Download Prior knowledge catch-up student sheet for Chapter 3 Quantitative

3 Quantitative chemistry
Prior knowledge catch-up
Previously you could have learned:

Atoms are tiny.

Nearly all of the mass of an atom is from the mass of the protons and neutrons.

The mass number of an atom is the number of protons plus the number of neutrons in an atom.

Mass is conserved in chemical reactions.

Balanced equations show the ratios in which substances react.
Key points to remember:
Atoms are very tiny. An atom is made up of three different particles, each of which has a different
charge and mass. These particles are called subatomic particles and are listed in the table below.
Name of subatomic particle
Relative mass
Relative charge
Proton
1
+1
Neutron
1
0
Electron
1
1800
(almost nothing)
−1
The protons and neutrons are found in the centre, in the nucleus of an atom, and as a result this is
where most of the mass is found. The electrons are found in paths called shells orbiting around the
nucleus as shown in the diagram below.
Figure 3.1: The modern-day model for atomic structure.
Each element in the periodic table has two numbers. The atomic number gives the number of
protons that an atom contains. It is the smallest number of the two. Atoms of different elements
have different numbers of protons. The mass number of an atom is the sum of the number of
protons and neutrons in an atom.
It is useful to remember
© Hodder & Stoughton Limited 2016
3 Quantitative chemistry
Prior knowledge catch-up
Atomic number = number of protons = number of electrons
Number of neutrons = mass number − atomic number
For example, the atomic number of sodium is 11 and the mass number is 23.
Number of protons = 11
Number of electrons = 11
Number of neutrons = 23 − 11 = 12
Chemical reactions can be represented using a formula to show reactants and products in a chemical
equation, with an arrow in between. An equation must be balanced with the same number of each
type of atom on each side of the arrow. To make equations balance, numbers may have to be added
in front of one or more of the symbols for the reactants and products.
For example in the equation Mg + O2 → MgO
There are two oxygen atoms on the left-hand side, but only one oxygen atom on the right-hand side.
The equation is balanced by adding a 2 in front of the MgO to balance the oxygen atoms, and a 2 in
front of the Mg to balance the Mg atoms as there are now two atoms of Mg in the product:
2Mg + O2 → 2MgO
This equation shows the ratio in which the substances react.
Figure 3.2: A particle diagram for the balanced equation above.
In a balanced chemical equation, the number of atoms of each element is the same on both sides of
the equation. This is because atoms cannot be created or destroyed. As there are the same atoms
present at the start and end of a chemical reaction, mass must be conserved in the reaction. In other
words, the total mass of the reactants must equal the total mass of the products. This is known as
the law of conservation of mass.
In the equation 2Mg + O2 → 2MgO, if 2.4 g of magnesium react with 1.2 g of oxygen then according
to the law of conservation of mass, 3.6 g of magnesium oxide are formed.
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3 Quantitative chemistry
Prior knowledge catch-up
Student worksheet
Questions
1 Name the two subatomic particles found in the nucleus of the atom.
2 What is the mass and charge of a proton? Choose one answer.
mass
charge
a)
1
−1
b)
1
0
c)
1
1800
+1
d)
1
+1
3 What is the definition of atomic number? Choose one answer.
a) the larger of the two numbers associated with an element.
b) the number of neutrons in an atom
c) the number of protons in an atom
d) the number of protons and neutrons in an atom.
4 Use your periodic table to complete the table below. The first row has been completed for you.
Element
Atomic
number
Mass
number
Number of
protons
Number of
electrons
Number of
neutrons
Al
13
27
13
13
14
Mg
H
Na
F
5 Balance the equation
Ca + HCl → CaCl2 + H2
6 If 64 g copper reacts with 16 g oxygen, what mass of copper oxide will be formed?
Now try and complete the questions from the beginning of Chapter 3 from the textbook.
1 What does the following equation tell you about the reaction:
CH4 + 2O2 → CO2 + 2H2O
2 What is the principle of conservation of mass?
© Hodder & Stoughton Limited 2016
3 Quantitative chemistry
Prior knowledge catch-up
3 In a reaction, hydrogen reacts with oxygen to make water and nothing else. What mass of water
will be made if 4 g of hydrogen reacts with 32 g of oxygen?
4 How many protons, neutrons and electrons are there in an atom of
39
19
K?
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