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.. Valency refers to the number of electrons in the outer shell, that need n eed 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
Ammonium Ion NH4
+
–2 –
Acetate Ion CH 3COO
Hydrogen Carbonate HCO3
–3
2– Carbonate Ion CO 3 2– Chromate Ion CrO 4
3–
Phosphate Ion PO 4
Blue indicates that
–
these should be known.
Hydroxide Ion OH
–
Dichromate Ion Cr2CO7
–
Sulfate Ion SO 4
–
Sulfite Ion SO3
Nitrate Ion NO 3 Nitrite Ion NO2
2– 2–
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 names like 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 com mon names were used, confusion would result. In 1787, in order to solve the problem a scientist named Lavoisier established the principles for a systematic naming process. There are however, some familiar compounds that are always referred to by their common name for example H 2O is always referred to as water, rather than di-hydrogen 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. –
2–
Likewise, Br (originally bromine) is called bromide, O (originally oxygen) is called oxide and 3–
N (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, CO 2 is called carbon dioxide
To avoid awkward pronunciation CO is called carbon m onoxide, not carbon mono-oxide.
Some examples of covalent compounds and their names ar e CO2 – carbon dioxide
CO – carbon monoxide
N2O5 – dinitrogen pentoxide
CCl 4 – carbon tetrachloride
NH3 – ammonia
CH4 – carbon 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 nu mber of atoms for products by placing whole numbers in front of the formula.
The law of conservation of matter The Law of of Conservation Conservation of of Matter Matter (or (or Law of Conservation of Mass) states that: matter cannot be neither created nor destroyed. Instead it can only be changed ch anged from one form to another. This means m eans 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 + O 2 → 2MgO
ii.
Iron (II) + Oxygen → Iron Oxide 2Fe + O2 → 2FeO
2. Metals with acids: METAL + ACID → SALT + HYDROGEN GAS This 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 HNO 3 (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 Chlori de + Hydrogen Gas Zn(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 Chloride + Water + Carbon Dioxide 2HCl(g) + ZnCO3 (s) → ZnCl2 (aq) + H2O(l) + CO2 (g)
ii.
Sulfuric Acid + Calcium Carbonate → Calcium Sulfate + Water + Carbon Dioxide H2SO4 (aq) + CaCO3 (s) → CaSO4 (aq) + H2O(l) + CO2 (g)
4. Neutralisation (acids with alkalis or bases): ACID + BASE (ALKALI) → SALT + WATER Neutralisation is an exothermic reaction, which r aises 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 + Water Ca(OH)2 (aq) + 2HNO3 (aq) → Ca(NO 3)2 (aq) + 2H2O(l)
Corrosion (rusting)
Corrosion refers 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 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 CO 2 and H 2O.
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 → AB Often two or more substances usually elements combine together, to form a single substance. These are called direct combination reactions combination reactions or synthesis synthesis reactions, reactions, and it has only one product, for example: i.
Iron (II) + Sulfur → Iron (II) Sul Sulfide + energy Fe(s) + S(s) → FeS(s)
ii.
Sodium + Chloride → Sodium Chloride + energy 2Na + Cl2 → 2NaCl + energy
Decomposition Word equation: AB → A + B Decomposition involves Decomposition 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 CaCO 3 (s) + heat → CaO (s) + CO2 (g)
ii.
Copper Carbonate + heat energy → Copper Oxide + Carbon Dioxide 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 + Chlorine AgCl2(s) + light → Ag (s) + Cl2 (g)
ii.
Silver Bromide + light energy → Silver + Bromine AgBr2(s) + light light → Ag(s) + Br2 (l)
Precipitation Word Equation: AB + CD → AD + CB Certain solutions when mixed react to product a su spension 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 Nitrate AgNO3 (aq) + NaCl(aq) → AgCl(s) + NaNO3 (aq)
