OCR GCSE Chemistry C7

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  • Created on: 05-03-17 19:20

C7 - The Chemical Industry

Some chemicals are produced in very large quantities. These are called bulk chemicals and include sulfuric acid, ammonia, sodium hydroxide and phosphoric acid. They are used in the manufacturing of other products.

Other substances are produced in much smaller quantities and these are called fine chemicals. These include drugs, fragrances and the flavourings, preservatives and additives used in food production.

There are laws and regulations which determine how a chemical factory is run. They cover the materials used, how they are transported, stored and used, the disposing of waste, the safety of employees and the protection of the environment.

A lot of research is done to develop a new product or a new process, such as using a catalyst to produce a substance more cheaply.

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C7 - Chemical Synthesis

There are 5 stages in manufacturing a chemical such as sulfuric acid:

1) Getting and preparing the materials, called the feedstock, needed for the process.

2) Reacting the raw materials to make products. This is called synthesis.

3) Separating the useful products.

4) Dealing with unwanted by-products and wastes.

5) Checking the purity of the product.

Materials are used up in every chemical process. Most of the stages use energy. Some stages may release pollutants into the environment unless this is prevented.

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C7 - Sustainability

To be sustainable a chemical process should be able to continue without damaging the environment and without using up resources that cannot be replaced. To help bring this about there are many questions to consider.

- Is the feedstock renewable?

- What happens to by-products and waste?

- How much energy is needed?

- What are the environmental impacts?

- Are there health and safety issues?

- Are there social or economic benefits to the process?

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C7 - Atom Economy

An important factor in making a process more sustainable is the atom economy for the reaction. The atom economy is the percentage of the mass of atoms in the feedstock that end up in the useful product.

If the atom economy is 100%, there are no by-products and nothing is wasted. This makes the process more sustainable.

Atom economy = mass in useful products / mass of feedstock x 100

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C7 - Energy Changes

An exothermic reaction gives out heat to its surroundings. The temperature rises. This can be shown by a energy level diagram where the reactants have more energy than the products.

In an endothermic reaction the products have more energy than the reactants. They get this extra energy by taking heat from the surroundings. The temperature of the surroundings fall.

Bonds between atoms change during reactions. Breaking bonds is an endothermic process. Energy is used to pull the atoms apart. When new bonds form the atoms are pulled together and energy is released, so making bonds is an exothermic process.

The bonds in reactants have to be broken first before new bonds can be formed in products. This means there is an endothermic stage at the start of all reactions. The energy that is put in at the start of a reaction is called the activation energy.

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C7 - Catalysts and Enzymes

Catalysts are used to speed up reactions in the chemical industry. They make some reactions possible that would otherwise be too expensive. Enzymes also make certain reactions go faster, They are protein molecules found in cells. Now the chemical industry are using enzymes as catalysts.

Catalysts and enzymes speed up chemical reactions. They do this by providing an alternative route for a reaction. Instead of having to react with each other the reactants first react with the catalyst. This reaction has a lower activation energy than the route without the catalyst. Therefore, less energy is needed to start the reaction.

Catalysts make reactions with good atom economy possible for the chemical industry with improves sustainability. Enzymes are very sensitive to reaction conditions. If the temperature rises too high, they will become denatured. The fact that enzymes work best at low temperatures benefits the sustainability. Less energy is needed to heat up the reactants than was needed for older catalysts.

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C7 - Reacting Masses

The number printed above the symbol of the element in the periodic table is called the relative atomic mass. The relative formula mass is the mass of the molecule or formula unit of a compound.

The ratio of a reactant to a product for a reaction can be calculated from the chemical equation using the relative formula masses. It is then possible to work out the mass produced by any mass of reactant.

  1. Write the balanced chemical equation for the reaction
  2. Underline the substances in the question (any excess substance can usually be ignored)
  3. Work out the relative formula mass for each underlined substance
  4. Multiply the Mr for each substance by the number in front of its formula in the equation, if there is one
  5. Work out the ratio of reactant to product (or product to reactant) for the required situation. (You want the unknown on the top!)
  6. Multiply the ratio by the known mass in the question.
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C7 - Alkanes

A hydrocarbon is a compound of hydrogen and carbon only. One family of hydrocarbons are called alkanes. Like all hydrocarbons, they burn in plenty of air to form carbon dioxide and water. Alkanes do not dissolve in water. They do not react with reactants dissolved in water.

