Writing chemical formulae

Valency

The ability of an element to combine with other elements to form compounds can be expressed by the term valency. Valency refers to the number of electrons in the outer shell, that need to be donated or gained to make a stable shell of an ion or atom, e.g. hydrogen’s valency is H+, lithium’s valency is Li+ and chlorine’s is Cl–.

In the case of ionic compounds, the valency of each element is the number of electrons lost or gained by the reacting metal and non-metal. It is therefore equal to the charge on each ion which is related to the electron configuration and the number of electrons in the valence shell.

Valencies of common polyatomic ions

+1 –1 –2 –3Ammonium Ion NH4

+ Acetate Ion CH3COO– Carbonate Ion CO32– Phosphate Ion PO4

3–

Hydrogen Carbonate HCO3

–

Chromate Ion CrO42– Blue indicates that

these should be known.Hydroxide Ion OH– Dichromate Ion

Cr2CO72–

Nitrate Ion NO3– Sulfate Ion SO4

2–

Nitrite Ion NO2– Sulfite Ion SO3

2–

Writing formula

The following rules should be used: All electrons must be accounted for as there can be no loose or unpaired electrons. The sum of positive and negative valencies of ionic compounds is zero.

Steps involved: Determine an element’s valency How many electrons are going to be lost or gained

Naming compounds

In the early days of chemistry, there was no system for naming of compounds. Chemists used common nameslike bicarbonate of soda, quicklime, milk of magnesia, Epsom salts and laughing gas to describe compounds. As the number of named compounds increased it was obvious that if such common names were used, confusion would result. In 1787, in order to solve the problem a scientist named Lavoisier established the principles for asystematic naming process.

There are however, some familiar compounds that are always referred to by their common name for example H2O is always referred to as water, rather than di-hydrogen monoxide.

Naming ionic compounds

To name an ionic compound: The positive ion (usually a metal) is named first, and the negative ion is second. A simple positive ion takes its name from its parent element. For e.g. Na+ is called sodium. A simple negative ion is named by taking the first part of the parent element’s name and adding the

suffix ‘-ide’.Cl– was originally a chlorine atom but is now an ion is given the new name chloride. Likewise, Br – (originally bromine) is called bromide, O2– (originally oxygen) is called oxide andN3– (originally nitrogen) is called nitride.

Naming covalent compounds

The naming of covalent compounds is similar to ionic compounds, even though there are no ions present: The first element in the chemical formula is named first, using the element’s full name. The second element is named as if it was a negative ion. Prefixes are used to show the number of atoms present. If the first element exists as a single atom, no prefix is used. For example, CO2 is called carbon

dioxide To avoid awkward pronunciation CO is called carbon monoxide, not carbon mono-oxide. Some examples of covalent compounds and their names are

CO2 – carbon dioxide CO – carbon monoxideN2O5 – dinitrogen pentoxide CCl4 – carbon tetrachlorideNH3 – ammonia CH4 – carbon tetrahydride, methane

Balancing equations

No atoms can be created or destroyed. They are just rearranged. You cannot change the valencies therefore you cannot change the formula.

Steps:1. Write the formulae for reactants and products.2. Make the number of atoms for reactants equal the number of atoms for products by placing whole

numbers in front of the formula.

The law of conservation of matter

The Law of Conservation of Matter (or Law of Conservation of Mass) states that: matter cannot be neither created nor destroyed. Instead it can only be changed from one form to another. This means that there must be the same number of each type of atom on each side of the equation. The atoms are simply being rearranged through the reaction process.

General equations

1. Oxidation (Corrosion) :METAL + OXYGEN → METAL OXIDE

Examples:i. Magnesium + Oxygen → Magnesium Oxide

2Mg + O2 → 2MgOii. Iron (II) + Oxygen → Iron Oxide

2Fe + O2 → 2FeO2. Metals with acids:

METAL + ACID → SALT + HYDROGEN GASThis type of reaction involves active metals such as Na, K, Hg, Ca etc. Unreactive metals such as Au (gold) do not react with hydrochloric acid (HCl), no matter how concentrated the acid is. Copper reacts with HNO3 (nitric acid).

