Chemistry 2

GCSE Edexcel C2 revision materials (additonal science)

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Precipitation reactions

A reaction in which an insoluble solid is produced from two soluble substances is called a precipitation reaction

A precipitate is an insoluble solid that emerges from a liquid solution. 

Example:

Lead nitrate (aq) + potassium iodide (aq) ---->lead iodide (s) + potassium nitrate (aq)

The lead nitrate and potassium iodide are salts which are soluble in water. Lead iodide is insoluble in water and appears as a yellow precipitate which is suspended in liquid.

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Ion tests

Flame tests are carried out on samples of ionic substances.

Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887) used simple spectroscopy- (analysis of the light on the spectrum) to study the light which was given off in flame tests by using some samples of mineral water. A prism was used to seperate the colours and observed a grey-blue colour which was new to them. They discovered a new element called caesium and later on they similarly used the same method and found rubidium (dark red). 

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Ion tests continued

How to carry out a Flame test:

step 1: Dip the flame test loop in acid, then hold the loop at the edge of the bunsen burner flame.

step 2: If the flame changes colour, repeat Step 1 until the loop is clean.

step 3: Dip the clean flame test loop in the sample, then hold the loop at the edge of the bunsen burner flame. Observe and record the flame colour. Repeat Steps 1-3 with each sample.

Precipitation test: to identify aqueous chloride ions, add a few drops of dilute nitric acid to the solution then shake. Then add a few drops of silver nitrate solution. Formation: white precipitate of silver chloride ions.

Sulfate ions test: add a few drops of dilute hcl acid to the solution, then shake. Then Barium solution is added. Formation: white precipitate of barium solution

carbonate test: Co2 gas produced bubbled in limewater thus turning milky. 

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Covalent bonds

Covalent bond: the bond formed when a pair of electrons is shared between two atoms (non-metal elements)

The electrons used in the bonds come from the outermost electron shells of the atoms.

Dot and cross diagrams- represent the electrons, so you can see which atom the electron is from.

Hydrogen molecule (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_109.gif)(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_111.gif)

Atoms can share more than one pair of electrons if it is required to complete their outermost shell. 2 pairs of shared electrons form a double bond.

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Properties of covalent substances

Simple molecular covalent substances-  this consists of gases e.g. hydrogen, oxygen, methane and Co2 and liquids such as water. They have low melting and boiling points because they have weak forces between the nearby molecules.

Current: the flow of charged particles. The atoms present in simple molecular covalent bonds have not lost or gained electrons, so no charged particles can move around. Therefore they are poor conductors of eletricity.

Some build up to form giant molecular covalent bonds. This includes billions of atoms joined together by convalent bonds. They tend to have high melting and boiling points as they are joined together by many atoms of strong covalent bonds, and so require a lot of energy to break the bonds.

Sand is built up of silicon and oxygen atoms joined together in large molecular structure. The sand particles are very hard. Carbon can produce 2 differnent types of giant molecular structure : diamond which is very hard and graphite which is quite soft. 

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Giant molecular covalent substances

Diamond and Graphite both have a very high melting and boiling points due to the strong covalent bonds between the carbon atoms .

Diamond:

  • very hard as the atoms are joined with strong covalent bonds. Used to make cutting tools.
  • Does not conduct electricity because there are no free electrons or charged patricles to move around.

Graphite:

  • easily rubs away in layers because although the covalent bonds within the layers are very strong, they have weak forces between the layers. It is soft enough to be used as a lubricant.
  • it conducts electricity because there is one electron from each carbon atom that can move along the layers. So Graphite can be used to make electrodes. 
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Miscible and Immiscible

Immiscible means that the liquids don't dissolve in each other – oil and water are an example. It is possible to shake up the liquids and get them to mix but they soon separate. Separating immiscible liquids is done simply using a separating funnel. The two liquids are put into the funnel and are left for a short time to settle out and form two layers. The tap of the funnel is opened and the bottom liquid is allowed to run. The two liquids are now separate.

Miscible liquids are harder to separate as they dissolve in each other. Miscible liquids are often separated using fractional distillation. This is possible as miscible liquids have different boiling points.

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Miscible and Immiscible continued

You need to be able to explain how nitrogen and oxygen are obtained from the air.

About 78 per cent of the air is nitrogen and 21 per cent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies:

1.    Water vapour condenses, and is removed using absorbent filters

2.    Carbon dioxide freezes at –79ºC, and is removed

3.    Oxygen liquefies at –183ºC

4.    Nitrogen liquefies at –196ºC

The liquid nitrogen and oxygen are then separated by fractional distillation.

