Types of chemical reactions

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1 PowerPoint to accompany Types of chemical reactions Chapters 3 & 16.1 M. Shozi CHEM110 / 2013

2 General Properties of Aqueous Solutions Solutions are mixtures of two or more pure substances. The solvent is present in greatest abundance in the solution. All other substances are in the solution are solutes. Aqueous solutions are those in which water is the dissolving medium.

3 Electrolytes A strong electrolyte dissociates completely when dissolved in water. NaCl Na + (aq) + Cl (aq) A weak electrolyte only dissociates partially when dissolved in water. CH 3 CO 2 H CH 3 CO 2 (aq) + H + (aq)

4 Electrolytes Substances that dissociate into ions when dissolved in water, eg. NaCl A non electrolyte may dissolve in water, but it does not dissociate into ions when it does so, eg. C 12 H 22 O 11

5 Exercise 1 Determine the calcium and nitrate concentrations in a M Ca(NO 3 ) 2 (aq) solution.

6 Precipitation Reactions Reactions that result in the formation of an insoluble product Pb(NO 3 ) 2 (aq) + 2KI(aq) PbI 2 (s) + 2KNO 3 (aq) A precipitate is an insoluble solid formed by a reaction in solution

7 Metathesis (Exchange) Reactions Reactions in which positive and negative ions appear to exchange partners KCl(aq) + AgNO 3 (aq) AgCl(s) + KNO 3 (aq) K + Ag + Cl NO 3

8 Predicting Precipitation Reactions The following must be noted: 1. Ions present in the reactants 2. Consider the possible combinations of the cations and anions 3. Use Table 3.1 (page 94) to determine whether the combinations are insoluble

9 Exercise 2 Using the solubility rules, predict whether the following salts are soluble, slightly soluble or insoluble. a) BaCl 2 b) MgCO 3 c) PbS d) Hg 2 Cl 2 e) Cu 3 (PO 4 ) 2

10 Ionic Equations Molecular equation full chemical formulae of the compounds involved: 2KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) Ionic equation indicates the form in which the compounds exist in an aqueous solution. 2K + (aq) + 2I - (aq) + Pb 2+ (aq) + 2NO 3- (aq) PbI 2 (s) + 2K + (aq) + 2NO 3- (aq) Net ionic equation shows only those ions that are taking part in the reaction: Pb 2+ (aq) + I (aq) PbI 2 (s) Spectator ions ions that don t take part in the reaction, Na + (aq) & NO 3 (aq)

11 Writing Net Ionic Equations 1.Write a balanced molecular equation. 2.Dissociate all strong electrolytes. 3.Cross out anything that remains unchanged from the left side to the right side of the equation, i.e. identify and cancel spectator ions. 4.Write the net ionic equation with the species that remain.

12 Exercise 3 Predict whether a reaction will occur in each of the following cases. If so, write a net ionic equation for the reaction. a) AlCl 3 (aq) + KOH(aq) b) K 2 SO 4 (aq) + FeBr 3 (aq) c) CaI 2 (aq) + Pb(NO 3 ) 2 (aq)

13 ACID-BASE REACTIONS ACIDS Arrhenius: Compounds that contain an ionisable H and able to ionise in aqueous solution to form H + or H 3 O + Strong acids - ionise completely in solution - strong electrolytes HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq) Weak acids - partially ionised in solution - weak electrolytes CH 3 CO 2 H(aq) + H 2 O(l) CH 3 CO 2 (aq) + H 3 O + (aq)

14 ACID-BASE REACTIONS BASES Arrhenius: Compounds that contain an OH group and are able to ionise in aqueous solution to form OH - Strong bases - ionise completely in solution - strong electrolytes NaOH(aq) Na + (aq) + OH (aq) Weak bases - partially ionised in solution - weak electrolytes NH 4 OH(aq) NH 4+ (aq) + OH (aq)

15 NEUTRALISATION REACTIONS When solutions of an acid and a base are combined, the products are a salt and water HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) (acid) (base) (salt) (water) Net ionic equation: H + (aq) + OH (aq) H 2 O(l)

16 GAS FORMING REACTIONS These are acid-base reactions which result in the formation of a gas Na 2 S(aq) + 2HCl(aq) H 2 S(g) + 2NaCl(aq) Net ionic equation: S 2- (aq) + 2H + (aq) H 2 S(g)

