Chemical Equations Chemical equations represent a chemical reaction. Be aware of symbols that represent gases, liquids, solids, and aqueous. The arrow represents yields, similar to equals in math. Reactants are the substances reacting. Products are the substances being produced. The reaction for solid zinc reacting with hydrochloric acid producing aqueous zinc chloride and hydrogen gas is represented by the following equation: Zn (s) + HCl (aq)  ZnCl 2 (aq) + H 2 (g)

Balancing We use coefficients in an equation to balance the amount of products and reactants in a reaction. You may change coefficients as often as necessary. You may NEVER change subscripts. There are no set rules for balancing you basically have to see what you are starting with (take inventory of all elements)and go from there. If you see a polyatomic ion on both sides treat it as one unit. If not you must break it up. Now lets balance the reaction between zinc and hydrochloric acid.

Balancing Practice 1) NaOH + KNO 3 --> NaNO 3 + KOH 2) C 2 H 4 + O 2 --> CO 2 + H 2 O 3) Fe + NaBr --> FeBr 3 + Na 4) (NH 4 ) 2 SO 4 + Mg(OH) 2 --> NH 4 OH + MgSO 4 5) NH 4 OH + H 2 SO 4 --> H 2 O + (NH 4 ) 2 SO 4 6) Na + H 2 O--> NaOH + H 2 7) Na 2 CO 3 --> Na 2 O + CO 2

Reaction Classification All chemical reactions can be placed into one of six categories. Here they are, in no particular order: Synthesis Decomposition Single Displacement Double Replacement Acid/Base Neutralization Combustion

Synthesis: A synthesis reaction is when two or more simple compounds combine to form a more complicated one. These reactions come in the general form of: A + B ---> AB One example of a synthesis reaction is the combination of iron and sulfur to form iron (II) sulfide: 8 Fe + S 8 ---> 8 FeS Practice: Write chemical equation that represents sodium oxide reacting with water to form sodium hydroxide:

Iron II Sulfide

Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These reactions come in the general form: AB ---> A + B One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas: 2 H 2 O ---> 2 H 2 + O 2 Practice: Potassium chloride and oxygen gas are produced from the decomposition of potassium chlorate. Write a chemical equation to represent this reaction.

Decomposition of Water

Single displacement: This is when one element trades places with another element in a compound. These reactions come in the general form of: A + BC ---> AC + B One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas: Mg + 2 H 2 O ---> Mg(OH) 2 + H 2 Practice: Chlorine gas reacts with solid sodium bromide to produce liquid bromine and solid sodium chloride. Write a balanced chemical equation.

Formation of Magnesium Hydroxide

Double displacement: This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form: AB + CD ---> AD + CB One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate: Pb(NO 3 ) KI ---> PbI KNO 3 Practice: Aqueous barium chloride reacts with aqueous copper II sulfate to produce barium sulfate solid and aqueous copper II chloride. Write a balanced chemical equation.

Lead iodide formation

Acid-base: This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H + ion in the acid reacts with the OH - ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water: HA + BOH ---> H 2 O + BA One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide: HBr + NaOH ---> NaBr + H 2 O Practice: Hydrochloric acid reacts with aqueous calcium hydroxide to produce water and calcium chloride. Write a balanced chemical equation.

Acid-Base Neutralization

Combustion: A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. An example of this kind of reaction is the burning of napthalene: C 10 H O 2 ---> 10 CO H 2 O Practice: Write a balanced chemical reaction for the combustion of methane.

Combustion of methane bubbles