AQA GCSE Chemistry Unit 3

  • Created by: Georgia
  • Created on: 15-05-13 15:05

The Early Periodic Table

During the 19th century, elements were classified on their properties and atomic weights.

Newlands (1864)
- Order of relative atomic mass.
- Followed the law of octaves where every 8th element seemed similar.
- Assumed all elements had already been discovered so didn't leave any gaps.
- Metals and non-metals were mixed and some elements were put in the same place.
- Ideas were not accepted by others.

Mendeleev (1869)
- Placed them in order of atomic mass.
- Arranged so that physical and chemical properties could be seen.
- Left gaps for undiscovered elements and predicted their properties.
- Not accepted as non-metals and metals in same group.
- Accepted because some discovered missing elements matched predictions.
- Basis for modern periodic table.

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The Modern Periodic Table

Elements arranged in proton numbers rather than atomic masses.
Groups of elements have similar chemical properties as they have the same number of electrons in their outer shells.

Metals react by losing electrons and reactivity increases as you go down the group.
Non-metals react with metals by gaining electrons and reactivity decreases as you go down the group.

The protons that have a positive charge in the nucleus attract the electrons which holds it in place. Larger atoms lose electrons more easily and gain electrons less easily.

An increased distance means the electron is getting further away from the nucleus
An increased shielding means there are inner electrons getting in the way
Both an increase in distance and shielding means...
= there's less attraction between the nucleus and electron
= a higher energy level electron is easily lost
= less likely to gain an electron

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Trends Within The Periodic Table

Alkali Metals (Group 1)
- Low melting and boiling point as you go down the group.
- React with water to produce hydrogen - alkaline solution containing the metal hydroxide.
- Form 1+ ions in reactions to make ionic compounds. Usually white and dissolve in water to give colourless solutions with a high pH.
- Reactivity increases going down the group.
- Low densities / Soft.

Halogens (Group 7)
- Form 1- ions in ionic compounds with metals.
- Form covalent compounds by sharing electrons with other non-metals.
- More reactive halogens can displace less reactive halogens from a solution of one of its salts.
e.g. Bromine displaces iodine because it is more reactive. Chlorine would displace both.
- Reactivity decreases going down the group.
- High melting and boiling points going down the group.
- Poor conductors of energy and electricity.

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Transition Elements

Found between groups 2 and 3.

- Good conductors of heat and energy.
- Hard and strong.
- High densities.
- High melting points.
- Less reactive.
- Can form ions with different charges that are often coloured.
- Make good industrial catalysts.

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Hard Water

Hard water is water that doesn't lather up easily with soap. This is because it contains dissolved compounds such as calcium and magnesium salts that react with the soap to form a precipitate called 'scum'.
However it may not be a problem when washing clothes because modern 'soapless detergents' do not produce scum in hard water.

One type of hard water can produce a 'scale' (calcium carbonate) when heated, reducing the efficiency of heating systems and kettles as it blocks the pipes. This is because scale is a thermal insulator (poor conductor of energy) so it takes longer to heat things up.

Pros: Calcium ions in drinking water help in the delvelopment of strong bones and teeth.
         It may also help to prevent heart disease.

Cons: Expensive because you need to use more soap.

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Removing Hardness

Soft water does not contain ions that produce scum or scale so hard water can be softened by removing these ions.

Temporary hardness = removed from water by heating it.
= This is because they contain hydrogen ions.
- However this could waste energy and be very expensive.

Permenant hardness = not changed by heating.

Two ways of softening any type of water:
- Adding sodium carbonate > reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate.
- Using commercial water softeners e.g. ion-exchange columns containing hydrogen or sodium ions > replace the calcium and magnesium ions when hard water passes through the column.

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Water Treatment

To make water fit to drink:
- Choose an appropriate source.
- Screened = passed through metal bars to remove twigs and leaves.
- Settlement tank = sand and soil settle out.
- Aluminium sulfate and lime = small dirt particles clump together and fall to the bottom.
- Filtered = Water is passed through a filter made of fine sand to remove remaining dirt.
- Chlorine = added to kill any of the remaining harmful bacteria.
- pH = corrected so it's neutral, then stored and pumped into homes.

