Chemistry Unit 3

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The periodic table

The early periodic table

Newlands' table put the elements in order of atomic mass but failed to take account of elements that were unknown at the time.

Mendeleev's periodic table  left gaps for the unknown elements, and so provided the basis for the modern periodic table.

The modern periodic table

The atomic (proton) number of an element determines its position in the periodic table.

The number of electrons in the outermost shell (highest energy level) of an atom determines its chemical properties

The group number in the periodic table equals the number of electrons in the outermost shell

We can explain trends in reactivity as we go down a group in terms of the distance between the outermost electrons and the nucleus and the nuber of occupied inner shells (energy levels) in the atoms.


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Group 1 - The alkali metals

Melting points and poiling points decrease going down the group.

All react with water to produce hydrogen and an alkaline solution containing the metal hydroxide.

They form 1+ ions in the reactions to make ionic compounds, These are generally white and dissolve in water, giving colourless solutions.

The reactivity of the alkali metals increases going down the group.

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The transition metals

Compared with the alkali metals, transition metals have much higher melting points and densities. They are also stronger and harder, but are much less reactive. They are good conductors of electricity and energy.

The transition elements do not react vigorously with oxygen or water.

A transition element can form ions with different charges, in compounds that are often coloured.

Transition elements and their compouns are important industrial catalysts.

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Group 7 - The Halogens

The halogens all form ions with a single negative charge in their ionic compounds with metals

The halogens form covalent compounds by sharing electrons with other non-metals

A more reactive halogen can displace a less reactive halogen from a solution of one of its salts.

The reactivity of the halogens decreases going down the group.

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Hard water

Hard water is water which contains dissolved compounds such as calcium and magnesium salts.

The calcium and/or magnesium ions in hard water react with soap producing a precipitate called scum.

One type of hard water can produce a solid (lime)scale when it is heated, reducing the efficiency of heating sytems and kettles.

Hard water is better than soft water for developing and maintaining teeth and bones. It may also help to prevent heart disease.

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Removing hardness

We can soften hard water by removing the calcium and magnesium ions.

Temporary hardness: Water that can have it's hardness removed by boiling it (type of hard water that causes scale)

Permanent hardness: Some ions from salts are not removed by heating, theis solutions cause permanent hard water but it can still be softened.

Using washing soda

When spdium carbonate is added to hard water a reaction takes place. Its soluble carbonate ions precipitate out calcium and magnesium ions forming insoluble carbonates. (Reaction similar to when scale is formed however this is quick and happens on demand)

Using an ion-exchanging column

A column usually contains a resin packed with sodium ions (sometimes hydrogen ions) which are swapped for the calcium and magnesium ions as the hard water rund through the column.

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Water treatment and issues

Water for drinking should contain only low levels of dissolved substanes and microbes.

Water is made fit to drink by filtering it to remove solids and adding chlorine to reduce the number of microbes.

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

Chlorine is added to water to sterilise it by killing microbes.

Fluoride helps to improve dental health however some argue against the fluoridation of public water supplies. For example, they think that people should have the right to choose if they want to take extra fluoride or not.

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Energy released by fuels

Energy released=mass of water heated x specific heat capacity of water x temperature rise

When fuels and food react with oxygen, energy is released in an exothermic reaction.

A simple calorimeter can be used to compare the enrgy released by different fuels or different foods.

We can calculate the energy change for reactions in solution by measuring the temperature change and using the equation Q=mc/\T

Neutralisation and displacement are examples of reactions that we can use this technique for.

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Calculations using bond energies

In an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken.

In an endothermic reaction, the energy released when new bonds are formed is less than the energy absorbed when bonds are broken.

We can calculate the overall energy change in a chemical reaction using bond energies.

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Tests for positive and negative ions

Positive ions:

  • Flame tests - dip nichrome wire loop in metal compound (after cleaning and dipping in hydrochloric acid)
  • Reaction with sodium hydroxide - aluminium, calcium and magnesium ions all form a white precipitate. (However with aluminium, the precipitate will dissolve if more sodium hydroxide is added - this isn't true with calcium and magnesium.)           Copper makes a blue precipitate, iron(ii) makes green and iron(iii) makes brown

Negative ions:

  • Carbonates - a dilute acid added to a carbonate will fizz and produce CO2. In lime water, the carbon dioxide reacts with calcium hydroxide to make a white precipitate (cloudy).
  • Halides - To test for bromide, chloride and iodide, add dilute nitric acid (to remove cabonate ions) then silver nitrate solution to see if a precipitate forms. Chloride gives white, Bromide gives cream and iodide ions give a pale yellow precipitate
  • Sulfates - First add dilute hydrochloric acid (to remove carbonate ions) then barium chloride solution (a white precipitate - insoluble salt, barium sulfate - means sulfate ions are present)
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Titrations

Titration is used to measure how much of an acid and an alkali is needed to react completely. Adding and acid to an alkali is a neutralisation reaction.

The end point of a reaction is the exact point at which the acid-alkali reaction is complete. (We use an indicator to show this)

To calculate the concentration of a solution, given the mass of solute in a certain volume:

1. Calculate the mass (in grams) of solute in 1cm3 of solution.

2. Calculate the mass (in grams) of solute in 1000cm3 of solution

3. Convert the mass (in grams) to moles.

To calculate the mass of solute in a certain volume of solution of known concentration:

1. Calculate the mass (in grams) of the solute there is in 1dm3 (1000cm3) of solution

2. Calculate the mass (in grams) of solute in 1cm3 of solution

3. Calculate the mass (in grams) of solute there is in the given volume of solution

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Altering conditions (reversible reactions)

Pressure can affect recersible reactions involving gases at equilibrium. Increasing the pressure favours the reaction with the least number of molecules of gas formed. Decreasing the pressure favours the reaction with the greater number of molecules of gas formed.

We can change the amount of products formed at equilibrium by changing the temperature at which we carry out a reversible reaction. Increasing the temperature favours the endothermic reaction. Decreasing the temperature favours the exothermic reaction.

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The Haber process

Ammonia is an important chemical for making products, including fertiliser.

Ammonia is made of nitrogen and oxygen in the Haer process.

We carry out the Haber process under conditions which are chosen to give a reasonable yield of ammonia as quickly as possible.

Any unreacted nitrogen and hydrogen are recycled in the Haber process.

The Haber process uses a pressure of around 200 atospheres to increase the amount of ammonia produced. Although higher pressures would produce more ammonia, they would make the chemical plant too expensice to build and run.

A temperature of about 450(degrees)C is used for the reaction. Although lower temperatures would increase the yield of ammonia, it would be produced too slowly.

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Alcohols

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

Alcohols are used as solvents and fuels, and ethanol is the main alcohol in alcoholic drinks.

Alcohols burn in air, forming carbon dioxide and water.

With sodium metal, alcohols react to form a solution, and hydrogen gas is given off.

Ethanol can be oxidised to ethanoic acis, either by chemical oxidising agents or by the action of microbes. Ethanoic acid is the main acid in vinegar.

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Carboxylic acids and esters

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

The homologous series of esters contain the -COO- functional group.

Solutions of carboxylic acids have a pH value less than 7. Carbonates gently fizz in their acidic solutions, releasing carbon dioxide gas.

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

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

Esters are volatile compounds used in flavourings and perfumes.

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