C6 Chemical Synthesis

C6 of the 6's exam

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Industrial Chemical Synthesis

The Chemical Industry makes useful products which have all been researched, formulated and tested by chemists.

  • Food additives
  • Cleaning and decorating products
  • Drugs- pharmaceutical industry
  • Fertilisers

Chemists have to work out how much to make of the chemicals, a way of producing heighest yeild with the lowest effect on the environment.

The chemical industry is huge. The chemicals can be produced on a large or small scale, but the scale of production doesnt mean they're is less improtant if produced on a small scale e.g. Sulfuric acid- used for loads of things so is produced on a huge scale. Pharmaceuticals are produced on a small scale, but its the largest share of industry. (page 69)

Pharmaceuticals- 37%   Other- 28%   Toiletries and cleaning- 12%  Plastic and rubber- 8%   Paint 8%   Agrochemicals- 3%  Dyes- 2%  Fertilisers- 1%  Fibres- 1%

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Acids and Alkalis

There's a sliding scale from very strong acids (pH0- red) to very strong alkali's (pH14- purple). Neutral (pH7- green).

Pure acid compounds can be:

  • Solids- citric acids
  • Liquids- nitric acids, sulfuric acids and ethanoic acids (acid in vinegar)
  • Gases- hydrogen chloride

Common alkali's:

  • Sodium chloride- used in bleach
  • Potassium hydroxide- used in alkaline batteries
  • Calcium hydroxide- used to neutralise acidic soils

Neutralisation reaction: Acid + Alkali = Salt + Water

Acidic compounds form aqueous hydrogen irons, H+(aq), in water.
Alkaline compounds form aqueous hydroxide irons, OH-(aq), in water.

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Acids reacting with Metals

Acid + Metal = Salt + Hydrogen       Hydrogen present- burning splint = squeeky pop

The more reactive the metal, the faster the reaction will go. Sodium will react explosively.

Copper doen't react with dilute acids as its less reactive than hydrogen.

The speed of a reaction is indicated by the rate at which the bubbles of hydrogen are given off.

Hydrochloric acid will always produce chloride salts.

e.g. HCl(aq) + Mg(s) = MgCl2(aq) + H2(g)

Sulfuric acid (H2SO4) will always produce sulfate salts.

e.g. 3H2SO4(aq) + 2Al(s) = Al(SO4)3(aq) + 3H2(g) 

Nitric acid (HNO3) will produce nitrate salts when neutralised.

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Oxides, Hydroxides and Carbonates

Metal oxides and metal hydroxides react with acids.

Neutralisation reactions:

Acids + Metal Hydrioxide = Salt + Water
Acid + Metal Oxide = Salt + Water
Acid + Metal Carbonate = Salt + Water + Carbon Dioxide

The combrination of metal and acid decides the salt.

Sulfate ion is SO4 2-
Hydroxide ion is OH -
Carbonate ion is CO3 2-

Hydrochloric acid + Sodium carbonate = Sodium chloride + Water +Carbon dioxide
2HCl(aq) + Na2Cl3(s) = 2NaCl(aq) + H2O(l) + CO2(g)

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Synthesising Compounds

There are 7 stages involved in chemical synthesis:

1. Choosing the reaction:
- Neutralisation- an acid and alkali react together to make a salt.
- Thermal Decomposition- Heat used to break up the compound into smaller substances.
- Precipitation- an insoluble solid formed when 2 solutions are mixed.
2. Risk assessment:
- Assessment of anything in the process that could cause injury.
- Involves- Identifying hazards, Assessing who might be harmed and what could reduce risks.
3. Calculating the quantities of reactants so you know how much raw material is needed so there is no waste- waste costs money.
4. Choosing the apparatus and conditions
5. Isolating the product- separating the products from the reaction mixture. This could be done by evaporation, filtration of drying.
6. Purification- crystallisation can be useful in the purification process
7. Measuring yield and purity- The yeild tells you the overall success of the process, the purity of the chemical has to be measured.

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Relative Formula Mass and Calculating Masses in Re

Relative atomic mass is the mass on the periodic table.

Relative atomic mass of a compound is just all the atomic masses added together (and not forgetting if there are more than 1 to multiply!)

