1. Chemical reactions
What is a chemical reaction?

2. Objectives Outcomes: To consider the features of chemical reactions;
To familiarise ourselves with the concept of conservation of mass
Outcomes:
You should be able to explain the features of chemical reactions and what is required for them to happen.
You must understand fully the concept of conservation of mass

3. Features
How do you know a chemical reaction has happened? (What might you observe)

4. What makes a chemical reaction happen?
What has to happen for a chemical reaction to occur?

5. Reactivity
Name very reactive elements.
Why are they so reactive?

6. Chemical reactions Reactants make Products
E.g. nitrogen + hydrogen = ammonia
Nitrogen and hydrogen are the reactants, ammonia is the product
What are the reactants in photosynthesis?
What are the products of combustion?

7. Unreactive
Name some unreactive elements. Why are they so unreactive?

8. What controls the number of particles that can combine?
Group 1 and 2
Groups 6&7
Transition metals

9. What happens Reactants combine/separate to form products.
We can describe them using equations
Equations can be word equations (general description) Or
Formulae
Equations with formulae must be balanced with the same number of atoms of each chemical on either side

10. Conservation of Mass
Matter cannot be created or destroyed (only changed)

11. What this means When you write an equation you must:
1. Account for all the elements involved
2. Make sure there are equal numbers of atoms of each element on either side of the equation;
3. Ensure you use the correct symbols and formulae
4. You may want to show that it is in solution with water: (aq) or it is a gas (g)

12. It is also important to know
1. What energy the reaction needs to get started : the activation energy
2. What energy is given out when bonds are broken
3. What energy is put in when bonds are made.
We will consider this when we discuss exothermic and endothermic reactions.

13. Balancing equations
1. The number of atoms on either side of the equation must be constant
2. The number of atoms of each element must be constant.
3. The number of particles may change
4. The number of chemicals may change.
5. The mass of either side will be the same.

14. To balance an equation NaOH + H2SO4 = Na2SO4 + H2O
1st count number of atoms of each element on each side Na O
H 3 2
S 1 1
Start with the Na. There are two in the products so make 2 in the reactants by putting a 2 in front of NaOH
This now gives you 4 H and 6 O which once you’ve taken off the 4 O in the sulphate ion allows for 2 molecules of water.

15. Balancing an equation Sodium + oxygen = sodium oxide Na + O2 = Na2O
(Balance this).
H2SO4 + Ca (OH)2 = CaSO4 + H2O
(and this)