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REACTING MASSES, AND CHEMICAL EQUATIONS

The Names of Compounds

The Names and Formula of Compounds

You can easily name the compound, just by following these rules:

  • When a compound contains a metal with a non-metal:

- The name of metal is given first.

- and then the non-metal, but

ending with "ide".

  • When a compound contains 2 non-metals:

- If it contains a hydrogen then that

is named first, otherwise the one

with the lower group is named first.

- and then the non-metal, but

ending with "ide"

Finding formulae from the structure of compound

The formula of a compound is related to its structure. For example:

Finding formulae from the structure of compounds

Valency

The valency of an element is the number of electrons its atoms lose, gain or share, to form a compound.

Valency

Writing formulae using valencies

Writing formulae using valencies

This is how to write the formula of a compound, using valencies:

1. Write down their symbols, in the same order as the elements in the name.

2. Write down the valencies of the two elements.

3. Add numbers after the symbols if you need to, to balance the valencies.

EXAMPLE

What is the formula of aluminum oxide?

1. AlO

2. Valencies: aluminum is 3; oxygen is 2.

3. Al O

= Al2O3

3 2

Examples of chemical reactions

1. When hydrogen reacts with oxygen it gives water. The formula is written like this:

H2 + 02 ----> H20

Equations for Chemical Reactions

Balancing Equations

Balancing Equations

Now to balance the equation you need to see the number of hydrogen and oxygen atoms on each side.

H2 + 02 ----> H20

= H2 + 02 ----> 2H20

= 2H2 + 02 ----> 2H20

Adding state symbols

Adding state symbols

You can show the states of the reactants and products by adding state symbols to the equations:

(s) for solid (l) for liquid

(g) for gas (aq) for aqueous solution

For the reactions below, the equation with state symbols are:

2H2 (g) + O2 (g) ------> 2H2O (l)

How to write the equation for a reaction

How to write the equation for a reaction

EXAMPLE

Q. Magnesium burns in oxygen to form magnesium oxide, a white solid. Write an equation for the reaction.

1. magnesium + oxygen -------> magnesium oxide

2. Mg + O2 -------> MgO

3. The equation is not balanced.

Mg + O2 -------> 2MgO

The equation is still not balanced.

2Mg + O2 -------> 2MgO

The equation is balanced.

4. 2Mg(s) + O2(g) -------> 2MgO(s)

Standard Carbon Atom

Scientist needed to compare the masses of atoms, so they chose an atom of carbon-12 to be the standard atom. They fixed its mass as exactly 12 atomic mass units.

Then they compared all the other atoms with this standard atom, and found values for their masses.

The Masses of Atoms, Molecules, and Ions

Ar and Isotopes

The atoms of an element are not always identical. Some may have extra neutrons. Different atoms of the same element are

called isotopes.

We need to take all the natural isotopes of an element into account, to

work out the relative atomic mass.

The relative atomic mass (Ar) of an element is the average mass of its isotopes compared to an atom of carbon-12.

Ar and Isotopes

Finding the masses of molecules

Finding the mass of molecules:

If the substance is made of molecules, its mass found in this way is called

the relative molecular mass, or Mr.

Finding the masses of molecules and Ions

Finding the masses of Ions

Finding the mass of a Ion:

An ion has the same mass as the atom from which it is made.

If the substance is made of ions, its mass is called the relative formula mass, which is also Mr for short.

Two Laws of chemistry

Some Calculations about Masses and %

1. Elements always react in the same ratio, to form a given compound.

EXAMPLE

when carbon burns in oxygen to form carbon dioxide:

6 g of carbon combines with 16 g of oxygen, so

12 g of carbon will combine with 32 g of oxygen.

2. The total mass does not change, during a chemical reaction.

EXAMPLE

12 g of carbon and 32 g of oxygen give 44 g of carbon dioxide.

Calculating Quantities

Calculating Quantities

Calculating quantities is quite easy, using the laws above.

EXAMPLE

Q1. 64 g of copper reacts with 16 g of oxygen to give the black compound copper(II) oxide.

a). What mass of copper will react with 32 g of oxygen?

Ans: - 64 g of copper reacts with 16 g of oxygen, so

C O

64 16 = 128g of copper

? 32

b). What mass of oxygen will react with 32 g of copper?

Ans: - 16 g of oxygen reacts with 64 g of copper, so

C O

64 16 = 8g of oxgyen

32 ?

Calculating the % composition of a compound

Calculating the % composition of a compound

The percentage composition of a compound tells you how much of each element it contains, as a percentage of the total mass. This is how to work it out:

1. Write down the formula of the compound.

2. Using Ar values, work out its molecular or formula mass (Mr).

3 Write the mass of the element as a fraction of the Mr

4. Multiply the fraction by 100, to give a percentage.

EXAMPLE

Calculate the percentage of oxygen in sulfur dioxide.

1. The formula of sulfur dioxide is SO2.

2. The Mr of the compound is 64 and the Mr of oxygen is 32

3. Mass of oxygen as a fraction of the total = 32/64.

4. Mass of oxygen as a percentage of the total = 32/64 x 100 = 50%

So the compound is 50% oxygen.

Calculating % purity

A pure substance has nothing else mixed with it.

But substances often contain unwanted substances, or impurities.

Purity is usually given as a percentage. This is how to work it out:

Calculating % purity

Example

Impure copper is refined (purified), to obtain pure copper for use in computers. 20 tonnes of copper gave 18 tonnes of pure copper, on

refining.

Q. What was the % purity of the copper before refining?

= %purity of the copper = 18 tonnes/20 tonnes x 100 = 90%.

So the copper was 90% pure.

Example

One More Thing

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