17 GAS FORMING REACTIONS Carbonates and hydrogen carbonates (or bicarbonates) will form CO 2 (g) when treated with an acid Na 2 CO 3 (s) + 2H + (aq) H 2 O(l) + CO 2 (g) +2Na + (aq) Net ionic equation: 2H + (aq) + CO 3 2- (aq) H 2 O(l) + CO 2 (g)

18 REDOX REACTIONS Reactions involve the transfer of electrons between reactants When a substance loses electrons, it undergoes oxidation: Ca(s) + 2H + (aq) Ca 2+ (aq) + H 2 (g) When a substance gains electrons, it undergoes reduction: 2Ca(s) + O 2 (g) 2CaO(s)

19 OXIDATION NUMBERS To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity using the following five sets of rules.

20 RULES FOR ASSIGNING OXIDATION NUMBERS 1. For an atom in its elemental form, the oxidation number is always zero. e.g. Na = 0 and Cl 2 = 0 2. For any monatomic ion the oxidation number equals the charge on the ion. e.g. Na + = +1 and Cl = -1

21 OXIDATION NUMBERS 3. Non metals usually have negative oxidation numbers: a) The oxidation number of oxygen is usually -2 except in the peroxide ion in which it has an oxidation number of 1. b) The oxidation number of hydrogen is +1 when bonded to a metal and -1 when bonded to a nonmetal. c) The oxidation number of fluorine is -1 in all compounds. The other halogens usually have an oxidation number of -1 unless combined with oxygen (oxyanions) where they have a positive oxidation number.

22 OXIDATION NUMBERS 4. The sum of the oxidation numbers of all atoms in a neutral compound is zero. Eg. Pb(NO 3 ) 2, KCl, H 2 SO 4 5. The sum of the oxidation numbers in a polyatomic ion atom equals the charge of the ion. Eg. NH 4+, NO 3, Cr 2 O 7 2

23 OXIDISING & REDUCING AGENTS OXIDISING AGENT Causes the oxidation of another substance Gains electrons, i.e. it is reduced Example: MnO 4 and Cr 2 O 2-7 REDUCING AGENT Causes the reduction of another substance Loses electrons, i.e. it is oxidised Example: NaH and CaH 2

24 Exercise 4 Identify the reducing and oxidising agents in the following reaction: 5SO MnO 4 + 6H + 5SO Mn H 2 O

25 OXIDATION & REDUCTION HALF -REACTIONS Separate the overall REDOX reaction oxidation and reduction half reaction: 0 +2 Zn(s) + Cu (aq) Zn 2+ 0 (aq) + Cu(s) Half Reactions REDOX reactions are linked by gain/loss of e - Oxidation Reaction: Zn(s) Zn 2+ (aq) + 2 e - Reduction Reaction: Cu 2+ (aq) + 2 e - Cu(s) Overall Reaction: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s)

26 Exercise 5 Show the oxidation and reduction that occur, and write the overall ionic equation for the reaction of Mn with lead(ii) nitrate solution to produce Pb(s) and Mn(NO 3 ) 2 (aq). Mn(s) + Pb(NO 3 ) 2 (aq) Pb(s) + Mn(NO 3 ) 2 (aq)

27 BALANCING REDOX REACTIONS HALF REACTION METHOD ACID MEDIUM Follow the steps below: 1. Write skeleton half reactions 2. Balance a) Any atoms other than H & O; b) O by adding H 2 O; & c) H by adding H + 3. Balance the charge by adding electrons 4. Multiply half reactions so that electrons cancel & add reactions together.

28 Exercise 6 Balance the following reaction in acidic medium: MnO 4 + C 2 O 4 2- Mn 2+ + CO 2

29 BALANCING REDOX REACTIONS HALF REACTION METHOD BASIC MEDIUM Follow the steps below: 1. Write skeleton half reactions 2. Balance a) Any atoms other than H & O; b) O by adding H 2 O; & c) H by adding H + d) H + by adding OH - to both sides of the equation 3. Balance the charge by adding electrons 4. Multiply half reactions so that electrons cancel & add reactions together.

