C4.0 – Stoichiometry

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This is a fairly confusing topic for students learning it for the first time. I would recommend using more resources to understand it as this topic forms the basis of everything you’ll be learning in chemistry.

1. Use the symbols of the elements to write the formulae of simple compounds.

Every element has it’s own symbol. The Periodic Table is labelled using these symbols. You need to learn some of these as it will be used to write chemical equations

The first 20 elements (Proton/atomic number, symbol, name):

1- H – Hydrogen       11- Na- Sodium
2- He- Helium           12- Mg- Magnesium
3- Li- Lithium           13- Al- Aluminium
4- Be- Beryllium       14- Si- Silicon
5- B- Boron                15- P- Phosophorous
6- C- Carbon              16- S- Sulfur
7- N- Nitrogen           17- Cl- Chlorine
8- O- Oxygen             18- Ar- Argon
9- F- Flourine            19- K- Potassium
10- Ne- Neon             20- Ca- Calcium

To be able to write down formulae, you need to know the valencies of each element. This is simple to find out.

For groups I to IV, the valency is the same as the group no. and no. of valence electrons.
For groups V to 0, the valency = 8 − group number (no. of valence electrons)

Group	I	II	III	IV	V	VI	VII	0
Valency	1	2	3	4	3	2	1	0

Don’t get confused between valence electrons and valency. Valence electrons are the number of electrons in an atom’s outer shell. Valency is the number of electrons an atom needs to gain or lose to become stable.

Let’s take the example of Magnesium chloride. Magnesium’s symbol is Mg and it is in Group II, so it’s valency is 2. Chlorine’s symbol is C and it is in Group VII (ignore yellow block), so it’s valency is 1.

Formula:

• write down the symbols in order: Mg Cl
• write down the valencies of the elements underneath the element:  Mg₂Cl₁ 
• swap around the valencies: Mg₁Cl₂
• cancel down the numbers to get the smallest whole number ratio when necessary: here, 1 and 2 are in it’s simplifies ratio, so no change needs to be made.

Thus the formula for Magnesium Chloride is MgCl₂

Some non-metals form oxides with different formulae. For example, C0 is carbon monoxide, CO₂ is carbon dioxide. SO₂ is sulfur dioxide and SO₃ is sulfur trioxide.
The formuale for some compounds containing only non-metals cannot be simplified by cancelling down. For example, H₂O₂ cannot be simplified as HO.

If a compound contains a metal and a non-metal, the metals symbol comes first and the non-metal changes into -ide. Examples: magnesium chloride, calcium oxide.

Naming non-metal compounds is more complicated. If a compound contains hydrogen, this comes first. If it doesn’t the non-metal with the lower group number comes first. Example: nitrogen dioxide (nitrogen is in group V and oxygen in group VI, so nitrogen comes first). If both are in the same group, the one further down the group, comes first. Example: sulfur dioxide (both are in group VI, but sulfur is below oxygen, so comes first).

Some compounds are better known differently. H₂O is water. NH₃ is Ammonia.

Some useful compound groups to remember are:

• OH – hydroxide    eg. sodium hydroxide NaOH
• NO₃ – nitrate         eg. magnesium nitrate Mg(NO₃)₂
• SO₄ – sulfate          eg. calcium sulfate CaSO₄
• CO₃ – carbonate    eg. sodium carbonate Na₂CO₃

2. Deduce the formula of a simple compound from the relative numbers of atoms present

You can tell how many atoms of each element are present in a compound. The number next to the element’s symbol indicates this. For example, a unit of Na₂SO₄ contains 2 sodium atoms, 1 sulfur atom and 4 oxygen atoms joined together.

3. Deduce the formula of a simple compound from a model or a diagrammatic representation.

You can deduce the formula of a compound by looking at its diagram. Take a look.

 

There is one Phosphorous atom and five Chlorine atoms. So this is clearly a PCl5 molecule.

 

There is one nitrogen atom and three hydrogen atoms. This is a NH₃ molecule.

4. Determine the formula of an ionic compound from the charges on the ions present.

In an ionic compound, the number of positive charges is balanced by the number of negative charges. We can work out the formula for a compound if we know the charges on the ions.

