Concept 7 “Chemical Reactions” Honors Chemistry 1

Writing a chemical equation. FOR A CHEMICAL EQUATION TO BE VALID: THE EQUATION MUST REPRESENT THE KNOWN FACTS. THE EQUATION MUST CONTAIN THE CORRECT FORMULAS OF THE REACTANTS AND PRODUCTS. THE LAW OF CONSERVATION OF MATTER MUST BE SATISFIED.

When balancing: BALANCE ATOMS THAT APPEAR ONLY ONCE PER SIDE FIRST. TREAT POLYATOMIC IONS AS UNITS IF THEY APPEAR ON BOTH SIDES OF THE EQUATION. BALANCE ANY REMAINING NON-H OR O ATOMS NEXT. BALANCE H NEXT. BALANCE O LAST.

AVOID MISTAKES-USE THE CORRECT CHEMICAL FORMULA! DON’T CHANGE SUBSCRIPTS!!!!

Three Types of Equations The first type of Chemical Equation is the word equation Word equations are expressed as you would speak a sentence. For example: HYDROGEN PLUS OXYGEN REACT TO FORM WATER.

Second type: The skeleton or unbalanced chemical equation Correct chemical formulas and symbols are substituted for the words. For the word equation: HYDROGEN PLUS OXYGEN REACT TO FORM WATER We write H → H 2 O

The third type is the balanced chemical reaction The law of conservation of matter must be obeyed. For every atom of oxygen on the reactant side, an atom of oxygen must be represented on the product side. To achieve this you add coefficients in front of the chemical formulas you need more of. DO NOT CHANGE SUBSCRIPTS

H 2 + O 2 → H 2 0 is not balanced Both sides have two H, but the product side only has one 0. We can't change the subscripts but we can have more than one H 2 O. Add another H 2 O then the 0 are balanced but we have two H on the reactant side and four on the product side. We balance the H by adding another H 2. This gives the balanced chemical equation…

2H 2 + O 2 → 2H 2 O This is a balanced chemical equation!

COMMON SYMBOLS USED IN CHEMICAL EQUATIONS It is often useful to know what state of matter or phase a reactant or product is in. They are not to be balanced. (aq) aqueous dissolved or made up in a water solution (I) exists as a liquid during this rxn (s) exists as a solid during this rxn (cr) exists as a crystal solid during the rxn (g) exists as a gas during the rxn (↓), precipitates falls out of the solution as a solid product (↑) gas product leaves the solution as a gas product → yields separates the reactants from the products indicates the direction of the rxn ↔ reversible rxn can go forwards or backwards

Five types of chemical reactions. Synthesis, composition or combination Analysis or decomposition Single replacement or displacement Double replacement or displacement Combustion

Synthesis, composition or combination A + X → AX RXN OF ELEMENT WITH OXYGEN → OXIDE RXN OF 2 NONMETALS → COVALENT COMPOUND RXN OF METAL WITH NONMETALS, NOT OXYGEN → SALTS RXN OF OXIDES AND WATER → HYDROXIDES

Analysis or decomposition AX → A + X DECOMP OF BINARY→ ELEMENTS DECOMP OF METAL CARBONATE → METAL OXIDE AND CO 2 DECOMP OF METAL HYDROXIDE → METAL OXIDE AND H 2 O DECOMP OF METAL CHLORATES → METAL CHLORIDE AND 0 2 DECOMP OF ACIDS → NONMETAL OXIDES AND H 2 O

Single replacement or displacement A + BX → AX + B or Y + BX → BY + X REPLACE. OF A METAL BY A MORE REACTIVE METAL Activity Series p288 REPLACE. OF H 2 IN H 2 0 BY A METAL → METAL HYDROXIDES AND H 2 REPLACE. OF H 2 IN AN ACID BY A METAL → SALT AND H 2. REPLACE. OF HALOGENS

Double replacement or displacement AY + BX → AX + BY FORMATION OF PRECIP.-ANIONS OF ONE REACTANT COMBINE WITH CATIONS OF THE OTHER REACTANT TO FORM AN INSOLUBLE OR SLIGHTLY INSOLUBLE COMPOUND. p 920 FORMATION OF GAS-WHEN THE PRODUCT PRODUCES A GAS WHICH IS INSOLUBLE IN THE NEW SOLUTION. THIS GAS WILL BUBBLE OUT OF SOLUTION.

Combustion C x H x + O 2 → CO 2 + H 2 O Hydrocarbon or carbohydrate and oxygen yields carbon dioxide and water Release large amounts of energy in the form of heat and light.

Reactions that form precipitates Complete ionic equation - break the chemical formulas into the ions they are formed from. Be sure to include symbols. Start with: AgNO 3 (aq) + NaCl(aq) → NaNO 3 (aq) + AgCl(s) Break it down into the aqueous ions: Ag +1 (aq) + NO 3 -1 (aq) +Na +1 (aq) + Cl -1 (aq) → Na +1 (aq) + NO 3 -1 (aq) + AgCl(s)

Spectator ions Spectator ions are the ions that don’t change from reactants to products. Ag +1 (aq) + NO 3 -1 (aq) + Na +1 (aq) + Cl -1 (aq) → Na +1 (aq) + NO 3 -1 (aq) + AgCl(s) Eliminate the ions that are un- changed (in red). Ag +1 (aq) + NO 3 -1 (aq) + Na +1 (aq) + Cl -1 (aq) → Na +1 (aq) + NO 3 -1 (aq) + AgCl(s)

Net ionic equation The net ionic equation is what remains after the spectator ions are removed. If no ions remain then the rxn will not occur. AgNO 3 (aq) + NaCl(aq) → NaNO 3 (aq) + AgCl(s) Becomes: Ag +1 (aq) + Cl -1 (aq) → AgCl(s)