Chemical reactions. Classifications Reactions in solution Ionic equations

Transcription

1 Chemical reactions Classifications Reactions in solution Ionic equations

2 Learning objectives Distinguish between chemical and physical change Write and balance chemical equations Describe concepts of oxidation and reduction Classify reaction according to types of reactants and products Distinguish among strong, weak and non-electrolytes Identify common acids and bases by from chemical formula Predict formation of precipitates by application of solubility rules Write total and net ionic equations from balanced molecular equations

3 Chemical vs physical redux Physical: No new substance! Chemical: New substance formed!

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5 Evidence for chemical reactions

6 The chemical equation aa + bb = cc + dd Reactant side coefficient ELEMENT or COMPOUND Law of Conservation of Matter states that matter is neither created nor destroyed Means: atoms on left equals atoms on right Product side

7 Chemical book-keeping Keys to balancing equations: Have I gained or lost any atoms? Put down the correct formula for each reactant or product Formulas cannot be changed in order to balance the equation

8 Reaction of hydrogen with oxygen to produce water: reactants are H 2 and O 2, product is H 2 O 2 H 2 + O 2 2 H 2 O The big number multiplies every atom after it Count the atoms: 4 H and 2 O The subscript only multiplies the atom before it 4 H and 2 O

9 Molecular representation of the reaction

10 Balance the equations A method of trial and error CH 4 + O 2 = CO 2 + H 2 O

11 Balance the equations CH 4 + O 2 = CO 2 + H 2 O CH 4 + 2O 2 = CO 2 + 2H 2 O C 3 H 8 + O 2 = CO 2 + H 2 O C 3 H 8 + 5O 2 = 3CO 2 + 4H 2 O N 2 + H 2 = NH 3 N 2 + 3H 2 = 2NH 3 Do balancing equation exercises

12 One approach to classification

13 Oxidation reduction: focusing on electrons Oxidation is loss of electrons Reduction is gain of electrons Oxidation is always accompanied by reduction The total number of electrons is kept constant Oxidizing agents oxidize and are themselves reduced Reducing agents reduce and are themselves oxidized

14 Redox in chemistry All reactions involve rearrangement of atoms and molecules Some reactions involve rearrangement of atoms and molecules and electrons Photosynthesis, respiration, combustion... These are called redox reactions Any reaction involving elements must be redox

15 Combination (synthesis) Element + element compound (redox) S + O 2 SO 2 Metal + nonmetal binary ionic compound Nonmetal + nonmetal binary covalent compound Compound + element compound (redox) CO + O 2 CO 2 Compound + compound compound SO 2 + H 2 O H 2 SO 3 reactions

16 Decomposition reactions Compound element + element (redox) HgO Hg + O 2 Compound element + compound (redox) PCl 5 PCl 3 + Cl 2 Compound compound + compound CaCO 3 CaO + CO 2

17 Single replacement (displacement) Element displaces another element from compound (redox) Cu + 2 AgNO 3 Cu(NO 3 ) Ag

18 Predicting single replacement reactions: the activity series Element higher in the will displace one lower in the series The element higher is a stronger reducing agent The element lower is a stronger oxidizing agent

19 Three types of double displacement reaction Compounds exchanging partners Usually ionic compounds in solution Precipitation Acid-base neutralization Gas formation

20 Precipitation Identify ions and swap them BaCl 2 + Na 2 SO 4 BaSO NaCl

21 Acid base neutralization: special case of double replacement BASE ACID SALT WATER KOH(aq) + HNO 3 (aq) = KNO 3 (aq) + H 2 O(l) Product is liquid water not a solid

22 Gas formation Product is either a gas or is unstable and decomposes to a gas CaCO 3 (s) + 2 HCl(aq) = CaCl 2 (aq) + H 2 O(l) + CO 2 (g)

23 Writing balanced molecular equations for double replacement reactions Use correct formulae Metal ion charge predicted from group number Use table for correct formula and charge for polyatomic ions Identify as solid (s), gas (g), liquid (l) or dissolved (aq) Balance: atoms (groups) on left = atoms (groups) on right

24 Balancing double replacement equations It s very much a matter of states show them! Pb(NO 3 ) 2 (aq) + 2KI(aq) = 2KNO 3 (aq) + PbI 2 (s) Balance polyatomic ions as units: Pb 2+, K +, I -, NO 3 - Left hand side Right hand side 1 Pb 2+ 1 Pb 2+ 2 NO NO K + 2 K + 2 I - 2 I -

25 Molecular equation for reaction of Na 2 SO 4 + Ba(NO 3 ) 2

26 Combustion Element or compound reacting with oxygen (redox) CH 4 + O 2 CO 2 + H 2 O Associated with production of heat and light Involves hydrocarbons (C x H y ), nonmetals (S) and metals (Mg)

27 Sorting solution reactions: Electrolytes: dissolved species Ionic compounds produce ions in solution (NaCl, NH 4 NO 3 etc.) Non-electrolytes: Covalent compounds do not produce ions in solution (CH 3 OH, C 6 H 12 O 6 etc.)