Alkanes have similar properties because of their similarities in structures. The bonds between the carbon and hydrogen atoms are single covalent bonds. The bonds are very hard to break, meaning there is a large activation energy needed to make the alkanes react with other substances. For this reason their reaction with substances dissolved in water are very slow. The alkanes are unreactive.

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C7 - Saturated and Unsaturated Molecules

Alkanes are said to be saturated molecules because all the bonds between the carbon atoms are single bonds. Some other molecules have double bonds between carbon atoms. These molecules are said to be unsaturated. Unsaturated compounds are more reactive because it is easier to break double covalent bonds.

Methane - CH4

Ethane - C2H6

Propane - C3H8

Butane - C4H10

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C7 - Alcohols

Methanol and ethanol are the simplest members of the alcohol group of compounds. They are not hydrocarbons because they contain oxygen atoms.

They are liquids at room temperature. Their uses include: Ethanol is used as a fuel, ethanol is used as a solvent, methanol is used as a raw material to make other substances e.g. drugs and plastics.

Alcohols burn in the air to form carbon dioxide and water. This -OH group is called the functional group of the alcohols and have higher melting and boiling points than the alkanes. They burn well in the air because they have a hydrocarbon chain before the -OH is added at the end.

A piece of sodium is placed in ethanol and it fizzes. A hydrogen gas is given off. The reaction is similar to, but slower than the reaction between sodium and water. This because water also has an -OH group. Sodium does not react with alkanes because they do not have functional group.

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C7 - Fermentation and Distillation

Fermentation is the process by which yeast produces ethanol from sugar. Yeasts are micro organisms. They use the sugar to get energy to grow and reproduce. Ethanol is produced as a waste product. Fermentation produces a dilute solution of ethanol. The solution is distilled to make it more concentrated. The ethanol boils off leaving most of the water behind. The ethanol vapour is then cooled and condenses to form a liquid. Distilled ethanol is called a spirit.

Methods of fermentating sugars and distilling ethanol industrially have developed. Alcohol drinks have become cheap and very common but ethanol is poisonous. Drinks containing ethanol make people drunk and can cause liver disease.

Fermentation makes use of enzymes produced by yeast cells. There are many different types of yeast which have enzymes that work best under slightly different conditions. Most yeast ferment sugar around 20-32degrees celcius. They are also most active at around 5PH in acidic solutions. Oxygen has to be excluded from the mixture as the reaction is an example of anaerobic respiration.

Sugar ---> ethanol + carbon dioxide

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C7 - Alternatives to Fermentation

Ethanol is a toxic compound and is poisonous to yeast as well as humans. As the ethanol concentration increases over 2 to 3 days, the yeast cells begin to die and fermentation slows. Drinks which contains a higher percentage of alcohol must have been distilled or have had distilled ethanol added to them.

Crude oil can be used as a raw material for making ethanol. Ethane, an alkane found in crude oil, is converted into ethene, which is also a hydrocarbon. Ethene then reacts with steam to form ethanol. This reaction needs a catalyst. A distillation process is used to separate the ethanol from the reactants and to purify it. The process for obtaining ethanol from ethene takes place in two stages. Ethene is formed by removing hydrogen from ethane using a catalyst and a high temperature. The reaction takes place at a high pressure aswell with phosphoric acid as a catalyst.

Converting ethene to ethanol is sustainable because there are no by products and it has a good atom economy. However, it is not sustainable because it uses crude oil which is non renewable and allot of energy is needed for conditions.


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C7 - Using Biomass for Ethanol

A lot of plant material is left over from agriculture and logging. This is called biomass. Bacteria have been genetically modified to do the job of turning biomass into useful materials like ethanol.

Biomass is largely made up of cellulose, the complex sugar that makes up plant cell walls. Genes from natural bacteria that attack cellulose have been transferred to a strain of E-coli bacteria. The biomass must be converted into a liquid by dissolving it in a solvent. Then the genetically modified e-coli bacteria begins to grow and multiply while they convert the cellulose to ethanol. This process takes place just above normal temperatures.

There have been problems in carrying out the process on a large scale so it is still not a major producer of ethanol. The only disadvantage is the chemicals needed to turn the biomass into a liquid.

