Chemical Reactions Ch. 8

Describing Chemical Change 8-1

Writing Chemical Equations Reactant = substance that undergoes a reaction Product = the new substance formed when reactants undergo a chemical change Ex: Fe+3 + O-2 Fe2O3 (rust) reactants products

Equations Word Equations: Ex: vinegar + baking soda sodium acetate + water + carbon dioxide Chemical Equations: Ex: HC2H3O2 (aq) + NaHCO3 (s) NaC2H3O2 (aq) + H2O (l) + CO2 (g) Shows the state of substance: aq – aqueous, s – solid, l – liquid, g- gas aqueous = solid dissolved in water.

Balancing Chemical Equations (REVIEW) Law of Conservation of Mass = matter is neither created nor destroyed… Thus, mass of products = mass of reactants Atoms in a chemical reaction don’t change, they rearrange! The same number of each kind of atom must be on the left side as well as the right! Ex: H2CO3(aq) H2O(l) + CO2(g) H = 2, C =1, O = 3 H = 2, C =1, O =3

A Balancing Act To indicate more than one unit of a compound, a coefficient is placed in front Ex: 2H2O H = 4, O = 2 NEVER change the subscripts of a compound, that would change its identity!! H2O is water, H2O2 is hydrogen peroxide Rule: start by balancing the elements that appear only once of each side of the equation. Ex: Balance oxygen last! H2CO3(aq) H2O(l) + CO2(g)

Writing Equations! Step 1: Write word equation Hydrogen + oxygen water + energy Step 2: Replace words with formulas H2(g) + O2(g) H2O(l) + energy Step 3: Balance the atoms on both sides! 2H2(g) + O2(g) 2H2O(l) + energy Step 4: Check it! H = 4, O = 2 H = 4, O =2

Example Write the balanced equation: Magnesium chloride + silver nitrate magnesium nitrate + silver chloride MgCl2 + AgNO3 Mg(NO3)2 + AgCl Now balance it! MgCl2 + 2AgNO3 Mg(NO3)2 + 2AgCl

You Try! Magnesium metal + water form solid magnesium hydroxide and hydrogen gas Mg(s) + H2O(l)  Mg(OH)2(s) + H2(g) Mg(s) + 2H2O(l)  Mg(OH)2(s) + H2(g) Potassium chlorate breaks down into potassium chloride and oxygen KClO3  KCl + O2 2KClO3  2KCl + 3O2 3. Solid potassium reacts with liquid water to produce aqueous potassium hydroxide and hydrogen gas. K(s) + H2O(l)  KOH(aq) + H2(g) 2K(s) + 2H2O(l)  2KOH(aq) + H2(g)

Types of Reactions 8-2

Categories of Reactions There are 5 types of reactions: Synthesis/Combination = 2 or more substances combine to form a single product A + B AB Ex: 2Na + Cl2 2NaCl Decomposition = a compound breaks down into 2 or more simpler substances AB A + B Ex: H2CO3 H2O + CO2 Sodium + Chlorine --- Sodium chloride

3. Single-Replacement = an element that is not combined replaces an element that is part of a compound -All compounds are aqueous! A + BC  AC + B Ex: Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2(aq) + Zn(s) Double-Replacement = different atoms in 2 different compounds replace each other -All reactants are aqueous -Products will include a gas, a ppt (precipitate), or H2O AC + BD  AD + BC Ex: AgNO3 + KCl  AgCl + KNO3

Combustion = substance rapidly combines with oxygen producing heat and light. -Reactants contain C,H,O -Products are CO2 and H2O vapor CxHy + O2(g)  CO2(g) + H2O(g) Ex: 5CH3OH + O2(g)  5CO2(g) + 8H2O(g)

You Try! First balance, then put into 1 of 5 categories: BaCl2 + Na2SO4 BaSO4 + NaCl BaCl2 + Na2SO4 BaSO4 + 2NaCl Double Replacement Mg + CuSO4 Cu + MgSO4 Single Replacement H2SO4 H2 + SO4 Decomposition Fe + O2 FeO 2Fe + O2 2FeO Synthesis Al + Cl2 AlCl3 2Al + 3Cl2 2AlCl3