Calculating masses in reactions:

1. Write out the balanced equation
2. Work out the relative formula mass- just for the 2 part you want.
3. Divide to get 1, then multiply to get all

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Isolating the Product

Isolating the product and purification use similar techniques.

Filtration- used if the product is an insoluble solid that needs to be separated from a liquid reaction mixture e.g. Pharmaceutical industry uses it to separate out aspirin.
- Can be used for purification e.g. spreading out solid impurities in the reaction mixture.

Drying- used to remove excess liquid.
- The product can be dried in a drying oven. Some simply heat, but others blow hot, dry air over the product.
- Also can be dried using desiccators- containers that contain chemicals like silica gel that remove water from the surroundings.

Evaporation and Crystallisation- Heating the solution causes the solute to evaporate leaving behind solid crystals of the product.
- Also used for purifying the product. The crystals have a regular structure that the impurities cant fit into.
- The process is repeated to improve the purity- recrystallization.

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Measuring Yeild

Percentage yield compares actual and theoretical yield.

Actual yeild- This is the mass of pure, dry product- calculated by weighing the dried product.

Theoretical yield- The maximum possible mass of pure product that could have been made.

Percentage yield- The actual yield of the product as a percentage of the theoretical yield.

% yield= (Actual yield(g) / Theoretical yield(g)) x 100

% yeild always less than 100% as some product lost along the way e.g. during purification and drying.

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Titration's are carried out using a burette and can be used to check the purity of acidic or alkaline products, they work using neutralisation reactions.

Titration's cant be carried out with solids- only liquids. So the solid product being tested needs to be made into a solution.

1. Crush solid into a powder.
2. Weigh out amount needed.
3. Add a solvent e.g. water or ethanol to dissolve the powder.
4. Swirl the titration  flask until all solid has dissolved.

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Some products need to be very pure.

The purity of a product will improve as it's being isolated.

To get a really pure product, earlier stages (filtration, evapouration and crystalisation) often need to be repeated.

Purification and measuring are important steps in the chemical industry..... Pharmaceuticals- drugs intended for human consumption need to be free from impurities- otherwise could do more harm than good.

Titration can be used to measure the purity of a substance.

(Read page 78)

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Rates of Reaction

The rate of a chemical reaction is how fast the reactants are changed.
- One of the slowest reaction is the rusting of iron and chemical weathering.
- Moderate speed- a metal reacting with an acid to produce a gentle stream of bubble.
- Burning is a fast reaction.

Controlling the rate of reaction is important in industry for safety and economic reasons: higher temp= bigger fuel bills  Faster rate= more product in less time.

An increased rate of reaction can be due to:

  • Increase in temperature. 
  • Increase in concentration.
  • Catalyst added.
  • Bigger surface area of reactant.
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Collision Theory

More collisions increases the rate of reaction.

This is increased by:
- Temperature....is increased = particles move faster so there will be more collisions and the particles will have more energy to make reaction happen.

- Increasing concentration....more concentrated means more particles of reactant in the same volume of water (or solvent)- this makes collisions more likely.

- Increasing the surface area....means the particles around the reactants will have more area to work on so there will be more collisions.

- Using a catalyst....is a substance which increases the speed of reaction, without being changed or used up in the reaction. Works by giving the reacting particles a surface to stick to where they can bump into each other.

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Measuring Rates of Reactions

Rate of reaction = Amount of reactant used or amount of product formed / Time

3 ways to measure the speed of a reaction:
1. Precipitation- When the product of the reaction clouds the solution.
- You observe a mark (normally a cross) through the solution and measure how long it takes to disappear (due to the solution going cloudy)
- Faster the mark disappears = quicker the reaction.
- Only works when the initial solution is see-through.
- People may not agree when the exact point the cross disappears.

2. Change in mass- measuring the speed of a reaction that produces a gas can be carried out using a mass balance.
- As the gas is released, the mass disappearing is easily measured on the balance.
- The quicker the reading drops, the faster the reaction.
- Rate of reaction graphs are easy to plot from this method.
-Most accurate.
- Negative- gas released into the room.

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Rate of Reactions Cont.

3. The volume of gas given off- involves the use of a gas syringe to measure the volume of gas given off.
- More gas given off during a given time interval, the faster the reaction.
- Quite accurate
- Have to be careful though- if the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe.

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