History of the Periodic Table
- In the Early 1800s, they could only go on Atomic Mass.
- Before, there was only two ways to categorise elements by:
1) Their physical and chemical properties 2) Their Relative Atomic Mass
- They had no idea of atomic structure or of protons or electrons(there was no atomic number). It was only in the 20th century, that they discovered protons and eletrons.
- The only thing they could measure was Relative Atomic Mass. When they arranged the Periodic table in this way, they noticed properties in the elements.
- Newland's Law of Octaves was able to arrange things more usefully in 1864. He noticed that every 8th element had similar properties. He was getting close but did not leave gaps for undiscovered elements. He was criticised because some elements didn't have similar properties. He also mixed up metals with non-metals.
- Dmitri Mendeleev left gaps and predicted New Elements. He arranged the elements in order of their atomic mass, and was able to leave gaps so the elements with the similar properties were in the same vertical groups. New elements fitted into his table.
The Modern Periodic Table
- After Mendeleev's work, the Periodic Table became more useful. It was able to predict properties of elements. In the late 19th century, scientists discovered protons, neutrons and electrons.
- The Modern Periodic Table is based on Electronic Stucture. The periodic Table is arranged in the order of atomic (proton) numbers. All the elements were put in groups.
- You can use the Periodic Table to work out the detailed arrangements of electrons.
- Electrons in an atom are set out in shells which correspond to an energy level.
- The maximum number of electrons that can occupy in each energy level is given by the simple formula 2 X n^2 where n is the number of energy levels.
- Elements in the same group have the same number of electrons in their highest occupied energy level. (Group 6 all have 6 electrons in their highest energy level)
- The Positive charge of the nucleus attracts electrons and holds them in place.
- The further away the electron from the nucleus is, the less the attraction.
- The attraction of the nucleus is even less when there are a lot of inner electrons. They get in the way of the nuclear charge, reducing the attraction. This effect is known as shielding.
- The combination of the two means that an electron in a higher energy level is more easily lost. Group 1 metals get more reactive as you go down the group.
- It also means that it is less likely to gain an electron. Group 7 elements get less reactive as you go down the group.
Group 1 - Alkali Metals
- They're all silvery solids.Their hydroxides dissolve in water to give an alkaline solution.
- As you go down the group, the alkali metals become: bigger atoms(extra full shell), more reactive(outer electron is lost more easily), higher density(atoms have more mass, the first three are less dense than water), lower melting & boiling points.
They are Lithium,Sodium,Potassium,Rubidium,Caesium and Fracium.
- Alkali metals need to be stored in oil.
- They must be handled with Forceps(they burn the skin).
- They all have one outer electron and they form a 1+ ion when they lose it.
- They always form ionic compounds.
- They react vigorously in water. They move around the surface and fizzes. The pH indicator turns purple.
- They produce Hydrogen(Potassium gets hot enough to ignite). A lighted splint will indicate hydrogen(squeaky pop sound).
- They form hydroxide in soution, i.e aqueous OH- ions.
Group 7 - The Halogens
- As you go down the group, the Halogens become less reactive and have higher melting & boiling points.
- They are non-metals with coloured vapours. Flourine is a poisonous yellow gas. Chlorine is a poisonous dense green gas. Bromine is a poisonous red-brown volatile liquid. Iodine is a dark grey crytalline solid or a purple vapour.
- They all form molecules which are pairs of atoms.
- They do both ionic and covalent bonding. They form 1- ions when they bond with metals. But they form covalent bonds with non-metals.
- Halogens react with metals to form Salts . They react with most metals including iron and aluminium to form metal halides.
- The more reactive Halogens will displace the less reactive.
- They are the typical metals in the middle of the periodic table. They have the properties which you would expect from a 'proper' metal.
- They're good conductors of both heat and electricity.
- They're dense, strong and shiny.
- They are less reactive than Group 1 metals. They don't react with water very much or oxygen.
