Ch. 8 – Chemical Reactions Intro to Reactions
Signs of a Chemical Reaction • Evolution of heat and light • Formation of a gas • Formation of a precipitate • Color change
Law of Conservation of Mass • mass is neither created nor destroyed in a chemical reaction • total mass stays the same • atoms can only rearrange 4 H 2 O 4 H 2 O 36 g 4 g 32 g
Chemical Equations A+B C+D REACTANTS PRODUCTS
Writing Equations 2H2(g) + O2(g) 2H2O(g) • Identify the substances involved. • Use symbols to show: • How many? - coefficient • Of what? - chemical formula • In what state? - physical state • Remember the diatomic elements.
Writing Equations Two atoms of solid aluminum react with three units of aqueous copper(II) chloride to produce three atoms of solid copper and two units of aqueous aluminum chloride. • How many? • Of what? • In what state? 3 2 Al (s) + 3 CuCl2 (aq) Cu (s) + 2 AlCl3 (aq)
Describing Coefficients: individual atom = “atom” covalent substance = “molecule” ionic substance = “formula unit” Describing Equations 3 molecules of carbon dioxide 2 atoms of magnesium 4 formula units of magnesium oxide 3CO2 2Mg 4MgO
Describing Equations Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g) to produce • How many? • Of what? • In what state? One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid one unit of aqueous zinc chloride and one molecule of hydrogen gas.
Ch. 8 – Chemical Reactions Balancing Equations
Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients to make #s 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. • If Hydrogen and Oxygen appear, balance them last. Balance Hydrogen before Oxygen. • Update ALL atom counts after adding a coefficient. • If an element appears more than once per side, balance it last.
Balancing Example Al + CuCl2 Cu + AlCl3 Al Cu Cl Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2 3 3 2 2 6 1 1 1 1 2 3 2 3 6 3
Balancing Example • Aqueous nitric acid reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water • (2,1,1,2) • Solid Calcium metal reacts with water to form aqueous calcium hydroxide and hydrogen gas. • (1,2,1,1)
Balancing example (hydrocarbons) • Ethane gas reacts with oxygen gas to form carbon dioxide and water. • Ethene gas reacts with oxygen gas to form carbon dioxide and water. • Butane gas reacts with oxygen gas to form carbon dioxide and water.
Ch. 8 – Chemical Reactions Types of Chemical Reactions
Combustion • the burning of a hydrocarbon in O2 to produce heat A + O2 B CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
Combustion • Products: • form CO2 + H2O • Form carbon monoxide and/or carbon if there is limited oxygen C3H8(g)+ O2(g) CO2(g)+ H2O(g) 5 3 4
Synthesis (Combination) • the combination of 2 or more substances to form a compound • only one product A + B AB
Synthesis (Combination) H2(g) + Cl2(g) 2 HCl(g)
Synthesis (Combination) • Products: • If products are ionic - cancel charges • If covalent - hard to tell Al(s)+ Cl2(g) AlCl3(s) 2 3 2
Decomposition • a compound breaks down into 2 or more simpler substances • only one reactant AB A + B
Decomposition 2 H2O(l) 2 H2(g) + O2(g)
Decomposition • Products: • binary - break into elements • others - hard to tell (more on this later) KBr(l) K(s) + Br2(l) 2 2
Synthesis Reactions to know • Almost all metals react with oxygen to form metal oxides. • Ex/ Magnesium reacts with oxygen to form Magnesium oxide
Synthesis reactions to know • Nonmetals also form oxides. • Sulfur reacts with oxygen to form Sulfur dioxide • Carbon reacts with oxygen to form Carbon dioxide
Synthesis reactions involving water to know • Oxides of active metals react with water to produce metal hydroxides. • Ex/ Calcium oxide reacts with water to form calcium hydroxide.
Synthesis reactions to know involving water • Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids (acids with oxygen). • Ex/ sulfur dioxide reacts with water to form sulfurous acid
Decomposition reactions to know • Metal carbonates break down to produce a metal oxide and carbon dioxide gas. • Ex/ calcium carbonate decomposes to produce calcium oxide and carbon dioxide.
Decomposition reactions to know • All metal hydroxides except those containing group 1 metals decompose when heated to yield metal oxides and water. • Ex/ calcium hydroxide decomposes to produce calcium oxide and water.
Decomposition reactions to know • Metal chlorates decompose to produce a metal chloride and oxygen. • Ex/ potassium chlorate decomposes in the presence of the catalyst Manganese dioxide to produce potassium chloride and oxygen.
Decomposition reactions to know • Sulfites decompose into the cation oxide and sulfur dioxide gas • Calcium sulfite decomposes to form calcium oxide and sulfur dioxide gas.
Decomposition reactions to know • Peroxides decompose into the oxide of the cation and oxygen gas. • Hydrogen peroxide decomposes to form water (dihydrogen monoxide) and oxygen gas. • Sodium peroxide decomposes to form . . .
Decomposition reactions to know • Certain acids decompose into nonmetal oxides and water. • Ex/ Carbonic acid is unstable and decomposes readily to produce carbon dioxide and water. • Sulfurous acid decomposes to produce sulfur dioxide and water.
Decomposition reactions to know • Ammonium salts decompose into ammonia and the acid of the anion. • Ex/ Ammonium acetate decomposes to form ammonia and acetic acid. • Ammonium nitrate decomposes to form ammonia and nitric acid.
Single Replacement • one element replaces another in a compound • metal replaces metal (+) • nonmetal replaces nonmetal (-) A + BC B + AC
Single Replacement Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)
Single Replacement • Products: • Metal atom metal cation(+) • Nonmetal atom nonmetal anion(-) • free element must be more active(check activity series) Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq) Br2(l)+ NaCl(aq) N.R.
Double Replacement • Occurs between two aqueous ionic compounds • ions in two compounds “change partners” • cation of one compound combines with anion of the other AB + CD AD + CB
Double Replacement Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)
Double Replacement • Products: • In order for a reaction to occur on of the following must occur • one product must be insoluble (a precipitate) (check solubility table) • A gas is produced • A covalent compound (molecular) is produced PbI2(s)+ KNO3(aq) 2 2 Pb(NO3)2(aq)+ KI(aq) NaNO3(aq)+ KI(aq) N.R.
Examples • Solutions of Hydrochloric acid and Sodium hydroxide are mixed • Solutions Rubidium carbonate and acetic acid are mixed
Writing net ionic equations • 1. Write the skeleton equation • Make sure charges ions are balanced. • 2. Write the total ionic equation • 3. Cancel out ions that appear on both sides • 4. These are called spectator ions. • Spectator ions do not take part in the overall net reaction.
Writing net ionic equations • A solution of Lead (II) nitrate is mixed with a solution of potassium iodide. • A solution of silver nitrate is mixed with a solution of sodium hydroxide. • View clip
Predicting products from reactions • If given only reactants • Figure out what type of reaction it is • Write the products. If one of the products is ionic, balance charges. • Balance the equation • Ex/ Hydrochloric acid is mixed with zinc metal.
Predicting (cont’d) • Calcium metal burns in the presence of oxygen. • Sodium carbonate is heated vigorously. • Potassium chlorate is heated vigorously. • Pentane burns in oxygen. • Aqueous silver nitrate is mixed with aqueous sodium chloride.
Predicting products • Hydrochloric acid is mixed with Sodium hydroxide. • Gold metal is dropped into hydrochloric acid.