1. Chemical changes occur when new substances are created. The original substance(s), called reactants, change into new substance(s) called products. Copper reacting in nitric acid Date: ______Science 10 4.3 Chemical Equations

2. 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

3. Conservation of Mass in Chemical Change A Chemical change means new compounds are created. – BUT no new matter is 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)!

4. Writing and Balancing Chemical Equations word equation – uses the names of the chemicals Example: 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. Counting Atoms to Balance an Equation Word equation: – Methane + oxygen  water + carbon dioxide Skeleton equation: – CH 4(g) + O 2 (g)  H 2 O (l) + CO 2(g) To balance the compounds, take note of how many atoms of each element occur on each side of the reaction arrow.

7. 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

8. Hints for Writing Chemical Equations –Several common covalent molecules containing hydrogen have common names that do not help in writing chemical formulas For example, –methane = CH 4 –glucose = C 6 H 12 O 6 –Ethane = C 2 H 6 –Ammonia = NH 3

9. Strategies for Balancing Equations – Balance compounds first, elements last – Only add coefficients; NEVER change subscripts! – Balance metals first, then nonmetals, then H & O – Polyatomic ions (such as SO 4 2– ) can often be balanced as a whole group – Always double-check after you think you are finished!

10. Balancing Equations Examples Balance the following: Fe + Br 2  FeBr 3 LeftRight 1.Balance Fe: 1 1 2.Balance Br 2 3 X3 X2 Br = 6 6 3. Balance Fe again X2 X2 2 2 2 Fe + 3 Br 2  2 FeBr 3

11. Balancing Equations Examples Balance the following: ___ C 2 H 6 + ___ O 2  ___ CO 2 + ___ H 2 O 2C 2 H 6 + 7O 2  4CO 2 + 6H 2 O Balance the following: __Sn(NO 2 ) 4 + __K 3 PO 4  __KNO 2 + __Sn 3 (PO 4 ) 4 3Sn(NO 2 ) 4 + 4K 3 PO 4  12KNO 2 + 1Sn 3 (PO 4 ) 4