Examples:i. Sodium + Hydrochloric Acid → Sodium Chloride + Hydrogen Gas

2Na(s) + 2HCl(aq) → 2NaCl(s) + H2 (g)

ii. Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen GasZn(s) + 2HCl(aq) → ZnCl2(s) + H2 (g)

3. Acids with carbonates: ACID + CARBONATE → SALT + WATER + CARBON DIOXIDE

Examples:i. Hydrochloric Acid + Zinc Carbonate → Zinc Chloride + Water + Carbon Dioxide

2HCl(g) + ZnCO3 (s) → ZnCl2 (aq) + H2O(l) + CO2 (g)

ii. Sulfuric Acid + Calcium Carbonate → Calcium Sulfate + Water + Carbon DioxideH2SO4 (aq) + CaCO3 (s) → CaSO4 (aq) + H2O(l) + CO2 (g)

4. Neutralisation (acids with alkalis or bases) : ACID + BASE (ALKALI) → SALT + WATERNeutralisation is an exothermic reaction, which raises the temperature of the solution.

Examples:i. Sodium Hydroxide + Hydrochloric Acid → Sodium Chloride + Water

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

ii. Calcium Hydroxide + Nitric Acid → Calcium Nitrate + WaterCa(OH)2 (aq) + 2HNO3 (aq) → Ca(NO3)2 (aq) + 2H2O(l)

Corrosion (rusting)

Corrosion refers to the reaction of a metal with gases in the air.

There are many methods to prevent corrosion such as galvanizing, chromium plating and tin plating.

Combustion

Combustion is any chemical reaction in which heat (and usually light) is produced, sometimes called burning, for example: 2Mg(s) + O2 (g) → 2MgO(s) + heat + white light.

Combustion reactions involve the burning of a usually organic substance with oxygen. Combustion of organic substances almost always produces CO2 and H2O. Rapid combustion produces flame and high temperatures e.g. fire Slow combustion produces low temperatures and no flames. Combustion reactions are an essential part of our lives: the burning of fossil fuels such as coal are

all combustion reactions which are vital for cooking food, warming houses and driving vehicles.

Other reactions

Combination or synthesis

Word equation: A + B → ABOften two or more substances usually elements combine together, to form a single substance. These are called direct combination reactions or synthesis reactions, and it has only one product, for example:

i. Iron (II) + Sulfur → Iron (II) Sulfide + energyFe(s) + S(s) → FeS(s)

ii. Sodium + Chloride → Sodium Chloride + energy2Na + Cl2 → 2NaCl + energy

Decomposition

Word equation: AB → A + BDecomposition involves the reaction of a single substance by which it breaks down into two or more simple substances. Decomposition reactions have only one reactant, and are caused by either light or heat. Decomposition caused by heat is called thermal decomposition, for example:

i. Calcium Carbonate + heat energy → Calcium Oxide+ Carbon Dioxide CaCO3 (s) + heat → CaO(s) + CO2 (g)

ii. Copper Carbonate + heat energy → Copper Oxide + Carbon Dioxide CuCO3 (s) + heat → CuO(s) + CO2 (g)

Some decomposition reactions can also be caused by light, for example:

i. Silver Chloride + light energy → Silver + ChlorineAgCl2(s) + light → Ag(s) + Cl2 (g)

ii. Silver Bromide + light energy → Silver + BromineAgBr2(s) + light → Ag(s) + Br2 (l)

Precipitation

Word Equation: AB + CD → AD + CBCertain solutions when mixed react to product a suspension in a liquid, an insoluble product, which is a compound, and is called the precipitate, for example

i. Silver Nitrate + Sodium Chloride → Silver Chloride + Sodium NitrateAgNO3 (aq) + NaCl(aq) → AgCl(s) + NaNO3 (aq)