Law of conservation of Matter • Mass is conserved in chemical reactions • Mass of products = mass of reactants 2H2 + O2→ 2H2O 4 amu + 32 amu = 2 (18 amu)
The atoms of each element are also conserved. • 2H2 + O2→ 2H2O 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→ 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.
CLASSIFYING CHEMICAL REACTIONS • Combination reaction (also called synthesis) • A + B → AB • Identified by two reactants only one product • Na + Cl2→ NaCl • CO2 + H2O → H2CO3
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 • H2CO3→ CO2 + H2O
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 • CaCO3 + HCl → CaCl2 + H2CO3 • 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.