Chemical Quantities and Aqueous Solution

Titration often in the lab, a solution’s concentration is determined by reacting it with another material and using stoichiometry – this process is called titration in the titration, the unknown solution is added to a known amount of another reactant until the reaction is just completed, at this point, called the endpoint, the reactants are in their stoichiometric ratio the unknown solution is added slowly from an instrument called a burette a long glass tube with precise volume markings that allows small additions of solution Tro, Chemistry: A Molecular Approach2

Acid-Base Titrations the difficulty is determining when there has been just enough titrant added to complete the reaction the titrant is the solution in the burette in acid-base titrations, because both the reactant and product solutions are colorless, a chemical is added that changes color when the solution undergoes large changes in acidity/alkalinity the chemical is called an indicator at the endpoint of an acid-base titration, the number of moles of H + equals the number of moles of OH  aka the equivalence point Tro, Chemistry: A Molecular Approach3

Titration Tro, Chemistry: A Molecular Approach4

Titration Tro, Chemistry: A Molecular Approach5 The base solution is the titrant in the burette. As the base is added to the acid, the H + reacts with the OH – to form water. But there is still excess acid present so the color does not change. At the titration’s endpoint, just enough base has been added to neutralize all the acid. At this point the indicator changes color.

Example The titration of mL of HCl solution of unknown concentration requires mL of M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution? The titration of a 20.0 mL sample of an H 2 SO 4 solution of an unknown concentration requires mL of a 0.158M KOH solution to reach the equivalent point. What is the concentration of the unknown H 2 SO 4 solution?

Gas Evolving Reactions Some reactions form a gas directly from the ion exchange K 2 S(aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 S(g) Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water K 2 SO 3 (aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 SO 3 (aq) H 2 SO 3  H 2 O(l) + SO 2 (g) Tro, Chemistry: A Molecular Approach7

NaHCO 3 (aq) + HCl(aq)  NaCl(aq) + CO 2 (g) + H 2 O(l) Tro, Chemistry: A Molecular Approach8

Compounds that Undergo Gas Evolving Reactions Tro, Chemistry: A Molecular Approach9 Reactant Type Reacting With Ion Exchange Product Decom- pose? Gas Formed Example metal n S, metal HS acidH2SH2SnoH2SH2S K 2 S(aq) + 2HCl(aq)  2KCl(aq) + H 2 S(g) metal n CO 3, metal HCO 3 acidH 2 CO 3 yesCO 2 K 2 CO 3 (aq) + 2HCl(aq)  2KCl(aq) + CO 2 (g) + H 2 O(l) metal n SO 3 metal HSO 3 acidH 2 SO 3 yesSO 2 K 2 SO 3 (aq) + 2HCl(aq)  2KCl(aq) + SO 2 (g) + H 2 O(l) (NH 4 ) n anionbaseNH 4 OHyesNH 3 KOH(aq) + NH 4 Cl(aq)  KCl(aq) + NH 3 (g) + H 2 O(l)

When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves Write a molecular equation for the gas-evolution that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite

Other Patterns in Reactions the precipitation, acid-base, and gas evolving reactions all involved exchanging the ions in the solution other kinds of reactions involve transferring electrons from one atom to another – these are called oxidation-reduction reactions also known as redox reactions many involve the reaction of a substance with O 2 (g) 4 Fe(s) + 3 O 2 (g)  2 Fe 2 O 3 (s) Tro, Chemistry: A Molecular Approach11

Combustion as Redox 2 H 2 (g) + O 2 (g)  2 H 2 O(g) Tro, Chemistry: A Molecular Approach12

Redox without Combustion 2 Na(s) + Cl 2 (g)  2 NaCl(s) Tro, Chemistry: A Molecular Approach13 2 Na  2 Na e  Cl e   2 Cl 

Reactions of Metals with Nonmetals consider the following reactions: 4 Na(s) + O 2 (g) → 2 Na 2 O(s) 2 Na(s) + Cl 2 (g) → 2 NaCl(s) the reaction involves a metal reacting with a nonmetal in addition, both reactions involve the conversion of free elements into ions 4 Na(s) + O 2 (g) → 2 Na + 2 O – (s) 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s) Tro, Chemistry: A Molecular Approach14