- They are harder, denser and stronger than Group 1 metals. They have a much higher melting point.
- Transition metals usually have more than one ion. The different ions form different-coloured compounds too.
- The compounds are very colourful due to the ion they contain.
- Transition Metals and their compounds all make good Catalysts.E.g. iron used in the Haber process for making ammonia.
- Their properties are due to the way their electron shell fills up.
- In an atom, as you get further away from the nucleus, energy levels get closer together until they start to overlap. E.g. potassium has 19 electrons but the last electron goes in the 4th shell and not the 3rd.
- First energy shell holds 2, second shell holds 8, third shell holds 18(but once it reaches 8 electrons, 2 goes into the fourth shell before continuing in the third shell).
Acids and Alkalis
- Arrhenius said Acids release Hydrogen Ions in water. This idea worked well but it only worked for acids and bases hat dissolved in water. There was too many exceptions which the hypothesis couldn't explain.
- Lowry and Bronsted said acids are Proton Donors. They came up with definitions, Soluble and Insoluble bases: Acids releases H+ ions (proton donors) and Bases accept H+ ions (proton acceptors). Their ideas were readily accepted because it explained the behaviour of acids and bases in solvents other than water. Also, they were an adaptation of an idea which already kinda of worked.
- Protons are Hydrated in water. For a substance to act as an acid or base, you usually need water. This is what happens:
In acidic solutions: The acid molecules dissociate, releasing a lot of H+ ions. These H+ ions(protons) become hydrated(surrounded by water molecules). The Protons are now given the name 'Hydrated protons' and can be represented by 'H+(aq)'. And it's these hydrated protons that make acids acidic.
In base solutions: Water molecules dissociate into H+ and OH- ions, although they almost never do in pure water. But, some base molecules, like ammonia can take hydrogen ions from water, causing more molecules to dissociate, and leaving an excess of OH- behind. Other bases like potassium hydroxide(KOH), releases hydroxide ions straight into the solution.
Acids, Alkalis and Titrations
- Strong acids ionise almost completely in water. This means almost every hydrgon atom is released to become a hydrated proton(loads of H+(aq) ions).
- The 'reversible reaction' symbol is used for weak acids.
- The pH of an acid or alkali is a measure of the concentration of H+(aq) ions in a solution. Strong acids typically have a pH of 1 or 2. A weak acid might be 4, 5 or 6.
- The pH of an acid or alkali can be measured with a pH meter or with universal indicator.
- There are strong and weak alkalis too. The hydroxides of sodium and potassium are strong, typically pH 13 or 14. Whereas ammonia solution is weak alkali with pH 9-10.
- A titration is used to find out exactly how much acid is needed to neutralise the alkali (or vice versa).
- You put some alkali in a flask and add an indicator depending on the strengths of the acids and alaklis.
- Phenolphtalein is used for a weak acid and strong alkali. Methyl orange is used for a strong acid and a weak alkali.
- If both the acid and alkali are strong then any acid-base indicator can be used.
- Add the acid to the alkali using a burette, giving the flask a swirl regularly. Go slowly when you think it is about to change.
- The indicator changes colour when all the alkali has been neutralised.
- The amount of acid needed is recorded. This process is repeated a few times.
- Concentration= moles/volume
- Example 1, if they ask for concentration in moles per dm^3.
- If you start off with 25 cm^3 of sodium hydroxide and the concentration is 0.1 moles per dm^3. It takes 30cm^3 of sulphuric acid(unknown concentration) to neutralise the sodium hydroxide.
- Write down the balanced equation and work out how many moles of the unknown stuff there was. Then work out the concentration(concentration=number of moles/volume)
- Concentration in grams per dm^3
- grams per cubic decimetre = grams per litre
- work out the relative formula mass for the acid
- convert the concentration in moles into concentration in grams
- Mass in grams = moles x relative formula mass
- Water cycle means water is endlessly recycled. The sun evaporates water from the sea.The water vapour is carried up as the warm air rises.As it cools from the lower part of the atmosphere at higher attitudes, it forms clouds.