Using this process has some advantages to its sustainability: It uses a renewable source, it disposes of waste material, conditions rely on little energy, high atom economy.

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C7 - Carboxylic Acids

Methanoic acid and ethanoic acid are two of a group of organic compounds called carboxylic acids. They are liquids. Methanoic acid is found in some insect and plant stings. Ethanoic acid is found in vinegar. All carboxylic acids have a sharp, acidic taste. Some carboxylic acids have unpleasant smells like old sweaty socks. Molecules of ethanoic and methanoic acid both belong to the functional -COOH group. The group is attached to a carbon chain like that in the alkanes.

Reactions that can take place from carboxylic acids:

- Ethanoic acid + METAL -----> METAL ethanoate + hydrogen

- Ethanoic acid + ALKALI -----> ALKALI ethanoate + water (neutralises with an alkali)

- Ethanoic acid + SOLVENT ---> SOLVENT ethanoate + carbon dioxide + water

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C7 - Weak Acids

There are many different types of vinegar. Some types are coloured and some are colourless, however they all contain ethanoic acid. This gives the vinegar a sharp taste. Vinegar is not harmful because it is dilute. But pure vinegar is corrosive and containers have warning signs.

What is a weak acid?

- When a piece of magnesium is put in hydrochloric acid it fizzes violently. A similar piece of magnesium in a dilute solution fizzes much more slowly even if there is the same amount of ethanoic acid present. The slower reaction shows the ethanoic acid is weaker. Also, weaker acids give off hydrogen gas when they react with metals (carbonates) much slower.

The pH of a solution is a good indicator of strong and weak acids. All acids produce H+ ions when dissolved in water. Strong acids split up completely in solution. With weak acids, only some of the molecules break up into ions so there are much fewer hydrogen ions in the solution of acid. This means the pH is lower.

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C7 - Esters

Each smell distinctive to products such as bananas are a mixture of chemicals which are called esters. This is formed when alcohols and carboxylic acids react. Esters have low melting and boiling points and they evaporate quickly at room temperature. The air carries the ester vapour meaning we are able to smell it.

A common ester is ethyl ethanoate. It has a pleasant smell and is used as a solvent. It is often a solvent for nail varnish remover and nail varnish in general. Esters similar are also used for paints, inks and other materials. Other esters are used for food flavourings. They are also often used in cheaper shampoos and shower gels. They are not used in expensive products e.g. perfumes because they can decompose on the skin.

An ester is made by combining a particular alcohol with a particular carboxylic acid.

Acid + alcohol ---> ester + water

The reaction is very slow so a catalyst is used. This is a strong acid e.g. phosphoric acid or sulfuric acid. The reactants are heated to increase speed of reaction.

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C7 - Making Esters

Synthesising ethyl ethanoate:

Ethanoic acid + ethanol --> ethyl ethanoate + water

The reaction is slow, so sulfuric acid is needed as a catalyst. Only a relatively small amount is needed however. Temperature is raised to increase speed of reaction. However, due to esters having low boiling points, it is heated under reflux. This means the mixture is boiled and the vapours cooled so they fall back to their molecules. After a period of time the ethyl ethanoate can be separated from the mixture, as in the following steps.

1) Distillation - Reaction mixture is boiled and vapours are directed into a collecting tube. Unfortunately, some water will distill over with the ethyl ethanoate. 

2) Removing acids -  Sodium carbonate reacts with the acids to give off co2 and forms salts which dissolve in the water, not ethyl ethanoate. This mixture is put in a tap funnel. shaken and allowed to settle. The ethyl ethanoate forms a layer ontop of the water which is separated from water layer.

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C7 - Making Esters (2)

3) Removing alcohol - Calcium chloride solution is added to the impure product. Any ethanol mixed with ethyl ethanoate dissolves in the calcium chloride. The mixture is put in a tap funnel and the lower layer is removed and discarded.

4) Drying - Pieces of anhydrous calcium chloride are added to the product. This absorbs any water that is still inside the product.

5) Second distillation - To separate the ethyl ethanoate from the solid drying pieces it is distilled again. The boiling point of pure ethyl ethanoate is 77degrees celsius.

