Early Periodic Table
The Development of the Periodic Table
In 1808 John Dalton suggests that all substances are made of atoms. He lists all known elements according to their atomic weights. However, some are inaccurate as he mistakenly included some compounds which he couldn't break down at the time.
In 1863, John Newlands arranged elements in order of increasing weights. He arranges them in 'octaves' because every eighth element seems to have similar properties. Newlands assumed incorrectly that all the elements had been discovered. Also, some of his groups had conflicting properties:
C and Si are similar but Ti is very different
In 1869 Mendeleev arranges elements in rows and columns in increasing atomic mass. He left gaps as he realised some elements hadn't been discovered, and predicted some of their properties. He swapped some elements over to ensure they fitted in the correct groups:
Ar and K were swapped so their properties fitted in their groups
Comparing Group 1 And Group 7
The Alkali Metals (Group 1):
- Have a low density so they float on water.
- React with non metals to form ionic compounds (their ions have a +1 charge).
- As you go down the group the melting point decreases and they become more reactive. This is because K has the structure 2,8,8,1. It's outer electron is further from the nucleus so it takes less energy to remove it and it is therefore lost more easily, unlike Li with a structure of 2,1 at the top of the group.
The Halogens (Group 7):
- Are non metals that have coloured vapours.
- React with metals to form ionic compounds (their ions have a -1 charge).
- As you go down the group the melting point increases and they become less reactive. This is because Cl has the structure 2,8,7. It gains an electron more easily because its outer shell is nearer the nucleus so less energy is required to attract one, unlike I with a structure of 2,8,18,18,7 at the bottom of the group.
Acids and Bases
Acids and Bases
An acid is a substance that produces H+ ions when it dissolves in water. An alkali is a soluble base that produces OH- ions when dissolved in water.
Bronsted and Lowry defined an acid as a proton donor and a base as a proton acceptor. They said a strong acid or alkali is one that ionises fully in water. Bronsted and Lowry had built on the ideas of Arrhenius, who claimed that when acids, bases and salts dissolve in water they seperate into charged particles called ions in the process of dissociation. Arrhenius's ideas weren't accepted because chemists did not think that molecules could split up and carry an electric charge.
The strength of an acid or an alkali can be tested by using pH indicator paper or pH universal indicator if they are in solution, or testing to see which ionises fully if they are not.
Calcium and Magnesium ions dissolved in water cause it to be 'hard'. Scum is a precipitate formed by the calcium/magnesium ions when they react with soap. Scale is the substance formed when hard water is boiled.
Hardness in water can be removed in two ways;
1) Washing soda can be added to the water. This contains sodium carbonate which precipitates out the calcium and magnesium ions as insoluble carbonates.
2) Using an ion-exchange column to remove the calcium and magnesium ions. Sodium ions are exchanged with the magnesium and calcium as the hard water passes through the column.
Water Filters at Home-
A water filter in a kettle contains a filter cartridge which has an ion-exchange resin, silver and activated carbon. The silver discourages the growth of bacteria, the carbon reduces the levels of chlorine and other impurites while the ion-exchange removes calcium. magnesium, lead and copper ions.
Large Scale Water Treatment-
- Water is passed through a grille made from metal bars that catch large objects.
- In a settlement tank sand and soil settle out.
- Aliminium sulfate and lime are added to the water. This makes small particles of dirt clump together and sink to the bottom of the water.
- The water is passed through a special filter made of fine sand which removes any remaining particles of mud or grit.
- A small amount of chlorine is added to kill any bacteria.
- The pH of the water is monitored and corrected so that it is neutral. It is then stored in large tanks ready to be pumped to homes, schools, offices and factories.
Bond breaking is an endothermic reaction because energy is required so it has to be taken in from the surroundings. Bond forming is an exothermic reaction as energy is released (no energy is required to make bonds).
The Activation Energy is the minimum amount of energy needed for a given chemical reaction to take place. A catalyst lowers the activation energy so that a higher proportionof reactant particles have sufficient energy to react.
Delta H is the heat energy change in a reaction. To work this out, use the equation
In an exothermic reaction Delta H is always negative because more enery is released when bonds are formed between atoms in the products than the energy needed when bonds are broken in the reactants and vice versa for an endothermic reaction.
Tests for Positive Ions
Group 1 and Group 2 metals can be identified using flame tests. Lithium burns bright red, Sodium golden yellow, Potassium lilac, Calcium brick red and Barium apple green.
We can also carry out precipitate tests using Sodium Hydroxide to identify different ions. Ca2+, Mg2+ and Al3+ ions all produce a white coloured precipitate (Al3+ redisolves in excess Sodium Hydroxide), Cu2+ produces a blue precipitate, Fe2+ produces a pale green precipitate and Fe3+ a red brown precipitate.
Ammonium ions produce the gas NH4+ when added to sodium hydroxide. It is an alkali.
Tests for Negative Ions
We can test for Halides by adding silver nitrate solution to them. Cl- produces a white precipitate, Br- a cream precipitate and I- a yellow precipitate.
Sulphate ions can be testd for with barium chloride. This will produce a white precipitate of Barium Sulphate.
Carbonate ions react with dilute acids. We can test the gas produced when they react by bubbling it through lime water to see if it turns cloudy. If the lime water does go cloudy then Carbon Dioxide gas has been produced.
When CuCO3 is heated it changes colour from green to black. ZnCO3 is a white powder but turns yellow when heated, and back to white when it cools.
Nitrate ions can be tested for by adding sodium hydroxide solution, adding a spatula of aliminium poweder, warming gently and testing the gas produced with damp red litmus paper. If the paper turns blue due to Ammonia gas then the ion is NO3-.