- When the condensed droplets get too big, they fall as rain.
- Then the water runs back to the sea. At some stage, it's going to come into contact with the rocks on the ground(water in different places dissolve different minerals). Then the cycle starts again.
- Water's a solvent- it dissolves many chemicals.
- Lots of substances dissolve in water, so it is sometimes called the universal solvent.
- Water dissolves most ionic compounds. Water molecules start to surround the ions, and disrupt the ionic bonding, so the solid structure of the ionic compound gradually falls apart.
- Water molecules are polar(they have a positive hydrogen side and a negative oxygen side). The slightly positive attract negative ions(vice versa).
- Salts of Sodium, Pottasium and Ammonium dissolve in water.
- All Nitrates dissolve in water.
- All chlorides apart from silver and lead dissolve in water.
- Sulfates do, apart from barium and lead. Calcium sulfate is only slightly soluble.
- Anything which is soluble dissolves(like sugar and not like sand).
- The solubility of a substance in a given solvent is the number of grams of the solute(usually a solid) that dissolve in 100g of the solvent(usually a liquid) at a particular temperature.
- The solubility of solvents usually increase with temperature.
- A saturated solution is one that cannot hold any more solid at that temperature and you have to be able to see solid on the bottom to be certain that it's saturated.
- A solubility curve plots mass of slute dissolved at different temperatures.
- All gases are soluble to some extrent.
- Chlorine water is a solution of chlorine and water(used as bleach).
- The amount of gas that dissolves depends on the pressure of the gas above it(higher the pressure, the more that dissolves).
- But gases become less soluble as the temperature of the solvent increases(opposite of solids).
- Hard water doesn't form a lather with soap, instead you get a nasty scum (unless you use more soap).
- Hard water also forms furring or scale(mosly calcium carbonate) on the inside of pipes, boilers and kettles. Badly scaled up pipes reduce the effiecency of the heating system and may need to be replaced.The problem is the hardness minerals.
- Most hard water is formed because it contains lots of calcium and magnesium ions. Hardness often comes from limestone, chalk and gypsum.
- Rain falling on some types of rocks can dissolve magnesium sulfate and calcium sulfate.
- Other calcium and magnesium come from a reaction. When carbon dioxide from the air dissolves in rain water,you get carbonic acid.
- It isn't all bad, Ca2+ ions are good for the teeth and bones.
- Scale inside the pipes form a proctective coating which stops posionous metal ions from getting into the drinking water. It also protects the pipes from rust.
- To remove the hardness, you can add sodium carbonate so the carbonate ions join onto the calcium or magnesium ions to make an insoluble precipitate. Or by ion exchange columns. Sometimes a water supply is fed through an ion exchange column to exchange sodium ions(or hydrogen ions) for calcium or magnesium ions. Scale is just calcium carbonate and can be dissolved by acid.
- Water must be free from poisonous salts and harmful microorganisms. Most of our drinking water comes from reservoirs.
- Water from the reservoirs goes through treatment works for treatment:
- water passes through a mesh screen to remove twigs.
- treated with chlorine or ozone to kill microorganisms.
- chemicals are added to make solids and microorganisms stick together and fall to the bottom.
- The water is filtered through gravel beds to remove all the solids.Nasty smells and odours can be removed by passing through 'activated carbon' filters or with 'carbon slurry'.
- The pH is corrected if it is too acidic or too alkaline.
- Water is chlorinated to kill off any harmful microorganisms left.
- To monitor water quality, water companies take samples of water. But some people aren't satified and buy filters containing silver or carbon. The carbon removes chlorine taste and the silver kills bugs.
- Totally pure water can be made by distillation. But this is too expensive and only used in chemistry labs.