Filter jugs at home (improve taste):
- Carbon = reduces levels of chlorine as well as pesticides and other organic impurities.
- Ion-exchange resin = removes calcium, magnesium, lead, copper and aluminium ions.
- Silver particles = discourage the growth of bacteria within the filter.

We can make pure water by distillation (boiling) but this requires large amounts of energy which makes it expensive.

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Water Issues

Chlorine = Added to water to steralise it by killing microbes.

Fluoridation of water

- Helps to improve dental health

FOR: - Nobody has proved there are any harmful effects (apart from flurosis).
         - Reduction in cavities on teeth as bacteria causing tooth decay are dying out.
         - Protects teeth of those with bad dental hygiene habits.
         - Fluoride is only added in tiny amounts anyway.
         - Bacterica associated with tooth decay could also cause heart disease.

AGAINST: - Teeth reflects bones > Flurosis could be a sign of the weakening of bones.
                - Benefit of flouridation for teeth is not significant.
                - Ethically wrong to give treatments without consent. Right to choose.
                - Excess fluoride effects the brain e.g. learning difficulties.
                - Can't set safe limits because you can't control people's intake.

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Energy Changes

When fuels and food react with oxygen, they release energy in an exothermic reaction. This energy is used to keep warm or for transport. Not all fuels release the same amount of energy when they burn. The energy released by an exothermic reaction heats up the water and its container. The temperature rise is proportional to the amount of energy released.

The actual energy released is related to the rise in temperature of the water in a calorimeter.
Measurement using a simple calorimeter are not accurate because of energy losses.
They can be used to compare the amounts of energ released.

Energy is measured in Joules (J):
energy released = mass of water heated x specific heat capacity of water x temperature rise
Q = mc^t

We can also use this equation to calculate the energy change for reactions in solution by measuring the temperature change. Neutralisation and displacement reactions are both example of reactions that we can use this technique for.
Reduce energy transfer to surroundings by insulating container e.g. polystyrene or a lid.

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Energy Level Diagrams

- Energy is taken in for the surroundings.
- Products are a HIGHER energy level than reactants.
- Temperature of surroundings DECREASES.

- Energy is released to suroundings.
- Products are at a LOWER energy lever than reactants.
- Temperature of surroundings INCREASES.

Activation energy = energy needed to start a reaction.

Catalysts = increase rate of reaction

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Fuel Issues

Consequences of burning fuels:
- Supplies of fossil fuels running out.
- Increase carbon dioxide emissions.
- Contributed to global warming.

Hydrogen can be used as a fuel in combustion engines.
hydrogen + oxygen > water
- No carbon dioxide emissions.
- Safety and storage issues.
- Can't be supplied using electrolysis as produced carbon dioxide and uses resources.

Hydrogen can also be used in fuel cells that produce electricity to power engines.
- Constant supply of hydrogen needed.
- Need to match performance, convenience and price of petrol and diesel cars.
- No carbon dioxide.
- More efficient

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Tests For Positive Ions

Flame tests = used to identify metal ions.
- Put compound on nichrome wire loop and hold loop on roaring blue flame.

lithium > crimson
sodium > yellow
potassium > lilac
calcium > red
barium > green

Sodium hydroxide
- Forms precipitates.

aluminium, calcium, megnesium > white
copper (||) > blue
iron (||) > green
iron (|||) > brown

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Tests For Negative Ions

Carbonates = react with dilute acids to form carbon dioxide.
- Carbon dioxide produces a white precipitate with water. This turns limewater cloudy.
- acid + carbonate > salt + water + carbon dioxide

Halides (chloride, bromide,iodide)
- Add dilute nitric acid to remove any carbonate ions before silver nitrate and precipitate is produced.

Silver chloride = white
Silver bromide = cream
Silver iodide = yellow

- Add hydrochloric acid before barium chloride.
- White precipitate is formed if sulfate is present.

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Titrations are used to find out exactly how much acid is needed to neutralise a quantity of alkali OR the concentrations of solutions.

