Chapter 3 Chemical reactions

What is a chemical reaction? ► The process that brings about a chemical change. ► The starting material in a chemical reaction is called the reactant and the substance formed is the product. ► Example: Na + H 2 O NaOH + H 2

► The mass is not changed during a chemical reaction. This is law Of Conservation Of mass. ► The numbers multiplying entire chemical equations are called stoichiometry coefficients of the substances.

Balancing a Chemical reaction ► When methane burns in air it forms carbon dioxide and water. ► CH 4 +O 2 CO 2 + H 2 O ► Step1 : C and h occur in two formulas and O occurs in three, we begin with C and H. The C atoms are already balanced. We balance the H by using a stoichiometric coefficient of 2 for H 2 O to give four H atoms on each side of the arrow: ► CH 4 +O 2 CO 2 + 2H 2 O

► Step 2 : Now only O remains to be balanced. Since there are a total of four O atoms on the right but only two on the left the O 2 needs a stoichiometry coefficient of 2. The result is ► CH 4 +2O 2 CO 2 + 2H 2 O

► Step 3 : Verify the equation is correct by counting the number of atoms of each element on each side of the arrow. There are one C atom, four H atoms, and four o atoms on each side of the arrow, so the equation is balanced. ► Since all reactants and products are gases we write ► CH 4 (g) +2O 2(g) CO 2(g) +2 H 2 O (g)

► A soluble substance is the one that dissolves in a specified solvent. ► An insoluble substance is the one that does not dissolve significantly in a specified solvent. Precipitation reactions

Electrolytes ► An electrolyte is a substance that dissolves to give a solution that contains ions; the ions allow the solution to conduct electricity. ► Strong electrolyte is a compound that dissolves to give a solution that contains mainly ions. Ex. Aq. Solution of NaCl or KNO 3. The ions can move in solution and conduct electricity.

► Weak electrolyte is a compound that dissolves to give a solution that contains more molecules and very few ions. We use the term “partially ionized”. ► Ex. Acetic acid.

► Non electrolyte is a substance that dissolves to give a solution that does not contain ions and therefore does not conduct electricity. ► Ex water. Tap water conducts electricity because it has dissolved ions like Ca 2+ Na +

Non Electrolyte

► WEAK ELECTROLYTE

► STRONG ELECTROLYTE

► A precipitation reaction occurs when solutions of two strong electrolytes are mixed and result in the formation of an insoluble solid. ► AgNO 3(aq) +NaCl (aq) AgCl (s) +NaNO 3(aq)

Net Ionic Equations ► A complete ionic equation for a precipitation reaction shows all the ions in solution explicitly. ► Ex. Ag + (aq) + NO 3 - +Na + +Cl - AgCl (s) + Na + (aq) +NO 3 - (aq)

► The complete ionic equation shows that dissolved Ag ions and Cl ions join to form insoluble AgCl. The Na and nitrate ions appear as both in the reactant and the products they play no direct role in the reaction. They are spectator ions. ► After canceling the spectator ions we get ► Ag + (aq) + Cl - (aq) AgCl (s)

► A complete ionic equation expresses a reaction in terms of the ions that are present in the solution. A net ionic equation is the chemical equation that remains after the cancellation of the spectator ions.

Class practice ► Example 1: CaCO 3 (s) + HCl ? ► Molecular Equation: CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(l) + CO 2 (g) ► Complete Ionic Equation: CaCO 3 (s) + 2H + (aq) + 2Cl-(aq) Ca 2+ (aq) + 2Cl-(aq) + H 2 O(l) + CO 2 (g) ► Net Ionic Equation: CaCO3(s) + 2H+(aq)  Ca 2+ (aq) + H2O(l) + CO 2 (g)

Reaction of acids and bases ► An acid is a compound that contains hydrogen and releases hydrogen ions in water. The acidic hydrogen atom in a compound is the hydrogen atom that can be released as H +. ► A base is a compound that produces hydroxide ions in water. ► Ammonia+ water gives ammonium ions and hydroxyl ions.

► A strong acid is almost completely ionized in aqueous solution and a weak acid is incompletely ionized in aqueous solution. ► Strong acids HCl+ H 2 O H 3 O +Cl- ► Weak acids are incompletely ionized in aqueous solution. ► Weak acids CH 3 COOH + H 2 O H 3 O +CH 3 COO-

► Strong acid Weak acid ex nitrous acid ► Ex. HCl

► Strong base is almost completely ionized in aqueous solution. NaOH will give Na ions and OH ions. ► Weak base is not completely ionized in aqueous solution. Ammonia is a weak base and when it dissolves in water it gives a solution in which exists almost entirely as ammonia molecules.

Neutralization ► The reaction between an acid and a base. ► Acid + base salt + water or other products.

Gas formation reactions ► In some reactions of acids and bases there is gas formation. ► 2NaCl (s) + H 2 SO 4(l) Na 2 SO 4(s) +2HCl (g)

Redox Reactions ► Oxidation and reduction: Loss of electron is oxidation and gain of electron is reduction. ► Here are some rules that are helpful in finding the oxidation number of a specific atom. ► The oxidation number of an atom in an elementary substance is 0. This means that the oxidation number of an O atom in O 2 is 0. ► The oxidation number of a Group IA atom in any compund is +1; The oxidation number of a Group IIA atom in any compund is +2. ► The oxidation number of fluorine is -1 in all of its compounds. ► The oxidation number of chlorine, bromine, and iodine is -1 in any compound containing only two elements. ► The usual oxidation number of oxygen in a compound is -2. The major exceptions of this rule are peroxides, like H 2 O 2, which have an oxidation number of -1. ► The oxidation number of hydrogen in most compounds is +1. ► The sum of the oxidation numbers in a compound is always zero. For something that is an ion consisting of two atoms (a polyatomic ion), the oxidation numbers add up to the charge on the ion.

► When we take an iron metal nail and dip it into a blue solution of copper (II) sulfate, the nail becomes covered with a reddish substance. This reddish substance is metallic copper. The equation for this is: ► 1.Fe(s) + CuSO 4 (aq) -> FeSO 4 (aq) + Cu(s) canceling spectator ions ► 2. Fe 0 (s) + Cu 2+ (aq) -> Fe 2+ (aq) + Cu 0 (s) ► 1. Fe(s) -> Fe+2(aq) + 2e- 2. Cu 2+ (aq) + 2e- -> Cu(s)

► Redox reaction