Ch. 11 Chemical Reactions Ch Describing Chemical Reactions

Vocabulary Reactants: original substances before the reaction (Rx) Products: new substances formed in the Rx ‘arrow’ → symbolizes chemical Rx

Definition of a Reaction  One or more reactants change into one or more products.  Reactants → products

1. Chemical Word Rx Hydrogen peroxide decomposes to form water and oxygen gas.  Hydrogen peroxide → water + oxygen

2. Chemical Equations They show  Chemical formulas  Number of atoms or molecules of reactants and products  Solid (s), liquid (l), gas (g), aqueous solution (aq)  Direction of the reaction → or ↔  What facilitates the Rx heat →

Examples  Fe + O 2 → Fe 2 O 3 Unbalanced skeleton equation Add physical states of the substances info:  Fe(s) + O 2 (g) → Fe 2 O 3 (s) Balanced with coefficients:  4Fe(s) + 3O 2 (g) → 2 Fe 2 O 3 (s)

Balancing Chemical Equations  Making a bicycle  Frame (F), Wheel (W), handlebar (H), pedals (P)  Bicycle: FW 2 HP 2 Rx:__F + _W + __H + __P → _ FW 2 HP 2 Balanced: Rx: F + 2W + H + 2P → FW 2 HP 2

C (s) + O 2 (g) → CO 2 (g) Name the reactants and products  Reactants C, O 2  Product: CO 2 Is the equation balanced? Meaning are the same # of elements on both sides of the arrow? 1 x C, 2 x O → 1 x C, 2 x O  yes

Rules for Balancing Equations 1.Determine the correct formulas for all reactants and products. 2.Write the skeleton equation, with the reactants on the left side, and the products on the right side of the yield arrow. 3.Count the # of atoms on both sides, counting polyatomic ions as single units if they appear unchanged on both sides of the arrow. 4.Balance single elements first by using coefficients. When no coefficient is used it is assumed to be 1. Never balance by changing the subscripts of a formula. 5.Check that each atom or polyatomic ion is equal on both sides of the reaction. 6.All coefficients have to be in the lowest possible ratio.

Practice  Hydrogen gas and oxygen gas react to form water 1/2. __H 2 (g) +__O 2 (g) → __H 2 O (l) 3.2H, 2O 2H, 1O 4. __H 2 (g) +__O 2 (g) → 2H 2 O (l) 3. 2H, 2O 4H, 2O 4. 2H 2 (g) +__O 2 (g) → 2H 2 O (l) 3. 4H, 2O 4H, 2O balanced!!!

Practice  Hydrogen peroxide solution (H 2 O 2 ) will decompose into water and oxygen gas 1/2:__H 2 O 2 (aq) → __H 2 O (l) +__O 2 (g) 3. 2H, 2O 2H, 3O 1/2: 2H 2 O 2 (aq) → __H 2 O (l) +__O 2 (g)  4H, 4O 2H, 3O 1/2: 2H 2 O 2 (aq) → 2H 2 O (l) +__O 2 (g) 3. 4H, 4O 4H, 4O balanced!!

Practice  Balance the following skeleton Rx  ___ Hf + ___N 2 → ___Hf 3 N 4