Formula and Equation writing

The ability to write formula and balance equations is an essential skill in chemistry and forms the starting point for solving most problems.

Writing Formula

Chemical compounds typically consist of one or two endings; -ide and -ate with the exception of hydroxides, a compound ending -ide will be made up of just two elements and those ending -ate will also contain the element oxygen (eg magnesium sulfate, MgSO4)

Chemical name often give an indication as to the formula of a compound. A good example of this is carbon dioxide (CO2). These prefixes can be used to quickly write a formula.Where a formula is not obvious, a system of combining powers can be used to work out the most likely ratio of elements. It begins by assigning combining powers to the columns in the periodic table.Transition metals (elements in the middle of the table) are able to show a variety of combining powers and such have Roman numerals that show the combining power (oxidation number) involved. For example the combining power of cobalt in cobalt IV chloride is 4, but 2 in cobalt II iodide.

Compound or polyatomic ions are groups of atoms that are covalently bonded but have an overall change. These species can also be assigned combining powers based on their charges. Some will become more familiar once the chemistry of acids has been studied.Having identified the elements involved and their combining powers, a crossing over rule can be used to construct the formula. In this method the powers are switched and if a common factor exists then the ratio of elements is cancelled down. Brackets are used, as in maths, for multiple groups of compound ions. For example, in magnesium hydroxide, Mg(OH)2, there are two hydroxide ions for every one magnesium ion.Balancing equations

Balanced equations show how chemicals react to form new products and contain the same number of each type of atom on both sides of the equation. Only whole numbers (coefficients) can be added to the front of formula to achieve this. It is also worth remembering that seven of the elements exist in their natural, uncombined forms as diatomic molecules (H2, O2, N2, F2, Cl2, Br2, I2).

An example of the steps used to balance an equation is shown below fir the reaction between iron and oxygen to form iron III oxide.

Step 1 Write in the symbols for the chemicals. Use combining powers where necessary. Looking at the equation it is apparent that the numbers of each type of atom on each side of the equation are not the same – it needs balancing.

Fe + O2 —-> Fe2O3

Step 2. The odd number of oxyge atoms on the right hand side of the equation and even number of the left will make balancing tricky. Address that problem first.

Fe + O2 —-> 2Fe2O3

Step 3. Now balance the Fe atoms

4Fe + O2 —->2Fe2O3

Step 4. And finally the O atoms.

4Fe + 3O2 —–> 2Fe2O3

State symbol should be included in balanced equations whenever possible. These letters in brackets indicate whether a chemical is solid (s), liquid (l), gas (g) or dissolved in water (aq). For the example above the equation then becomes.

4Fe(s) + 3O2(g) —–> 2 Fe2O3(s)



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