1. 4.3 Chemical Equations A Chemical change means new compounds are created. Law of Conservation of Mass: matter is not created or destroyed; atoms are just rearranged. mass of reactants = mass of products number of each = number of each atom in reactants atom in products If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O 2 ) = mass of products (exhaust)!

2. Provincial Exam Question

3. Chemical reactions can be written in different ways. –A word equation: Nitrogen monoxide + oxygen  nitrogen dioxide –A symbolic equation: 2NO (g) + O 2(g)  2NO 2(g) State of matter - Letters indicate the state of each compound (aq) =aqueous/ dissolved in water (s) = solid (l) = liquid (g) = gas Coefficients - indicate the ratio of compounds in the reaction - here, there is twice as much NO and NO 2 than there is O 2

4. Writing and Balancing Chemical Equations word equation – uses the names of the chemicals Example: K + O 2  K 2 O Potassium metal + oxygen gas  potassium oxide

5. Writing and Balancing Chemical Equations A skeleton equation shows only the formulas of the elements/compounds –Shows atoms, but is not balanced K (s) + O 2 (g)  K 2 O (s) A balanced chemical equation shows the correct number of each atom –Balancing ensures that the number of each atom is the same on both sides of the reaction arrow –Always use the smallest whole number ratio 4K (s) + O 2 (g)  2K 2 O (s)

6. Hints for Writing Chemical Equations –Use the chemical symbol to represent single elements. Be careful of diatomic and polyatomic elements such as O 2, P 4 and S 8 The “special seven” are all diatomic elements (hockey stick & puck) –H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2

7. Balancing Equations Examples Balance the following: __Sn(NO 2 ) 4 + __K 3 PO 4  __KNO 2 + __Sn 3 (PO 4 ) 4 Balance the following: ___ C 2 H 6 + ___ O 2  ___ CO 2 + ___ H 2 O

8. Provincial Exam Question Take the Section 4.3 Quiz