handle; they are often melted down (for example, molten sulphur for the production of sulphuric acid is often transported by ship as a molten liquid). Solids can also be mixed with water to form a watery slurry that can be pumped along pipelines.A vast proportion of the organic compounds produced today are derived from oil and natural gas.

Fixed Costs- costs occurred by the company irrelevant of the amount of product formed. Heat Exchanger- a vessel in which heat is transferred from one medium to another.

La Chatelier’s Principle- if a dynamic equilibrium is disturbed by changing the conditions, then the position of equilibrium will move to counteract that change

Optimum Conditions- the best environment for a process to maximise the yield. Pilot Plant- a scaled down version of what the full scale plant is anticipated to be. Rate- a measure of how fast a reaction occurs. Raw Materials- the natural resources that are the basic starting materials from which the

products are made. Recycling- the unreacted feedstock is separated from the reaction mixture, processed and

then fed into the reaction again. Slurry- a watery mixture of insoluble material suspended in water. Variable Costs- costs related to the unit of production; they change according to how much

substance is produced. Yield- the amount of product formed.

Aspects of a Chemical Plant:

Waste Disposal and Pollution

Pollution can be defined as any process which results in the increase of harmful substances or harmful factors (noise, radiation etc.) in the environment.

Waste chemicals can be dealt with in one of three possible ways: Dumping the waste in the nearest convenient environment (for example, the atmosphere,

lake, river or in the ocean). This approach is based on the idea that “the solution to pollution is dilution”. This was once the most common method of disposal; however it is now very unpopular.

Containing the waste in purpose built ponds/heaps. This is often chosen for wastes such as those produced by the mining industry. This form of disposal can be unsightly, and can prevent the land being used for other more productive purposes, such as farming.

Chemical/mechanical treatment. The waste products can be treated to make them suitable for disposal into the environment. For example, liquids have to meet specific pH requirements before being pumped into the rivers.

Recycling involves reclaiming the unreacted materials and using them again in the manufacturing process.

Separating the unreacted feedstock is not an easy task; it is important that impurities do not get recycled along with the useful substances. If the impurities are also recycled, then their concentrations will build up in the reaction step, which could result in interference with it.

Energy can also be recycled; the heat produced by one reaction can be transported (via a heat exchange) to another part of the plant that requires energy.

This can save energy costs and thus make the product more cost efficient.

Plant Construction Materials

It is essential to chose construction materials that don’t react with the feedstocks, catalysts, solvents, or products involved in the chemical process.

The wrong choice can result in lower efficiency, hazardous reactions, and product contamination.

In the manufacture of phenol from benzene, a substance called cumene hydroperoxide is produced.

Cumene hydroperoxide is explosive in the presence of cobalt, nickel and zinc; for this reason it is imperative that these materials are absent form the equipment used in that particular part of the plant.

It is also important that construction materials are resistant to corrosion by the chemicals flowing through them. Glass-lined vessels, alloys, or glass reinforced plastics are frequently used in place of steel when there is a corrosion risk.

Safety

Personal safety is rated very highly in the chemical industry. In a typical production site, there will be eye-baths, showers, toxic gas refuges, breathing

apparatus, emergency control rooms, and on large sites, the company’s very own fire brigade, ambulance service and a well-equipped medical centre.

Some reagents used by a chemical plant may be hazardous to the people living in the surrounding areas; where this occurs, the chemical plant works closely with the local authority and emergency services to ensure that emergency procedures are carefully planned and rehearsed at regular intervals.

An analysis of all possible hazards and an examination of safety are applied to any proposed or modified chemical plant.

A Hazard and Operability Study (HAZOP) is a systematic procedure frequently carried out to carry such an analysis.

In the HAZOP, every valve, pipe, pump etc. is examined and the risk associated with its failure is assessed and minimised by design.

The layout of the chemistry plant is carefully designed to reduce the risk of a serious catastrophe.

Location of the Site

The choice of location of a chemical plant is usually related to the source of the raw materials needed.

Transportation of raw materials can be very expensive and so would increase the costs associated with production.

Availability of water and power may also be important in deciding where to build the chemical plant; for electrolytic processes, the availability of large power supplies is crucial.

Sometimes, chemical plants are built near existing works; this may provide the plant with a specialised labour force, shared canteen facilities etc., and shared feedstock. Chemical processes sharing feedstock in this way are said to be part of an integrated plant.

A good transport and communications network also plays a role in deciding where to build a new chemical plant.

Costs

The profit generated is the difference between the selling price and the costs of production. Therefore, to make a decent profit, it is vital that costs are reduced to a minimum.

Some major costs (research and development, plant design, construction and start up etc.) are incurred before the plant starts production.

Sales of the products have to cover these costs and the cost of production before a profit can be made.

Fixed costs have to be paid no matter how much product is made; for example, the cost of construction (capital cost), and work force.

Variable costs depend upon the level of production. If there is no production then there are no variable costs; the greater the production, the higher the costs. Variable costs include the cost of raw materials, effluent control, distribution etc.

Bonding, Structure and Properties:

The properties of substances are determined by their bonding and structure. Bonding means they way the atoms are held together. Structure means the way the atoms are arranged.