A PIPETTE is used to measure out a fixed volume of solution.
A BURETTE is used to measure the volume of solution added. This adds the acid gradually to the mixture.

The point at which an acid-alkali reaction is complete is called the end point of the reaction.
We use an indicator to show this as it will change colour when the reaction is complete.

1dm3 = 1000cm3

In a reversible reaction, the products of the reaction can re-act to reform the original reactants. In a closed system, the rate of the forward and reverse reactions are equal at equilibrium. Changing the reaction conditions can change the amount of products and reactants in a reaction mixture at equilibrium. We need conditions that give as much product as possible.

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Chemical Equilibrium

Pressure and temperature can affect reversible reactions involving gases at equilibrium.
Changing the pressure effects the equilibrium only if there are different numbers of molecules of gases on each side of the equation.

If the forward reaction produces more molecules of gas...
... an increase in pressure decreases the amount of products formed.
... a decrease in pressure increases the amount of products formed.
If the forward reaction produces fewer molecules of gas...
... an increase in pressure increases the amount of products formed.
... a decrease in pressure decreases the amount of products formed.

If the forward reaction is exothermic...
... an increase in temperature decreases the amounts of products formed.
... a decrease in temperature increases the amount of products formed.
If the forward reaction is endothermic...
... an increase in temperature increases the amount of products formed.
... a decrease in temperature decreases the amount of product formed.

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Plants need nitrogen to grow and ammonia is an important chemical for making other products including fertilisers.

The Haber Process
- A way of turning nitrogen in the air into ammonia.

The raw materials for making ammonia are:
1. Nitrogen in the air.
2. Hydrogen from natural gas (containing methane CH4)
- The nitrogen and hydrogen are purified.
- They are passed over an iron catalyst at high temperatures (450 C)
- The pressures are about 200 atmospheres.
- The conditions are chosen to give a reasonable yield of ammonia as quick as possible.
- The product of this reversible reaction is ammonia.
- We remove the ammonia by cooling the gases so the ammonia liquifies.
- Any unreacted nitrogen and hydrogen are recycled back into the Harbour Process so they have the chance to react again.

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Economics of the Harbour Process

Effect of Pressure:
- Uses a pressure of around 200 atmospheres to increase the amount of ammonia produced.
- Higher pressures would produce more ammonia BUT they would make the chemical plant too expensive to build and run.

Effect of Temperature:
- Temperature is 450 C.
- A lower temperatire would increase the yield of ammonia but it would be produced too slowly.

A catalyst speeds up the reaction so ammonia is formed more quickly BUT does not affect the yeild of ammonia.

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Alcohols are used as solvents, fuels and of course in alcoholic drinks.

The homologous series of alcohols contain the -OH functional group.

Methanol, ethanol, and propanol:
- dissolve in water to form a neutral soution.
- react with sodium to produce hdrogen.
- burn in air.

Etanol can be oxidised to ethanoic acid, either by chemical oxidising agents or microbial action. Ethanoic acid is the main acid in vinigar.

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Carboxylic Acids

Ethanoic acid is a member of the carboxylic acids.

The homologus series of carboxylic acids contain the -COOH functional group.

Solutions of carboxylic acids have a pH value less than 7.

Aqueous solutions of weak acids have a higher pH value than solutions of strong acids with the same concentration.

Carboxylic acids:
- dissolve in wanter to produce acidic solutions.
- Carbonates gently fizz in their acidic solutions releasing carbon dioxide gas.
- react with alcohols in the presense of an acid catalyst to produce esters.
- do not ionise completely when dissolved in water and so are weak acids.

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Ethyl ethanoate is the ester produced from ethanol and ethanoic acid.

Esters are made by reacting a carboxylic acid and an alcohol together with an acid catalyst.

They have the functional group -COO-.

They a volatile compounds with distinctive smells and are used in flavourings and perfumes.

In the future, the use of biofuels such as ethanol and esters, could help society as crude oil supplies run out.

Future used of biofuels might conflict with the need to feed the world as the land used for biofuel crops could be used for food crops.

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