Describing & Balancing Chemical Reactions

Describing a Chemical Reaction A chemical reaction = the process by which one or more substances is changed into one or more different substances. Reactants = the original substance(s) that react Products = the substances that form from a reaction In a chemical reaction, symbols and formulas represent the identities and relative amounts of reactants and products in a chemical reaction.

Indications of a Chemical Reaction There are 4 observable changes that indicate that a chemical reaction is taking place: Energy released as light or heat Production of a gas Formation of a precipitate Precipitate = a solid produced in a chemical reaction Color change

Precipitate Video https://www.youtube.com/watch?v=2EQznGPZY5A

Characteristics of Chemical Equations They must represent known facts. All reactants and products must be identified. They must contain the correct formulas for the reactants and products. Use charges and oxidation states to determine the chemical formulas of the products from the reactants Recall that charges must cancel—must make sense

Characteristics of Chemical Equations The law of conservation of mass must be satisfied. The same number of each type of atom must appear on each side of the chemical equation Coefficients = whole numbers that are placed in front of a formula to balance the chemical equation. Ex: 2H2O

Symbols Used in Chemical Equations + if more than one reactant or product is present  “yields,” “reacts to produce,” “produce,” or “form” ( ) Heat is added (s) Reactant or product is a solid or a precipitate (l) Reactant or product is a liquid (g) Reactant or product is a gas (aq) Reactant or product is aqueous – dissolved in water

Writing Chemical Formulas Chemical reactions can be written as: Word equations: methane + oxygen  carbon dioxide + water Formula equations: CH4 (g) + O2 (g)  CO2 (g) + H2O (g) (not balanced) CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)

Sample Problem #1 Word Equation: Write the word and formula equations for the reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in water). Word Equation: Formula Equation: Balanced Formula Equation: Sodium oxide + water  sodium hydroxide Na2O (s) + H2O (l)  NaOH (aq) (unbalanced) Na2O (s) + H2O (l)  2NaOH (aq)

Significance of a Chemical Equation Chemical reactions contain quantitative information: Coefficients indicate relative amounts of reactants and products. (moles) The relative masses of the reactants and products can be determined.

Significance of a Chemical Equation The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction. 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

Balancing Chemical Equations Steps for balancing chemical equations: Balance the different types of atoms one at a time. Balance polyatomic ions that appear on both sides of the equation as single units. Balance any metals next. Balance H then O atoms after all other elements have been balanced.

Sample Problem #2 __Zn (s) + HCl (aq)  ZnCl2 (aq) + ___ H2 (g) Zinc reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas. Write and balance the equation. __Zn (s) + HCl (aq)  ZnCl2 (aq) + ___ H2 (g)

Practice ___C2H4(g) +___ H2(g)  ____ C2H5

Practice Balance and Translate to a chemical sentence: ___BaCl2(aq) + ___Na2(CrO4)(aq).  ___Ba(CrO4)(s) + ___NaCl(aq)

Practice Hydrazine, N2H4, is used as rocket fuel. Hydrazine reacts violently with oxygen to produce gaseous nitrogen and water. Write the balanced chemical equation.

Practice Solid aluminum metal reacts with aqueous zinc chloride to produce solid zinc metal and aqueous aluminum chloride. Write the balanced chemical equation.