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Chromotography

Chromotography can be used to find out which colourings are in substances and seperate the mixtures.

Some of the coloured compounds dissolve better within a solvent more than others. This is because they different solubilities. This difference allows the seperation of different compounds of mixtures in chromotography.

Simple chromatography is carried out on paper. This paper with the seperated components is called chromotogram. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

Rf vaule is the distance the compounds has risen divided by the distance the solvent has risen.

Rf= distance moved by compound ÷ distance moved by solvent 

The Rf value of a specific compound does not change, if the conditions are the same.

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Chromotography terms

Solvent: the  liquid which dissolves the solute. solute + solvent ------>solution

Solute: the solid or liquid that dissolves in a given solvent to form a solution.

Solution: the clear mixture that forms when a solute dissolves in a given solvent. 

Solubility: a measure of the amount of substance that will dissolve in a certain volume of solvent.

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Metallic Bonding and transition metals

Metallic bonding

The useful properties of metals consist of malleability and the ability to conduct electricity.

The particles in a metal are held together by strong metallic bonds. It takes a lot of energy to separate the particles. That is why they have high melting points and boiling pointsA metal consists of of a regular arrangement of positive ions surronded by a sea of negative delocalised elctrons which means that they are not located in a specific atoms. These electrons come from the outer shell of the atom and can move around freely throughout the metal.

The delocalised electrons between the positive metal ions hold the structure together by strong electrostatic forces.  The delocalised electrons move freely between the positive ions, in all directions. If a voltage is applied across a piece of metal, the electrons start to move to one direction. This movement is an electric current.


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Metallic Bonding and transition metals continued

Metals is a layer of position ions can slide over each other easily when a large force is applied on the piece of metal. The ions are still held together by the sea of electrons, so the metal spreads instead of breaking. It is malleable.

Transition metals

Most metals are transition metals. They are located in the middle block of the periodic table. Most transition metals have high melting points and form coloured compounds.

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Alkali metals

The Alkali metals is found in group 1 in the periodic table. They are solid at room temperature but have low melting points. They are all soft metals and can be cut with a knife. The atoms in the alkali metals are held together by metallic bonding.

They all react with water to form a metal hydroxide (alkaline) and hydrogen gas.

Lithium (S) + water (L) -----> lithium hydroxide (AQ) + hydrogen (G)

The equations for the reactions of the other alkali metals are simliar, because they all form ions with a charge of +1.

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Alklai metals continued

Reactivity

The reactivity of the alkali metals increases down the group. When they are put in water:

  • lithium floats on the surface and fizzes
  • sodium melts from the heat produced by the reaction and whizzes around on the surface as a molten ball, and sometimes the hydrogen produced catches fire
  • Potassium reacts even faster and the hydrogen burns with a lilac flame.

HIGHER TIER

The elements towards the bottom of the group have more electrons, and so they have more electron shells in their atoms. The outer electrons are further from the nucleus, and so the force between the negatively charged nucleus is weaker. It is much easier to remove the outer electrons from the caesium atoms than it is to remove the outer electron from a lithium atom, so caesium is much more reactive than lithium.

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Halogens

The halogens are the elements in group 7 on the right side of the periodic table. They have similar reactions to each other due to the fact that if they gain one electron, it can complete their outer electron shells( charge of (-1). They all react with metals to form compounds called hallides.

Fluorine is the most reactive halogen,  and the reactivty decreases as you go down the group.

At room temperature:

  • Fluroine (F)   Gas      pale yellow
  • Chlorine (Cl)  Gas      yellow-green
  • Bromine (Br)  Liquid  red-brown 
  • Iodine     (I)    Solid    grey         
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Noble gases

The noble gases are in group 0 of the periodic table. They are inert (they do not react)  compared with the other elements and are very difficult to react with anything. 

All these elements in group 0 have a full outer electron shells. This means that as other elements in other groups gain full electron shells by forming ions or covalent bonds. It makes it very hard to react at all.

The noble gases have the following properties in common:

  • They are non-metals
  • They are very unreactive gases
  • They are colourless
  • They exist as single atoms (they are monatomic)
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Periodic table

Group 1 - alkali metals, group 7 - halogens, group 0 - noble gases. Transition metals are between group 2 and 3.  (http://www.bbc.co.uk/schools/gcsebitesize/science/images/38_modern_periodic_table.jpg)

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