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Contents Mathematics Fermat Physics Wordlist Notes Chemistry Quick Facts Reference LINKS

Contents

Chemistry
Relative Atomic Mass
Moles
Percentage of an element in a compound
Concentration of elements in solutions
Types of Chemical Reactions
Electrolysis and Electroplating
Composition of air and the earth's crust: Pie charts


Chemistry


Relative Atomic Mass:
The Relative Atomic Mass, (RAM), of an atom is the average mass of its isotopes as compared with 1 atom of 126C, a carbon atom. The RAM is made up of the mass of protons + neutrons in the atom's nucleus. The mass of electrons is negligible, and therefore excluded. 1 carbon atom is used as the standard for comparison with other types of atoms. It is assigned a mass of 12 units, based on 6 protons + 6 electrons. Hydrogen has only 1 proton and a RAM of 1 unit.

As many elements also have isotopes (atoms with varying numbers of neutrons), the RAM takes an average mass calculation. e.g. Out of every 4 chlorine atoms, 3 have a mass of 35 and 1 has a mass of 37. The average mass of chlorine was calculated as 35.5. Ions have the mass as atoms.

Table of common elements and compounds

A formula mass, or relative molecular mass (RMM) for a compound or diatomic or other type of molecule such as H2, is found by adding up the masses of the component atoms.

Moles are the RAM or RMM in grams of a substnce. 1 mole of carbon would be 12g of carbon, and 1 mole of sodium would be 23g. 1 mole of any substance contains 6.02 x 1023 atoms or molecules of that substance. For iodine in its diatomic state, 1 mole of I2 contains 6.02 x 1023 diatomic molecules, and 1 mole of helium would be 4g and contain 6.02 x 1023 atoms.
Avogadro's Constant = 6.02 x 1023 = 1 mole

To find an unknown mass of a given number of moles:
Mass (in grams) = Mass of 1 mole x Number of moles
e.g. Find the mass 0.5 moles of bromine molecules, Br2:
A bromine molecule is diatomic, and its formula mass is 160. So 0.5 x 160g = 80g

To find an unknown number of moles in a given mass, first convert the mass into grams:
Number of moles = Mass (g) / Mass of 1 mole
e.g. Find the number of moles in 64g oxygen, O2:
O2 is diatomic, and its formula mass is 32. Therefore 1 mole = 32g. So 64g / 32 = 2 moles in sample.


Percentage of an element in a compound:
To find the percentage or proportion of an element in a compound as a fraction of the total mass, first find the formula mass of the compound, then the RAM of the desired element:
Mass of element / Total mass of compound
e.g. Find the proportion of hydrogen and carbon in methane:
The formula mass is CH4 made up of 1C + 4H = 1 x 12 + 4 x 1 = 16. Mass of carbon = 12/16 = 3/4. Mass of hydrogen = 4/16 or 1/4 as a fraction of total mass. To obtain a percentage simply multiply by 100:
Percentage of carbon in methane = 3/4 x 100 = 75%
Percentage of hydrogen in methane = 1/4 x 100 = 25%

e.g. Find the percentage of nitrogen in the fertiliser, ammonium nitrate:
First work out the formula mass:
The RAM's are N = 14, H = 1, O = 16
Ammonium nitrate = NH4NO3
2n = 2 x 14 = 28 +
4H = 4 x 1 = 4
3O = 3 x 16 = 48
RMM = 80
The mass of nitrogen in the formula is 28. This as a fraction of the total = 28/80, and as a percentage = 28/80 x 100 = 35%


The concentration of solutions:
The concentration of a solution is the amount of solute (in grams or moles) dissolved in 1 dm3 (1 lt or 1000 cm3).

e.g. Find the concentration in moles of 2.5 grams of copper (II) sulphate dissolved in 1 dm3 of water. First find the formula mass:
Formula of copper (II) sulphate = CuSO4.5H2O, made up of:
1Cu = 1 x 64 = 64 +
1S = 1 x 32 = 32
9O = 9 x 16 = 144
10H = 10 x 1 = 10
RMM = 250 and 1 mole = 250g
2.5g = 2.5/250 = 0.01 mole in this solution. Because the solution is 1 dm3, this can be written 0.01 M (M = mole per dm3. If there were 250g in 1 dm3 of solution, it would be a molar solution as it would contain 1 mole in 1 dm3.


Compound Formulae
A compound's formula gives the proportion of atoms and moles in the compound. CO2, the formula for carbon dioxide, tells us that 1 carbon atom combines with 2 oxygen atoms, and hence, 1 mole of carbon atoms combines with 2 moles of oxygen. Changing moles to grams, 12g carbon combine with 32g oxygen. NH3, the formula for ammonia tells us that 1 mole of nitrogen atoms combine with 3 moles of hydrogen atoms, or 14g nitrogen combine with 3g hydrogen.

Working backwards to find a formula from a given mass, first convert the mass to grams. Convert this to moles of atoms. Work out the proportion of atoms and obtain the formula using RAM's. This is the empirical method.

