CHAPTER 9 CHEMICAL REACTIONS AND EQUATIONS

Law of conservation of Matter Mass is conserved in chemical reactions Mass of products = mass of reactants 2H 2 +O 2 →2H 2 O 2H 2 +O 2 →2H 2 O 4 amu + 32 amu = 2 (18 amu)

The atoms of each element are also conserved. 2H 2 +O 2 → 2H 2 O 4 H atoms, 2 O atoms → 4 H atoms and 2 O atoms 4 H atoms, 2 O atoms → 4 H atoms and 2 O atoms

BALANCING CHEMICAL EQUATIONS When you are given an equation to balance the chemical formulas will be correct. DO NOT CHANGE THESE. H2+O2 → H2OH2+O2 → H2OH2+O2 → H2OH2+O2 → H2O Note (diatomic elements are marked with a green X on the periodic chart at the front of the room.) Balance equations by placing coefficients in front of the formulas. Balance only one element (or polyatomic ion) at a time until the entire equation is balanced.

Complete balancing worksheet

CLASSIFYING CHEMICAL REACTIONS Combination reaction (also called synthesis) A+B → AB A+B → AB Identified by two reactants only one product Na+Cl 2 → NaCl Na+Cl 2 → NaCl CO 2 + H 2 O → H 2 CO 3

Decomposition Reaction Exact opposite of combination reaction One compound is broken down “decomposed” into two. Often energy has to be added to make this happen. AB → A + B AB → A + B H 2 CO 3 → CO 2 + H 2 O H 2 CO 3 → CO 2 + H 2 O

Single Replacement Reactions One element kicks another element out of a compound and takes its place. A + BX → AX + B A more reactive metal will replace a less reactive metal or hydrogen. (See metal activity series page 295) A more reactive nonmetal will replace a less reactive nonmetal (compare electronegativity)

Double Replacement Reaction Compounds with positive and negative groups exchange partners. AX + BY → AY + BX CaCO 3 + HCl → CaCl 2 + H 2 CO 3 The 4 ions of the starting materials are all dissolved in water and no reaction will take place unless: –A molecular compound is formed (like water) –A precipitate is formed (check the solubility of possible products) –A gas that leaves the solution is formed

When will a reaction not take place? A metal will not react with a metal. A noble gas will not react with anything. A less reactive element will not replace a more reactive element.