Three main factors are important in deciding the properties of a substance:

1. Type of particles it contains:e.g. atoms, ions or molecules:- If ions are present then it is capable of being an electrolyte. Ions or polar molecules might dissolve in water.

2. Type of bonding:Could be ionic, covalent, metallic or weak intermolecular. Stronger bonds results in higher Tm

and Tb and greater hardness. For example, silica, SiO2, has strong covalent bonds linking every atom to several others forming a giant covalent structure. The atoms in silica are very hard to separate, and therefore it is very hard and difficult to melt. Carbon dioxide on the other hand has strong covalent bonds between the C and O atoms, but only weak intermolecular forces between each CO2 molecule. The molecules are therefore easily separated and so CO2 has a low melting/boiling point.

3. Arrangement of the particles:Can be referred to as structure.i)one dimensional chains e.g. Polyetheneii) two dimensional sheets e.g. claysiii) three dimensional arrangements e.g. diamond, silica.

Giant Lattice Molecular (Covalent)Ionic Covalent Metallic Macromolecular Simple Molecular

What substances have this sort of structure?

Compounds of metals and non-metals.

Some elements in group IV and some of their compounds.

Metals Polymers Some non-metal elements and some metal/non-metal compounds.

Examples. Sodium chloride, calcium oxide.

Silicon(IV) oxide, diamond, graphite.

Copper, iron. Polyethene, proteins.

Carbon dioxide, chlorine, water.

What type of particles does it contain?

Ions Atoms Positive ions surrounded by delocalised electrons.

Long chained molecules.

Small molecules.

How are the particles bonded together?

Strong ionic bonds.

Strong covalent bonds.

Strong metallic bonds.

Weak intermolecular forces between molecules, strong covalent bonds within molecules.

Weak intermolecular forces between molecules, strong covalent bonds within each molecule.

Properties:Melting/boiling point.

High. Very High. Generally High.

Moderate(often decomposes on heating)

Low.

Hardness Hard, but brittle.

Very hard. Hard, but malleable.

Soft, but often flexible.

Soft

Electrical Conductivity.

Conducts when molten or dissolved in water.

Do not normally conduct.

Conduct Do not normally conduct.

Do not conduct.

Solubility in water.

Often soluble.

Insoluble. Insoluble. Sometimes Soluble.

Usually Soluble.

Solubility in non-polar solvents (benzene, hexane, etc).

Insoluble. Insoluble. Insoluble. Sometimes soluble.

Sometimes Soluble.

Equilibria and Concentrations:

Consider the following general reaction:aA (aq )+bB(aq)⇌cC (aq )+dD(aq )

The equilibrium law states that:

K c=[C ]c [D ]d

[A ]a [B ]b

This constant, Kc, is the equilibrium constant for the reaction at a specified temperature.

The letter K is used to represent all equilibrium constants. When the expression is written in terms of concentrations, we write Kc.

For example, for the equilibrium reaction:

N2 (g)+3H 2 (g)⇌2NH3 (g )K c=[NH 3]

2

[N2]1[H 2]

3

Kc is a measure of how far a reaction proceeds. If an equilibrium mixture is composed largely of reactants, then the value of Kc is small. If the equilibrium mixture is composed largely of products, then the value of Kc is large.

Units of Kc:

Worked Example -

K c=[NH3 ]2

[N2 ]1 [H 2 ]3units=¿¿

Calculations Involving Kc:

Calculate the value of Kc at 763K for the following reaction:H 2 (g)+ I 2 (g )⇌ 2HI (g )

Given the following data:[H2(g)] = 1.92 moldm-3

[I2(g)] = 3.63 moldm-3

[HI(g)] = 17.8 moldm-3

K c=[HI ]2

[ I 2 ] [H ¿¿2]=17.82

1.92×3.63=45.5 (nounits )¿

If we were to add more hydrogen to temporarily give a higher concentration and if there were no change at all then the value of Kc would become much smaller than it should be.

However, the equilibrium shifts as a result of this addition of hydrogen, the equilibrium shifts to the right.

This alters the concentrations of the I2 and HI, thus after calculation you would find that Kc will have been restored.

Altering concentrations does not affect the value of Kc.

Changing the Temperature:

An exothermic reaction:

N2 (g)+3H 2 (g)⇌2NH3 (g )∆ H=−92kJmol−1

An increase in temperature causes the equilibrium to shift to the left, in the endothermic direction, and as a result the concentration of ammonia drops and the concentration of nitrogen and hydrogen rise. Therefore Kc drops.

An exothermic reaction:

N2O4 (g)⇌2NO2 (g )∆ H=+57kJ mol−1

On increasing the temperature the equilibrium shifts to the right, in the exothermic direction, the concentration of NO2 increases and that of N2O4 decreases. Therefore Kc is higher.

Changes in temperature do alter the value of Kc.

Changes in pressure:

We already know that increasing the pressure moves the equilibrium to the side of the equation with fewer gas molecules as this tends to reduce pressure.

Decreasing the pressure moves the equilibrium to the side of the equation with more gas molecules as this tends to increase the pressure.

