Starter What are the formulas for the following compounds: What are the symbols for the following elements: Sodium Oxygen Chlorine Iron Copper What are the formulas for the following compounds: Water Potassium hydroxide Magnesium oxide Potassium iodide Caesium oxide Na O Cl Fe Cu 1. H2O 2. KOH 3. MgO 4. KI 5. Cs2O

Key facts In chemical reactions new substances are made. A reaction can be summarised in a word equation or a symbol equation. The total mass of reactants is the same as the total mass of products in a reaction A symbol equation must be balanced as no atoms are created or destroyed. The conservation of mass in a chemical reaction is an essential prerequisite to understanding why we need to balance equations.

Balancing equations C2O X CO4 X C2O2 X 2CO2 You can never change a formula when balancing an equation. You can only put a number in front of a formula. Lets consider how to show 2 molecules of CO2: C2O X CO4 X C2O2 X 2CO2 This is an animated slide. When you press the left hand mouse button once, the first piece of information will appear; press the button again and the next will appear etc. This animation can be removed to have all text displayed by going into slide show, custom animation, highlight each piece of text in turn and click (left-hand mouse key) remove.

Symbol equations No atoms are created or destroyed in a chemical reaction. There should be the same number of atoms of each type of element each side of the equation. Magnesium + Oxygen Magnesium oxide Write the balanced symbol equation below explaining how you balance it at the same time (2Mg +O2  2MgO)

2 Mg + O2  MgO 2  Symbols Mg + O2

Balanced or unbalanced?

Al + 2 Cl2 AlCl3 Atom Type Left hand side Right hand side Balanced? Al   Cl

2Na + 2Cl2 2NaCl Atom Type Left hand side Right hand side Balanced?  

You try – balance these equations C + O2 CO2 H2 + O2 H2O N2 + H2 NH3 CH4 + O2 CO2 + H2O Already balanced 2H2 + O2  2H2O 2N2 + 3H2  2NH3 CH4 + 2O2  CO2 + 2H2O

State Symbols H2 O2 CO2 H2O C E.g. (s) (l) (g) (aq) State that the temperature should be room (25oC). H2(g) O2(g) CO2(g) H2O(l) C(s)

What is wrong with this equation? Lithium was added to water in a school lab. Below is the symbol equation for the reaction – or is it? Li(s)+ H2O(s)  LiO(aq) + H2(g) This reaction would make lithium hydroxide. Water is a liquid at room temperature. Lithium oxide immediately reacts with water to make lithium hydroxide therefore this phase of lithium oxide is not possible. Extension – get the students to complete the correct symbol equation for this reaction. http://www.youtube.com/watch?v=8ypUVpwgcAA