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C7 - Fats and Oils

Fats and oils are very similar substances and are a special group of esters. Organisms use fat as a way of storing energy. Plants make oils as a store of energy mainly for their seeds, when the seeds become new plants they use the energy in the oil for growth. Vegetable oils are a very good source of energy for animals. Animals store spare energy from foods as fats and oils. During the summer animals may build up a layer of fat. they use it for the winter time where food is scarce. However storing too much fat can lead to you becoming overweight.

Like all esters, fats and oils are a combination of a carboxylic acid and an alcohol. The carboxylic acids are called fatty acids. The alcohol in fats and oils is called glycerol. It is because it has 3 -OH groups. This means that fats and oils have three fatty acid chains attached to the glycerol molecule.

The fatty acids that make up most vegetable oils are unsaturated. This means they contain some carbon to carbon double bonds. Most of the fatty acids that make up animal fats are saturated. They all have single bonds. Therefore, oils are thought to be healthier but too much consumption of any two can lead to obesity.

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C7 - Reversible Reactions

Most chemical reactions go in one direction. Some also go backwards, we say thse reactions are reversible. If you keep all the substances in a closed container, after a short amount of time the rate at which the solid is decomposing is the same as the rate at which the two gases are combining. (an example) This reaction reaches a state of equilibrium. The symbol ⇄ shows not only the reaction is reversible but that the reaction can reach equilibrium, when the amounts of reactants and products will each stay the same.

Even at equilibrium the forward amd backward reactions are still going on. This is called dynamic equilibrium. They are happening at the same rate so reactants are being formed as fast as they are being used up. 

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C7 - The Haber Process

Nitrogen is the most important element needed by plants to help them grow. There is plenty of nitrogen in the air but plants cannot use nitrogen in the form of gas due to it being unreactive.

Nitrogen + hydrogen --> ammonia

The ammonia produced is reacted with nitric acid to make the ammonium nitrate used in fertilisers. Factories make ammonia using the haber process. Nitrogen is taken from the air. Hydrogen is produced by reacting methane (natural gas) with steam.

The reaction used in the haber process is exothermic, slow and reversible. The reaction is sped up by using an iron catalyst. A high temperature and pressure is also used to increase speed of the reaction. A high temperature and pressure also affects the amount of ammonia formed when the reaction reaches equilibrium. Higher pressure squeezes the molecules together, producing more ammonia. Heating the mixture encourages endothermic backwards reaction, reducing ammonia production. The gases do not spend enough time in the reaction vessel to reach equilibrium. This is not a problem because left over reactants are recycled through the process and none is wasted.

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C7 - Alternatives to Haber Process

The choice of the conditions for the haber process reaction is a compromise between yield, speed and cost. Using a high temperature speeds up the reaction but produces less ammonia and would cost more fuel. High pressure helps the speed but it has a high maintaining cost and there is a risk of leaks.

Artificial fertilisers have increased the yield of crops, but they do cause problems. They can be washed into rivers and encourage algae growth. Bacteria reproduce under ideal conditons under the algae taking in oxygen from the water. This leads to fish and other animals being unable to breathe. Organic fertilisers such as manure are better for crops. They are longer lasting and are less likely to be washed away.

Bacteria that live in the soil and in the roots of some plants naturally 'fix' nitrogen from the air. The bacteria produce special enzymes that catalyse the reaction of nitrogen from the air. Producing copies of these enzymes may make it easier to produce fertilisers cheaply, at room temperature and average pressure. Alternatively, crops may be genetically modified so that either they produce the enzymes to fix nitrogen or they allow bacteria to live in the roots. No fertilisers needed.

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C7 - Paper and Thin-Layer Chromatography

In any form of chromatography there are two parts. There is a stationary phase which most of the time is paper. Then there is a part that moves called the mobile phase, which carries the sample along.

Paper chromatography is done with sheets of special paper similar to filter paper. The paper fibres have a coat of water. This is the stationary phase. In thin-layer chromotography which is mainly used for analytical purposes, a plate of glass coated with silica or alumina is the stationary phase. Spots of the sample are put on a line about from 1 or 2cm from the end of the plate or paper. The solvent slowly rises up the stationary phase, different substances travel different distances.

The Rf value of a substance is a measure of its position on a chromatogram.

Rf value = distance moved by sample/distance moved by solvent

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