Even though changes in pressure affect the position of the equilibrium, the value of Kc will stay constant.

A catalyst does not affect Kc, it only alters the rate at which the equilibrium is attained.

Le Chatelier’s Principle Revised:

Position of equilibrium can be altered by changing the concentration of solutions, the pressure of gases or the temperature.

The principle states that if a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible.Concentration:

Concentration change Equilibrium shiftIncreases reactants To the right (decrease reactants)Increase products To the left (decrease products)Decrease reactants To the left (increase reactants)Decrease products To the right (increase products)

Pressure:

Pressure change Equilibrium shiftIncreasing To the side with fewer gas moleculesdecreasing To the side with more gas molecules

Temperature:

Temperature change Equilibrium change ExampleIncrease Shifts in the direction of the

endothermic reaction. ( mixture becomes darker brown)

exothermic→brown gas colourless

2NO2(g) ⇌ N2O4(g)

Endothermic ←decrease Position of equilibrium shifts in the

direction of the exothermic reaction. (mixture becomes lighter brown)

Table to show what affects Kc:

Change in Composition Kc

Concentration Changed UnchangedPressure May change UnchangedTemperature Changed ChangedCatalyst Unchanged Unchanged

Haber process:

The raw materials are air and natural gas. From these, a feedstock of nitrogen and hydrogen is made.

N2 (g)+3H 2 (g)⇌2NH3 (g )∆ H=−92kJmol−1

The usual conditions for this process are: iron catalyst, temperature of 450℃ and 200atm pressure.

These represent compromise values in order to produce a reasonable yield at an acceptable rate of attainment of equilibrium.

Increasing the temperature speeds up the rate at which equilibrium is achieved but decrease the yield.The increase in temperature means more molecules have the necessary activation energy when they collide.The yield decrease as the position of the equilibrium shifts to the left because the forward reaction is exothermic.

Increasing the pressure increases both the yield and the rate, but it is expensive (capital costs of the high pressure plant) and the running costs are high (electricity).The position of the equilibrium shifts to the right where there are fewer gas molecules; also there will be more collisions.

Using a catalyst speeds up the rate at which equilibrium is achieved.

Nitrogen Chemistry:

Group V and Nitrogen

Group V of the periodic table shares similar characteristics of the other groups of the p-block; at the top of the groups are the non-metals (nitrogen and phosphorus) and at the bottom of the group are the metalloids (antimony and bismuth).

Atoms of the Group V elements can form three covalent bonds by sharing the three unpaired p-electrons.

This gives them compounds in which the Group V element has an oxidation state of +3 or -3. The lone pair of electrons that each element has allows them to form dative bonds. This

enables them to form compounds in which their oxidation state is five. Nitrogen and phosphorous are very important elements within group five; they are

constituent elements in living things; they are both essential for healthy plant growth.

Nitrogen

Nitrogen is the most abundant gas in our atmosphere; however it is extremely unreactive. This low reactivity arises due to the strong triple bond between nitrogen atoms in

N2 molecules.

In order for nitrogen to react, the triple bond between the nitrogen atoms must be broken, or partially broken.

This requires large amounts of energy, thus the activation energy for reactions involving nitrogen is very high.

During thunderstorms, the highly energetic lightening flash can provide enough energy to form nitrogen oxides from atmospheric oxygen and nitrogen.

Once reacted, it can form many useful compounds, for example ammonia, nitrogen oxides and nitrates.

Ammonia

Ammonia is an example of a nitrogen hydride. The lone pair of electrons on the ammonia allows it to acts as a nucleophile (donating a pair

of electrons to a positively charged carbon atom), as a base (forming a dative bond with an H+ ion) and as a ligand.

Nitrogen Oxides

Nitrogen forms many different oxides, all of which are gaseous. Some of the more important ones are: Nitrogen monoxide (NO), a colourless gas which turns to brown NO2 in air. It is produced in

combustion processes, especially from internal combustion engines. It is also formed in thunderstorms, and in the soil by denitrifying bacteria.

Nitrogen dioxide (NO2) is a brown gas; it is toxic and is also formed in combustion processes and thunderstorms.

Dinitrogen oxide (N2O) is a colourless gas formed in the soil by denitrifying bacteria.

Nitrates

There are two types of nitrate ions in the nitrogen cycle: nitrate (III), NO2-, and nitrate (V),

NO3-.

Their names are distinguishable from one another by their oxidation states.

The Nitrogen Cycle:

Almost all of the nitrogen in the soil is present in complex organic molecules and so is not available to plants.

However, various processes convert the unreactive atmospheric nitrogen and organic nitrogen compounds into ammonium and nitrate ions which are available to plants.

The main processes are shown in the nitrogen cycle on the next page:

Improving food Production:

Plants need the right conditions to grow well:

Fertilisers add nutrients for plant growth. Manure adds organic matter for plant growth and soil improvement. Lime or chalk added to the soil will alter the pH of the soil. Pesticides can increase crop yields. They need to be biodegradable so they do not

accumulate in food chains. GM crops can be developed to give crops with desirable properties.

Look at F334 and F332 notes on atom economy and percentage yield.