Chemical Equations Represent Chemical Reactions An equation must

Chemical Equations Represent Chemical Reactions

An equation must: § Represent known facts (be chemically possible) § Contain CORRECTLY written formulas § Comply with the Law of Conservation of Mass This is why we must balance them!

Law of Conservation of Mass § Mass is neither created nor destroyed in a chemical reaction total mass stays the same n atoms can only rearrange n 4 H 36 g 2 O 4 g 32 g 4 H 2 O

Chemical Equations A+B C+D REACTANTS PRODUCTS

Chemical Equations § Reactants – substances you begin with and are written on the LEFT side of the arrow. § Products are the substances you end up with, and are written on the RIGHT side of the arrow.

Chemical Equations

Synthesis § Two substances come together to form one product. § A + B AB § Examples: § 2 H 2 + O 2 2 H 2 O § 2 Na + Cl 2 2 Na. Cl

Decomposition § One compound breaks down into two or more elements or compounds. § AB A + B § Examples: § Ca. CO 3 Ca. O + CO 2 § 2 Ag 2 O 4 Ag + O 2

Single Replacement § Atoms of one element replace the atoms of another element in a compound. § AX + B AB + X § Examples: § Mg. Br 2 + Cl 2 Mg. Cl 2 + Br 2 § Ca. O + Mg Mg. O + Ca

Double Replacement § Ions are exchanged between two compounds in a reaction § AX + BY AY + BX § Examples: § Pb. Cl 2 + Li 2 SO 4 Pb. SO 4 + 2 Li. Cl § 6 KOH + Co 3(PO 4)2 2 K 3 PO 4 + 3 Co(OH)2

Combustion § O 2 reacts with something to produce H 2 O and CO 2 § Cx. Hy + O 2 H 2 O + CO 2 § A + O 2 AO § Examples: § CH 4 + 2 O 2 2 H 2 O + CO 2 § 2 Mg + O 2 2 Mg. O

Three Types of Equations § Skeleton equation (unbalanced) Al + Cl 2 Al. Cl 3 § Balanced equation 2 Al + 3 Cl 2 2 Al. Cl 3 § Word equation Aluminum plus chlorine yields aluminum chloride

Diatomic Elements 7 elements that, when not part of a compound, ALWAYS exist as a “pair” Br I N Cl H O F (write with a 2 subscript) Ex: Br 2

Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients to make # of atoms on both sides equal. Coefficient subscript = # of atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

Helpful Tips § Balance one element at a time. § Update ALL atom counts after adding a coefficient. § If an element appears more than once per side, balance it last. § Balance polyatomic ions as single units. § “ 1 SO 4” instead of “ 1 S” and “ 4 O”

Helpful Tips § If there is no way to balance with a whole number, put a decimal in as the coefficient and then double everything. § When balancing a chemical equation you may add coefficients but you may NOT change subscripts. Subscripts are part of the compound and cannot be changed.

Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2 Al + 3 Cu. Cl 2 3 Cu + 2 Al. Cl 3 2 1 Al 1 2 3 1 Cu 1 3 6 2 Cl 3 6

Examples: Al + Cl 2 Al. Cl 3

Examples: C 2 H 6 + O 2 CO 2 + H 2 O

Examples: Na + H 2 O Na. OH + H 2

Examples: Barium chloride reacts with sulfuric acid to produce barium sulfate and hydrochloric acid.

Examples: Hydrogen peroxide (H O ) decomposes to 2 2 produce oxygen and dihydrogen monoxide.

Examples: Iron (II) phosphate and lithium sulfide yields lithium phosphate and iron (II) sulfide.

More Categories of Reactions: *Reactions can be classified further into 3 categories (in addition to S, D, SR, DR, C) 1) acid - base reaction 2) precipitation reaction 3) reduction - oxidation reaction

Acid - Base Reactions: *Is a type of DOUBLE REPLACEMENT reaction. *Is called Neutralization: The reaction between a traditional acid and base to produce a salt and water.

*Traditional Acid: A compound that STARTS WITH HYDROGEN, and when dissolved in water, provides a H +1 ion. *Traditional Base: A compound that ENDS WITH HYDROXIDE, and when dissolved in water, provides a OH -1 ion. *Salt: A compound consisting of the cation from a base and the anion from an acid. HX (aq) + ZOH (aq) ZX (aq) + H 2 O (l) ( ) + ( )

Acid - Base Reactions: Example: nitrous acid reacts with sodium hydroxide to produce sodium nitrite and water

Acid - Base Reactions: Example: HCl. O 2 (aq) + Sr(OH)2 (aq) Sr(Cl. O 2)2 (aq) + H 2 O (l)

Precipitation Reactions: *Is a type of DOUBLE REPLACEMENT reaction. * Precipitate: A solid (insoluble) product formed in a reaction of aqueous solutions. AB (aq) + CD (aq) AD (s) + CB (aq)

Precipitation Reactions: Example: Pb(C 2 H 3 O 2)2 (aq) + KI (aq) KC 2 H 3 O 2 (aq) + Pb. I 2 (s)

Precipitation Reactions: Example: A calcium chloride solution and a sodium carbonate solution are mixed and solid calcium carbonate and aqueous sodium chloride are produced.

Reduction - Oxidation Reactions: *Can be SYNTHESIS, DECOMPOSITION, COMBUSTION, or SINGLE REPLACEMENT. *Is called Redox: A reaction in which there are changes in charge/oxidation number of reactants and products. * Oxidation Is Loss of electrons, Reduction Is Gain of electrons (OIL RIG)

Reduction - Oxidation Reactions: * For a Red-Ox reaction to be valid, both reduction and oxidation must occur. *When Oxidation occurs, the charge becomes more positive if you compare left to right side. *When Reduction occurs, the charge becomes more negative if you compare left to right side.

Reduction - Oxidation Reactions: *Basic rules for assigning oxidation numbers: 1) the charge/oxidation number on all elements (or diatomic elements) is zero. 2) for any monatomic ion, the oxidation number is the same as the charge on the ion. 3) the sum of the oxidation numbers in an ionic or molecular compound is zero. (this is why compounds are neutral)

Reduction - Oxidation Reactions: *HINT: The easiest way to recognize a Red-Ox reaction is if a substance is elemental (or diatomic) on one side and part of a compound on the other side of the reaction arrow.

Reduction - Oxidation Reactions: Example: Ag (s) + O 2 (g) Ag 2 O (s) (also Synthesis)

Reduction - Oxidation Reactions: Example: Water, when electrolyzed, produces hydrogen and oxygen (also Decomposition)

Reduction - Oxidation Reactions: Example: Zn (s) + H 2 SO 4 (aq) Zn. SO 4 (aq) + H 2 (g) (also Single Replacement)

Reduction - Oxidation Reactions: Example: Methane burns in oxygen to produce carbon dioxide and water (also Combustion)