Oxidation and Reduction in order to convert a free element into an ion, the atoms must gain or lose electrons of course, if one atom loses electrons, another must accept them reactions where electrons are transferred from one atom to another are redox reactions atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced Tro, Chemistry: A Molecular Approach15 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s) Na → Na e – oxidation Cl e – → 2 Cl – reduction Leo Ger

Electron Bookkeeping for reactions that are not metal + nonmetal, or do not involve O 2, we need a method for determining how the electrons are transferred chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reaction even though they look like them, oxidation states are not ion charges! oxidation states are imaginary charges assigned based on a set of rules ion charges are real, measurable charges Tro, Chemistry: A Molecular Approach16

Rules for Assigning Oxidation States rules are in order of priority 1. free elements have an oxidation state = 0 Na = 0 and Cl 2 = 0 in 2 Na(s) + Cl 2 (g) 2. monatomic ions have an oxidation state equal to their charge Na = +1 and Cl = -1 in NaCl 3. (a) the sum of the oxidation states of all the atoms in a compound is 0 Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0 Tro, Chemistry: A Molecular Approach17

Rules for Assigning Oxidation States 3. (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion N = +5 and O = -2 in NO 3 –, (+5) + 3(-2) = (a) Group I metals have an oxidation state of +1 in all their compounds Na = +1 in NaCl 4. (b) Group II metals have an oxidation state of +2 in all their compounds Mg = +2 in MgCl 2 Tro, Chemistry: A Molecular Approach18

Rules for Assigning Oxidation States 5. in their compounds, nonmetals have oxidation states according to the table below nonmetals higher on the table take priority Tro, Chemistry: A Molecular Approach19 NonmetalOxidation StateExample FCF 4 H+1CH 4 O-2CO 2 Group 7ACCl 4 Group 6A-2CS 2 Group 5A-3NH 3

Practice – Assign an Oxidation State to Each Element in the following Br 2 K + LiF CO 2 SO 4 2- Na 2 O 2 Tro, Chemistry: A Molecular Approach20

Oxidation and Reduction Oxidation and Reduction Another Definition oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction Tro, Chemistry: A Molecular Approach21 CH O 2 → CO H 2 O – oxidation reduction

Oxidation–Reduction oxidation and reduction must occur simultaneously if an atom loses electrons another atom must take them the reactant that reduces an element in another reactant is called the reducing agent the reducing agent contains the element that is oxidized the reactant that oxidizes an element in another reactant is called the oxidizing agent the oxidizing agent contains the element that is reduced Tro, Chemistry: A Molecular Approach22 2 Na(s) + Cl 2 (g) → 2 Na + Cl – (s) Na is oxidized, Cl is reduced Na is the reducing agent, Cl 2 is the oxidizing agent

Identify the Oxidizing and Reducing Agents in Each of the Following 3 H 2 S + 2 NO 3 – + 2 H +  S + 2 NO + 4 H 2 O MnO HBr  MnBr 2 + Br H 2 O Tro, Chemistry: A Molecular Approach23

Combustion Reactions Reactions in which O 2 (g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation-reduction reactions Tro, Chemistry: A Molecular Approach24 2 C 8 H 18 (g) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O(g)

Combustion Products to predict the products of a combustion reaction, combine each element in the other reactant with oxygen Tro, Chemistry: A Molecular Approach25 ReactantCombustion Product contains CCO 2 (g) contains HH 2 O(g) contains SSO 2 (g) contains NNO(g) or NO 2 (g) contains metalM2On(s)M2On(s)

Practice – Complete the Reactions combustion of C 3 H 7 OH(l) combustion of CH 3 NH 2 (g) Tro, Chemistry: A Molecular Approach26

Practice – Complete the Reactions C 3 H 7 OH(l) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O(g) CH 3 NH 2 (g) + 3 O 2 (g)  CO 2 (g) + 2 H 2 O(g) + NO 2 (g) Tro, Chemistry: A Molecular Approach27