The charge on the ions will be the elements’ valencies. For metals, this charge will be positive, for non-metals it is negative.

The ionic charges of some compound groups:

• • NH₄ : 1+
  • OH:  1-
  • NO₃:  1-
  • SO₄:  2-
  • CO₃:  2-
  • HCO₃:  1-

To find the formula, for example, of aluminium nitrate:

• write down the symbols:  AlNO₃
• write down the ionic charge of each element below its symbol: Al³⁺NO3^–
• swap the numbers: Al(NO₃)₃
• balance the number: no simplifying required here

5. Construct and use word equations.

Chemical reactions are shown by chemical equations. It’s simple!

Magnesium + Oxygen = Magnesium Oxide
Zinc + Chlorine = Zinc Chloride

6. Construct and use symbolic equations with state symbols, including ionic equations.

These word equations can be written using their symbols. We can use state symbols after each element’s symbol to show what state each element is in.

(s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution- dissolved in water.

Example: Zn(s) + H₂SO₄(aq)   ——–> ZnSO₄(aq) + H₂(g)

An ionic equation is a special form of symbol equation that shows only those ions that react.

• Step 1: write down the balanced ionic equation with the state symbol.(Balancing equations will be covered next).

BaCl₂(aq) + Na₂SO₄(aq)   ———–>  2NaCl (aq) + BaSO₄(s)

• Step 2: write down the ions present in the equation. Only the aqueous compounds are split into ions. Not solids, liquids and gases.

Ba²⁺(aq) + 2Cl^–(aq) + 2Na⁺(aq) + SO₄^2-(aq)  ————–>  2Na⁺(aq) + 2Cl^–(aq) + BaSO₄(s)

• Step 3: cross out the ions that are same on both sides of the equation.

Ba²⁺(aq) + 2Cl^–(aq) + 2Na⁺(aq) + SO₄^2-(aq)  ————–>  2Na⁺(aq) + 2Cl^–(aq) + BaSO₄(s)

• Step 4: write down only the reactants and products that are left.

Ba²⁺(aq) + SO₄^2-(aq) —————->   BaSO₄(s)

7. Deduce the balanced equation for a chemical reaction, given relevant information.

Equations need to be balanced, that is, the same number of atoms present in the reactants should be present in the product.

Before we jump into balance, you should know that some gaseous elements exist as diatomic molecules. They have two atoms per molecule. Examples : hydrogen H₂; nitrogen N₂; oxygen O₂; flourine F₂; chlorine Cl₂; bromine Br₂; and iodine I₂. Whenever these elements come in equations you write them with a 2 at the end.

• Step 1: write down the formula:

H₂ + O₂ —————–> H₂O

• Step 2: Count up the atoms of each element

There are two hydrogen toms on both sides. There are two oxygen atoms on the left side but only only on the right. This is not a balanced equation.

• Step 3: Balance the equation by putting a number in front of one of the reactants or products. The number in front will multiply all the way through the molecule.

H₂ + O₂ ——————>  2H₂O

Now the oxygen atoms are balanced, but the hydrogen atoms are not!! There are four hydrogen atoms on the right (2 x H₂ = 4 hydrogen atoms).

• Step 4: keep balancing until you get all the atoms balanced

2H₂ + O₂   ————–> 2H₂O

Now the atoms are all balanced. There are two oxygen atoms on both sides and four hydrogen atoms on both sides! This has now become a balanced equation!

8. Define relative atomic mass, A_r.

Scientists have given a carbon-12 atom a mass of exactly 12 units. The mass of all other atoms is found by comparing their mass with the same number of carbon-12 atoms. This mass is called relative atomic mass.
The average mass of naturally occuring atoms of an element on a scale where the carbon-12 atom has a mass of exactly 12 units, is called relative atomic mass (A_r).

9. Define relative molecular mass, M_r, as the sum of the relative atomic masses.

Relative molecular mass is the sum of all the relative atomic masses of the atoms shown in the formula of a molecule.

 

 

Notes submitted by Lintha

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