Chapter 6: Chemical Formulae & Equations

Question 1

How are cations found in the Periodic Table?

Answer 1

Cations are formed from metals. (left side of ‘staircase line’)
Group 1 cations:
- The elements in Group 1 all have one valence electron
- When forming ions, they will lose their valence electron to form ions with a charge of 1+

Group 2 cations:
- The elements in Group 2 all have two valence electrons
- When forming ions, they will lose their valence electrons to form ions with a charge of 2+

Group 13 cations:
- The elements in Group 13 all have three valence electrons
- When forming ions, they will lose their valence electrons to form ions with a charge of 3+

Question 2

How are anions found in the Periodic Table?

Answer 2

Anions are formed from non-metals. (right side of ‘staircase line’)
Group 15 anions:
- The elements in Group 15 all have five valence electrons
- When forming ions, they will accept three electrons to form ions with a charge of 3-

Group 16 anions:
- The elements in Group 16 all have six valence electrons
- When forming ions, they will accept two electrons to form ions with a charge of 2-

Group 17 anions:
- The elements in Group 17 all have seven valence electrons
- When forming ions, they will accept one electron to form ions with a charge of 1-

Question 3

What are some symbols of ‘special cations’?

Answer 3

Iron (II), Iron (III): Fe2+, Fe3+
Lead (II): Pb2+
Copper (I), Copper (II): Cu+, Cu2+
Zinc: Zn2+
Silver: Ag+
Ammonium*: NH4+
Hydrogen: H+

Question 4

What are some symbols of polyatomic anions?

Answer 4

*Sulfate: SO4^2-
*Nitrate: NO3^-
*Carbonate: CO3^2-
*Hydroxide: OH-
Nitrite: NO2^-
Sulfite: SO3^2-
Hydrogen carbonate: HCO3^-
Phosphate: PO4^3-

Question 5

Golden rule of ‘Balancing-Charges’ Method

Answer 5

If the ‘+’ charges and the ‘-‘ charges cancel out immediately, write down the chemical formula of the ionic compound directly by simply ignoring the charges.

*Note the use of bracket to indicate more than one of the same polyatomic ion.

Question 6

How are chemical formulae written?

Answer 6

• The symbol of the metallic element is written first.
• The number of atoms is written as a subscript to the right of the atom’s symbol.
• The subscripts are the simplest set of whole numbers.
• Do not write the subscript ‘1’ .
• If there is more than one polyatomic ion, the formula of that ion is placed within brackets.

Question 7

How to name an ionic compound?

Answer 7

A compound formed from only two elements has name that ends with ‘ide’ .

Compounds that contain the hydroxide ion (OH-) is named a hydroxide.

Compounds containing negatively charged polyatomic ions usually have names ending with ‘-ate’ .

Question 8

How to name anions?

Answer 8

The names of anions usually end with ‘-ide’

Examples:
fluorine = fluoride
hydrogen = hydride
oxygen = oxide

Question 9

What are some important rules to observe when writing chemical formulas?

Answer 9

1. The positive ion (usually a metal ion) is written first followed by the negative ion (usually a non-metal ion)
2. The overall charge of a compound should be zero.
3. Brackets are used to group polyatomic ions.
4. It is not necessary to write the subscript ‘1’ .

Question 10

What are word equations and chemical equations?

Answer 10

Word equation:
- Using names of compounds to form equations.

Chemical equation:
- Using chemical formulae of compounds to form equations.

A chemical equation shows what happens in a chemical reaction. It tells us:
- which reactants and products are involved in the reaction;
- the relative amounts of reactants and products;
- the physical states of the reactants and products of the reaction.

Balancing a chemical equation:
A balanced chemical equation must contain equal numbers of atoms of each element on both sides of the equation.
- The reactant(s) are written on the left-hand side of the equation.
- The product(s) are written on the right-hand side of the equation.
- The chemical equation is balanced by adding a number in front of the chemical formula. This is the same as multiplying the formula by that number.
- The state symbols
(s) - solid
(g) - gas
(l) - liquid
(aq) - aqueous solution

Question 11

What are state symbols and what are some substances that are in certain states?

Answer 11

Solid (s) substances:
- Metals, ionic compounds at r.t.p (insoluble in water)

Liquid (l) substances:
- Water, mercury, molten solids

Gas (g) substances:
- Gaseous substances eg. carbon dioxide, hydrogen, oxygen, etc

Aqueous (aq) substances:
- Ionic compounds (dissolved in water), acids/alkalis

Question 12

What are the *solubility rules?

Answer 12

All S.P.A.N salts (aqueous) dissolve in water
- Sodium
- Potassium
- Ammonium
- Nitrate

All chlorides are soluble (aqueous) except:
- Silver
- Lead (II)
(‘lao sai’)

All sulfates are soluble (aqueous) except:
- Calcium
- Lead (II)
- Barium
(‘CLB’)

All carbonates (s) are insoluble except:
- Sodium
- Potassium
- Ammonium
(‘SPA’)

Question 13

What are the steps to write balanced chemical equations?

Answer 13

1. Write down formulae of reactants and products
2. Check the number of atoms of each element on both sides of the equation
3. Balance the equation by placing numbers in front of the formulae of the substances in the equation. The number ‘1’ is omitted
4. Write the state symbols. s: solid, l: liquid, g: gas, aq: aqueous

Question 14

What is an ionic equation and what are the steps to write it?

Answer 14

An ionic equation is a simplified chemical equation that shows the reactions involving ions in aqueous solution.

Spectator ions:
- In chemical reactions, there are ions that do not take part in the chemical reaction.
- Such ions are called spectator ions.

Steps to write an ionic equation:
1. Write a balanced chemical equation including state symbols
2. For substances in aqueous state, rewrite them in terms of ions
3. Cancel out spectator ions
4. Write the ionic equation (whatever that’s remaining)