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Grade 12 Physical Sciences chemistry revision
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Saturday X-tra X-Sheet: 2
Revision of Grade 11 Chemical Change
Key Concepts
This lesson focuses on the following:
• Representation of chemical change
• Writing down chemical formulae of both reactants and products
• Role of kinetic energy in chemical reactions
• The collusion theory
• Energy changes during chemical reactions and graphs
• Types of chemical reactions
Terminology & definitions
• Collision theory – molecules and atoms must have enough kinetic energy and the
right orientation to react chemically
• Reactants – chemical entities in the form of atoms or molecules that interact
together to form a product
• Products – chemical substances formed when a chemical reaction takes place
• Exothermic reaction – a chemical reaction that liberates/gives off energy
• Endothermic reaction – a chemical reaction that takes in energy
• Heat of reaction ( H) – the energy difference between products and reactants
• Activation energy – energy needed to start a chemical reaction
• Catalyst – a substance that speeds up a chemical reaction. It does so by lowering
the energy hill (amount of activation energy needed to initiate a chemical reaction).
• Activated complex – a state during a chemical reaction where the reactants have
enough energy to be able to react with each other.
X-planation of key concepts and terminologies
Chemical change is represented by a chemical equation. When sodium reacts with
oxygen to form sodium oxide, the following equation is used to represent the chemical
change that occurrs from the reactants to the formation of products:
sodium + oxygen sodium oxide.
This chemical equation can be written in symbol form:
Na(s) + O2(g) Na2O(s).
It is important to show the states (phases) in which the reactants and products occur.
(s) for solid, (g) for gas, ( ) for liquid and (aq) for substance reacting in the form of
solutions where water is the solvent.
Whilst it may be easier to understand where the chemical formulae for the reactants
come from; that sodium is represented as Na on the periodic table and that oxygen is
represented as O and that it exists as a diatomic molecule, hence the O2, it is often
difficult to tell what the chemical formula of the product is going to be. In this case the
chemical formula of the product is Na2O. Let’s look at where this comes from: Ions
formed from atoms of elements in group1 will always carry a charge of 1+. Thus
sodium ion will be represented as Na1+ or Na+. On the other hand, ions of non metals
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from group 6 will carry a negative charge given by 6 – 8 = - 2. Thus the charge on an
oxygen ion will be O2-. The numerical values of the charges carried by the ions are
used to tell the resulting chemical formula. This is done by swapping these numerical
values between the reacting ions and placing them as subscripts on the product
molecule formed. Thus for sodium Oxide, the chemical formula becomes, Na2O as
shown above.
Example 1:
Write the balanced chemical equation for the reaction of aluminium and chlorine to
produce aluminium chloride.
Solution:
Step 1: The chemical equation in words is:
Aluminium + Chlorine Aluminium Chloride.
Step 2: Write the formula of all reactants and products
Aluminium: Al
Chlorine: Cl2 (Remember: Chlorine is diatomic_
Aluminium chloride: AlCl3
Aluminium is a metal found in group 3 in the periodic table and will loose three
electrons and so the charge on Aluminium will be 3+. Chlorine comes from group 7, its
charge in the ion form will be 7 – 8 = -1. Thus the chloride ion will be C 1-. When the
numerical values of the two ions are swapped to find the resulting chemical formula of
the product, the following is found to be the chemical formula of Aluminium Chloride;
AlC 3.
Step 3: Write the chemical equation in symbols:
A (s) + C 2(g) A C 3(s).
Step 4: Balance the equation
2 A (s) + 3 C 2(g) 2A C 3(s).
Example 2
Write down the chemical formulae for:
(a) cobalt (II) chloride
(b) iron (III) oxide
(c) magnesium oxide
(d) ammonium nitrate
(e) ammonium sulphate
(f) calcium hydroxide
Example 3
Write down the balanced chemical equation for the reaction when zinc reacts with
hydrochloric acid to produce zinc chloride and hydrogen gas.
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