32g sulphur combine with 32g oxygen to form sulphur dioxide. Find the formula from masses:
RAM's: S = 32, O = 16
32/32 = 1 mole of S atoms combines with 32/16 = 2 moles O atoms
1 atom of S combines with 2 atoms of O
The formula for sulphur dioxide is SO2.

A formula can also be found by experiment. First find the masses of the individual elements that combine. Convert to grams, and then to moles. Obtain the formula using the RAM's.

Magnesium burns in oxygen to form magnesium oxide. First, a ribbon of magnesium is placed in a container of known mass, say 25.2g. This is weighed again and the new weight noted: mass of container + mass of Mg ribbon = 27.6g. Heat is applied and oxygen allowed into the container at intervals, and the container resealed. When the burning is complete, the sealed container is allowed to cool and weighed again. The final result is noted as 29.2.
Mass of Mg ribbon = 27.6g - 25.2g = 2.4g
Mass of MgO = 29.2g - 25.2g = 4.0g
Mass of O = 4.0 - 2.4 = 1.6g
From this: 2.4g magnesium combines with 1.6g oxygen. The RAM's are Mg = 24, O = 16. Converting the masses to moles: 2.4/24 = 0.1 moles magnesium combine with 1.6/16 = 0.1 moles oxygen to form MgO.


Physical and chemical change:
Changes occur when substances are heated, mixed with other substances or dissolved. A change, such as a change of state where no new chemical substance is formed, is a physical change. If the change produces a new chemical substance, then it is a chemical change. In a mixture where there is no chemeical change, the particles are mixed closely together. In a reaction where a new compound is formed, ionic or molecular bonding occurs. A reaction often takes in or gives off energy such as heat or light. If heat is taken in during a reaction, it is endothermic. If heat is given out, the reaction is exothermic. A chemical change is usually difficult to reverse, requiring several reactions to do so. A physical change is usually easy to reverse.

Equations for chemical reactions:
An equation can give the proportion of reactants and their state and the resultant compound and its state. Because atoms are not lost or destroyed during a reaction, only rearranged, the equation must balance for each type of atom in turn. The equation for a reaction gives the proportion of reactants in moles and grams. The total mass of the components does not change during a reaction.

View list of example equations


Gases:
At room temperature and pressure (rtp), 1 mole of any gas has a volume of 24 dm3 (24 lt or 24,000 cm3). 24 dm3 is the molar gas volume. e.g. 12g carbon reacts with 24 dm3 oxygen to form 24 dm3 carbon dioxide (volumes measured at rtp).


Types of chemical reactions: Decomposition occurs when a substance or compound breaks down into 2 or more simpler substances. Thermal decomposition is decomposition due to heat. Light may also cause decomposition of some substances.

Precipitation occurs when two solutions react to create a solid product or precipitate.

Combustion occurs when a substance reacts in oxygen, giving out heat and light.

Reduction and oxidation, or 'Redox' reactions occur when a substance loses or gains oxygen. Reduction and oxidation occur together. One substance e.g. copper (II) oxide gets reduced as oxygen is drawn off by hydrogen. The hydrogen in turn becomes oxidised as it acts as the reducing agent. Mangesium burning in oxygen will form magnesium oxide. The magnesium becomes oxidised during this reaction while the oxygen is reduced. The magnesium atoms each lose 2 electrons resulting in ions, and the oxygen atoms each gain 2 electrons. So a substance is oxidised if it loses electrons during a reaction, or reduced if it gains electrons.

Reversible reactions are reactions that can build up a compound out of simpler substances, or breakdown the compound into simpler substances depending on conditions.


Conductors and non - conductors:Electricity is a stream of moving electrons. They flow from positive to negative. Conductors allow this flow to pass through them. The only solids that are conductors are the metals and graphite. Molecular substances are non - conductors. Ionic substances don't conduct when solid, but do conduct as liquids as the ions are freed to move.

When ionic substances conduct electricity, they decompose. Decomposition due to electricity is electrolysis, and the conducting ionic substance is the electrolyte. All ionic substances can be electrolysed as molten, or fused liquids. The elerolyte always decomposes. Electrodes used to connect the electrolyte into a circuit are usually graphite or platinum, and don't react with either the electrolyte or products of the electrolysis. Positive ions always go to the cathode (-ve electrode), and the negative ions go to the anode (+ve electrode).

The difference between conductors and electrolytes:
Conductors are elements and don't decompose when they conduct. Electrolytes are molten ionic compounds, and they decompose when they conduct.

The products of electrolysis are given off at the electrodes and can be separated off. Uses include the purification of metals or other substances and electroplating. Electroplating is used to plate one metal in another, e.g. nickel in silver: The silver is made the anode, silver nitrate solution is used as the electrolyte, and the nickel is used as the cathode. The silver anode dissolves forming positive ions in solution. These ions are attracted to the negative cathode where they receive electrons. They form a layer on the nickel cathode.

To plate an object in a metal, use the object as the negative electrode and the metal as the positive electrode which dissolves. A solution of the metal is used as electrolyte.


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