History Of The Periodic Table
Newlands arranged the elements in order of atomic weight and identified that every eighth element had similar properties. This doesn’t quite work on the modern periodic table because we have now got the noble gases too. Mendeleev did the same kind of thing but arranged the elements so that the “similar elements” were in vertical groups. His main claim to fame was that he left gaps where elements had not yet been discovered. The most famous example is the gap left for what we call Germanium (he called it eka-silicon). Chemists were pleased to discover the element had the properties that Mendeleev had predicted. Later, with the discovery of the details of atomic structure we can understand that it is better to arrange the elements in order of atomic number rather than in order of atomic mass. For most of the elements the order is the same but a major difference occurs with potassium and argon. If the elements were in order of mass, argon (noble gas, very unreactive) would be in the same group as sodium while potassium (a very reactive metal) would be in the noble gases. This problem is overcome when the order is based on the number of protons in the atom. Since this links to the number of electrons, which in turn links to the chemical properties, this is a more sensible arrangement.
Modern Periodic Table
Metals on the left and centre, non-metals on the right. The pattern of reactivity changes down the group is different for the metals and the non-metals but this is not just being awkward, it depends on the kind of reaction that metals and non-metals do. Metals usually react by losing one or more electron(s) to form positive ions. Non-metals usually react by gaining one or more electron(s) to form negative ions. A reactive metal is one which can lose electron(s) easily. A reactive non-metal is one which can gain electron(s) easily. Electrons are held most tightly when they are close to the nucleus. Small atoms have a strong pull on their electrons. Large atoms have a weak pull on their electrons. A small metal atom hangs on tightly to its electrons and so is UNREACTIVE. A large metal atom hangs on weakly to its electrons and so is REACTIVE. Reactivity of metals increases down the group. A small non-metal atom attracts electrons really strongly and so is REACTIVE. A large non-metal atom attracts electrons weakly and so is UNREACTIVE. Reactivity of non-metals decreases down the group.
Alkali Metals and Halogens
The alkali metals (NOT alkaline) are all part of Group I.
They are fairly soft metals (can be cut with a knife)
They get softer as you go down the group.
The melting point decreases as you go down the group.
The reactivity increases as you go down the group.
They all react with water to give hydrogen and the metal hydroxide
The halogens are all part of Group VII.
They are all diatomic molecules (two atoms per molecule)
The melting point increases as you go down the group.
The halogens react with many metals to form a halide salt.
The reactivity decreases as you go down the group
Transition elements have a confusing name.
They are not halfway between metals and non-metals (even though they are in the middle of the periodic table between groups II and III). They are fully metals.
They are usually hard and high melting point.
Their compounds are usually coloured.
The metals and their compounds are useful as catalysts.
Acids and Bases
The Lowry-Bronsted theory of acids and bases depends on the giving and receiving of hydrated hydrogen ions. An H+ ion is a hydrogen atom that has lost an electron.
Since a hydrogen atom is only made of one proton and one electron, if it loses one electron it is just a proton!
This very small particle with a full positive charge is strongly attracted to water molecules and so we ought to talk of the “hydrated hydrogen ion” or the “hydrated proton”...H+(aq) or H3O+(aq). All acids have at least one hydrogen atom in them that can be released as a hydrated hydrogen ion. The releasable hydrogen ion is shown in these examples in bold type
eg hydrochloric acid (HCl), sulfuric acid (H2SO4), ethanoic acid (CH3COOH) All bases have the ability to accept a hydrated hydrogen ion.
eg Sodium hydroxide contains OH- which can accept a hydrated hydrogen ion to form water. Ammonia (NH3) can gain a hydrated hydrogen ion to form ammonium (NH4+). A base that is soluble in water is called an alkali.
Strong and Weak Acids
When you are given a drink of orange squash, most people will ask if you would like to have it strong or weak. In a chemistry sense, this is wrong. They ought to be asking whether you like it concentrated or dilute because they want to know if you want a large amount of orange squash and a little bit of water or a little squash and a lot of water.
The words “strong” and “weak” refer to the properties of acids. A strong acid is totally broken down in water. A good example is hydrochloric acid. There are no HCl molecules in a bottle of hydrochloric acid. They have all broken down to form H+(aq) and Cl-(aq).
With ethanoic acid, many of the molecules stay as CH3COOH with only a few becoming CH3COO- and H+. Ethanoic acid is a weak acid.
A substance that changes colour depending on the pH is called an indicator.
There are two types of indicator:
Single indicators show only one colour change and are used to decide if the pH is higher or lower than a certain value. This makes them useful for titrations.
Litmus is a common one (red in acid and blue in alkali).
Methyl Orange (red in acid and orange in alkali) and Phenolphthalein (colourless in acid and purple in alkali) are two others that are often used.
Mixed indicators (like Universal Indicator or Full Range Indicator) show a wide range of colours and can be used to decide the pH of a given solution. They are not much use for titration work.
The idea is to find out the concentration of one solution if we know the concentration of the other. We need to know the volume of each solution that just rreacts together exactly with none of either left over.You can do the calculation using moles (see next sheet). If we know that the ratio of acid to alkali is 1:1 we can simply use:
Concentration of A x Volume of A = Concentration of B x Volume of B
One solution is measured out (with a pipette or a measuring cylinder) into a conical flask.
A few drops of a suitable indicator are added.
The other solution is added from a burette.
The titration is stopped when the indicator JUST changes colour. Choosing an indicator is a little bit tricky.
For a strong acid: strong alkali titration, almost any indicator will do.
For a weak acid: strong alkali titration, use Methyl Orange.
For a strong acid: weak alkali titration, use Phenolphthalein.
Remember we are using strong and weak in their correct meanings (nothing to do with concentration)