30 Exercise 7 Balance the following reaction in basic medium: MnO 4 + C 2 O 4 2- Mn 2+ + CO 2

31 PEROXIDE CHEMISTRY General formula M 2 O 2 Oxidation state of O = -1 In limited oxygen supplies: M 2 O (small amounts of Li 2 O 2 from Li). In excess oxygen: Li and Na form the peroxide, M 2 O 2. K, Rb and Cs form the superoxide MO 2.

32 DISPROPORTIONATION REACTION A redox reaction where an element is simultaneously oxidised and reduced. Example: Decomposition of hydrogen peroxide H 2 O 2 2 H 2 O + O 2-1 reduced to -2-1 oxidised to 0

33 DISPLACEMENT REACTION A redox reaction where one element in molecular/atomic form reacts with either an acid or metal salt Mg(s) + 2 HCl(aq) MgCl 2 (s) + H 2 (g) 33

34 TITRATION THEORY The analytical technique in which one can determine the concentration of a solute in a solution.

35 SOME TERMINOLOGY Standard solution A solution of accurately known concentration. Analyte The substance being analysed in an analytical procedure. Indicator An intensely-coloured, organic dye that exhibits different colours in acidic & basic medium End Point the volume of titrant added at which the indicator changes colour Equivalence Point the volume of titrant at which stoichiometric amounts of reactant have been added.

36 Exercise 8 A ml solution of sodium thiosulphate (Na 2 S 2 O 3 ) of concentration M is titrated with ml of I 2 solution and the following reaction takes place: 2Na 2 S 2 O 3 (aq) + I 2 (aq) Na 2 S 4 O 6 (aq) + 2NaI(aq) Calculate the concentration of the I 2 solution in: a) mol L -1 b) g dm -3

37 Exercise 9 A piece of iron wire has a mass of g. It is dissolved in acid and then reduced from Fe 3+ to Fe 2+. The whole solution was titrated against M K 2 Cr 2 O 7 solution of which ml was required for complete reaction. The balanced reaction equation is: Cr 2 O 7 2 (aq) + 14H + (aq) + 6Fe 2+ (aq) 2Cr 3+ (aq) + 6Fe 3+ (aq) + 7H 2 O(l) What is the percentage purity of the iron wire?

38 PRIMARY STANDARDS Highly purified compounds Reference material for titrimetric methods of analysis Preparation Accurate mass dissolved in accurate amount of H 2 O Resulting solution s concentration is accurately known Properties It is obtained in pure and stable form and dissolves completely It does NOT absorb H 2 O or CO 2 from the air It has an accurately known molar mass It reacts quickly and completely with the sample

39 Exercise 10 A particular solution of NaOH is supposed to be approximately 0.1 M. To determine the exact molarity of the NaOH(aq), a g sample of KHP (KHC 8 H 4 O 4, g mol -1 ) is dissolved in water and titrated with ml of the NaOH(aq). What is the accurate molarity of the NaOH(aq)? KHC 8 H 4 O 4 (aq) + NaOH(aq) KNaC 8 H 4 O 4 (aq) + H 2 O(l)

40 BACK TITRATION METHOD Indirect titration method Direct reaction is slow, lack of indicator, analyte is insoluble, analyte contains impurities Involves: Addition of excess reagent Analyzing the amount of this that remains unreacted Concentration of analyte is calculated backwards

41 Examples Calcium in antacid tablets or chalk Dissolved in excess acid for complete reaction Excess acid titrated with base Ammonia in cleaning agent Forms cloudy solution in water (NH 4 OH) Excess NH 3 titrated with acid

42 Exercise 11 A student reacts aspirin (C 9 H 8 O 4, g mol -1 ) with ml of M NaOH. He then titrates the excess NaOH with ml of M H 2 SO 4. Calculate the mass (in grams) of aspirin that originally reacted with NaOH. Reaction 1: C 9 H 8 O 4 + 2NaOH C 6 H 5 O 3- + CH 3 COO - + 2Na + + H 2 O Reaction 2: 2NaOH (excess) + H 2 SO 4 Na 2 SO 4 + H 2 O

Acids, Bases and Titrations Chapter 4 M. Shozi CHEM110 / 2014

Acids, Bases and Titrations Chapter 4 M. Shozi CHEM110 / 2014 ACID-BASE REACTIONS ACIDS Arrhenius: Compounds that contain an ionisable H and able to ionise in aqueous solution to form H + or H 3 O + Strong