Ch. 11 Chemical Reactions

Ch. 11.1 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 + O2 → Fe2O3

Unbalanced skeleton equationAdd physical states of the substances

info: Fe(s) + O2(g) → Fe2O3 (s)

Balanced with coefficients: 4Fe(s) + 3O2 (g) → 2Fe2O3 (s)

Balancing Chemical Equations

Making a bicycle Frame (F), Wheel (W), handlebar (H),

pedals (P) Bicycle: FW2HP2

Rx:__F + _W + __H + __P → _FW2HP2

Balanced:

Rx: F + 2W + H + 2P → FW2HP2

C (s) + O2 (g) → CO2 (g)

Name the reactants and products Reactants C, O2 Product: CO2

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 Equations1. 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 water1/2. __H2 (g) +__O2 (g) →__H2O (l)

3. 2H, 2O 2H, 1O4. __H2 (g) +__O2 (g) →2H2O (l)

3. 2H, 2O 4H, 2O4. 2H2 (g) +__O2 (g) →2H2O (l)

3. 4H, 2O 4H, 2O balanced!!!

Practice Hydrogen peroxide solution (H2O2) will

decompose into water and oxygen gas1/2:__H2O2 (aq) →__H2O (l) +__O2 (g)3. 2H, 2O 2H, 3O1/2: 2H2O2 (aq) →__H2O (l) +__O2 (g) 4H, 4O 2H, 3O1/2: 2H2O2 (aq) → 2H2O (l) +__O2 (g)3. 4H, 4O 4H, 4O balanced!!

Practice

Balance the following skeleton Rx ___ Hf + ___N2 → ___Hf3N4