28 Electrolytes: distinguishing by strength All soluble substances that produce ions are electrolytes Strong electrolytes are characterized by complete dissociation in water Weak electrolytes dissociate to a much smaller extent.

29 Strong, weak or non electrolyte? All soluble salts are strong electrolytes Strong acids and bases are strong electrolytes Weak acids and bases are weak electrolytes Insoluble compounds are non-electrolytes Molecular compounds are non-electrolytes

30 Classification of electrolytes Strong electrolytes ACIDS: HCl, HBr, HI HClO 4, HNO 3, H 2 SO 4 SALTS: KBr, Na 3 PO 4 BASES: NaOH, Ba(OH) 2 Weak electrolytes ACIDS: HF, H 3 PO 4, CH 3 CO 2 H SALTS: None BASES: NH 3 Nonelectrolytes Molecular covalent compounds: H 2 O, CH 3 OH, C 12 H 22 O 11 (sucrose) Most organic compounds and INSOLUBLE salts

31 Flow chart for determining type of electrolyte Yes 1. Soluble in H 2 O? No Yes 2. Acid or base? No Weak 3. Weak or strong? Strong Ionic 3. Ionic or covalent? Cov Weak electrolyte Strong electrolyte Nonelectrolyte

32 Recognizing acids and bases Acids usually have H at the beginning of the formula HCl Bases usually have OH in the formula NaOH Not in organic compounds though - CH 3 OH Acid formula Name Base formula Name HCl Hydrochloric acid NaOH Sodium hydroxide H 2 SO 4 Sulfuric acid KOH Potassium hydroxide H 3 PO 4 Phosphoric acid Ba(OH) 2 Barium hydroxide HNO 3 Nitric acid NH 3 Ammonia HClO 4 Perchloric acid (CH 3 ) 3 N Trimethylamine CH 3 CO 2 H HCO 2 H Acetic acid Formic acid Citric acid

33 The strong acids and bases Strong acids (Only six) Strong bases (g1a and g2a) HCl Hydrochloric acid LiOH Lithium hydroxide HBr Hydrobromic acid NaOH Sodium hydroxide HI Hydroiodic acid KOH Potassium hydroxide HNO 3 Nitric acid Ca(OH) 2 Calcium hydroxide H 2 SO 4 Sulfuric acid Sr(OH) 2 Strontium hydroxide HClO 4 Perchloric acid Ba(OH) 2 Barium hydroxide

34 Solubility rools Group I and ammonium (NH 4+ ) compounds soluble Nitrates (NO 3- ), acetates (CH 3 CO 2- ) soluble Chlorides, bromides and iodides generally soluble {except Pb(II), Ag(I) and Hg(I)} Sulphates (SO 4 2- ) generally soluble (except g2a and Pb 2+ ) Carbonates (CO 3 2- ), phosphates (PO 4 3- ) generally insoluble (except gia) Hydroxides (OH - ), sulphides (S 2- ) generally insoluble (except gia and giia)

35 Total ionic equations Pb(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) = 2KNO 3 (aq) + PbCrO 4 (s)

36 Total ionic equation Dissolved substances: Strong electrolytes show as ions Weak or non- electrolytes show as molecular formula Pb 2+ (aq) + 2NO 3- (aq) + 2K + (aq) + CrO 4 2- (aq) = 2K + (aq) + 2NO 3- (aq) + PbCrO 4 (s)

37 Net ionic equations Spectator ions are those ions that do not undergo a change; they do not participate in the chemical change and are the same on both sides of the equation Remove all spectator ions from the equation Pb 2+ (aq) + 2NO 3- (aq) + 2K + (aq) + CrO 4 2- (aq) = 2K + (aq) + 2NO 3- (aq) + PbCrO 4 (s)

38 Net ionic equations Pb 2+ (aq) + CrO 4 2- (aq) = PbCrO 4 (s) Mass and charge must still balance, although overall charge may not be neutral in a net ionic equation

39 Net ionic equation for reaction of Na 2 SO 4 + Pb(NO 3 ) 2