Balancing Compounds
- Covalent and ionic compounds have a total charge of zero, or neutral.
- In general:
  - Groups I, II and III ions have charges of +1, +2, and +3 respectively.
  - Groups V, VI and VII ions have charges of -3, -2 and -1 respectively.
- Net charges of the ions are written in superscript behind the atomic symbols. For example: a magnesium ion of +2 charge is written as Mg²⁺.
- Ionic compounds would require both anions and cations to be properly balanced so the compound has a total charge of 0. For example:
  - Magnesium bromide, MgBr₂ has 1 magnesium ion of +2 charge, and 2 bromide ions of -1 charge, thus balancing the ion.
  - Aluminium sulfate, Al₂(SO₄)₃, has 2 aluminium ions of +3 charge (+6 total), and balanced by 3 sulfate ions of -2 charge (-6 total).
- Some ions are made up of a few different elements, and these are considered polyatomic ions. Some examples include:
  - Phosphate ion PO₄^3-
  - Sulfate ion SO₄²⁻
  - Carbonate ion CO₃²⁻
  - Nitrate ion NO₃⁻
  - Ammonium ion NH₄⁺
  - Chromate ion CrO₄^2–
  - Dichromate ion Cr₂O₇^2–
  - Permanganate ion MnO₄^–
Relative atomic/molecular mass
- Relative atomic mass is denoted by A_r, and it is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12. As learnt in previous chapters, the mass of all elements are measured by the relative atomic mass.
  - For example, A_r of hydrogen is 1, oxygen is 16, carbon is 12.
- Relative molecular mass, M_r, is used to denote the sum of the relative atomic mass of the atoms in a covalent molecule. For ionic compounds, relative formula mass, also denoted as M_r, is used instead.
  - For example, the relative formula mass of NaCl is the sum of relative atomic mass of Na and Cl, which is 23 + 35.5 = 58.5.

Finding the chemical/empirical formula of a compound
- The chemical formula of a compound can be determined by the mass of each element present in the compound.
  - For a compound with 24g of carbon and 64g of oxygen, we know that carbon has an A_r of 12 and oxygen has an A_r of 16.
  - Dividing 24 by 12 and 64 by 16, we get 2 units of carbon and 4 units of oxygen, which can be simplified to 1 unit of carbon and 2 units of oxygen to give CO₂ (carbon dioxide).
  - This method also gives the empirical formula of the compound, which is the simplest whole number ratio of the different atoms or ions in a compound.
- The chemical formula of a compound can also be found using the structural formula/diagram of the compound.
  - Let’s use this compound as an example:
  - By counting the respective number of different elements, we can determine that this compound has a chemical formula of: CH₄ON₂.
  - For easier counting, use a different coloured pen to circle the different elements to prevent careless mistakes when counting.
Word equations and chemical equations
- Word equations are used as a simple method to clearly display the reactants and products of a chemical reaction.
  - Let’s use photosynthesis as an example:
  - In a word equation, the reactants are placed on the left side, while the products are placed on the right side. The conditions for the reaction to occur (in the presence of light and chlorophyll) are written in the centre: on top and bottom of the arrow respectively.
- On the other hand, chemical equations show the full chemical reaction, showing the chemical formulae of the reactants and products and balancing both sides of the equation.
  - Using photosynthesis again as the example, we can see that the equation on both sides are balanced:
  - For a chemical equation, it retains most of the characteristics of the word equation: reactants on the left, products on the right, and conditions in the centre.
  - As seen in this reaction, the total number of atoms of hydrogen, carbon and oxygen are equal on both sides of the equation: There’s a total of 12H, 18O and 6C. As seen here, 6 molecules of water and 6 molecules of carbon dioxide are reacted to form one molecule of glucose and 6 molecules of oxygen gas.
  - The states of the reactants and products are also noted: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous, which means it is dissolved in water.
Ionic equations
- An ionic equation is a chemical equation where the chemicals are written as dissociated ions.
- Let’s use the reaction of sodium chloride and silver nitrate as an example:
  - Firstly, we dissociate all the ionic compounds into their individual ions, giving the complete ionic equation:
  - Next, we remove the spectator ions which are not participating in the chemical reaction and cancel them off. In this case, since the sodium and nitrate ions remain aqueous and does not combine like the chloride and silver ions, they can be removed
  - As a result, we obtain the net ionic equation as seen here:
    Ag⁺ (aq) + Cl^– (aq) → AgCl (s)
- Another example of an ionic equation can be seen here with the reaction of lead nitrate and sodium iodide:

3.01 - Stoichiometry

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