Aqueous Reactions and Solution Stoichiometry (continuation)

Transcription

1 Aqueous Reactions and Solution Stoichiometry (continuation)

2 1. Electrolytes and non-electrolytes 2. Determining Moles of Ions in Aqueous Solutions of Ionic Compounds 3. Acids and Bases 4. Acid Strength 5. Neutralization Reaction

3 Electrolytes and non-electrolytes Substance, which dissolve to give a solution that conduct electricity due to the ions they contain are called electrolytes Those substances that give aqueous solutions which do not conduct electricity because the solute remains molecular are called non-electrolytes

4 Ionic Compounds in Water (Electrolytes) The conduction process in the solution is due to the free movement of ions when the compound dissolves in water There are two ways to provide mobile ions for conducting purposes. 1. Dissociation of Ionic Compounds These ions are not the result of a chemical reaction, they are the result of a dissociation of the compound into ions that compose the solid. H2O + NaCl( s) Na ( aq) + Cl ( aq) 2. Ionization of Polar Covalent Molecular Substances

5 In the case when dissolves the covalent compound no ions are formed, the molecules just disperse throughout the solvent- Non-electrolyte is formed O sugar( s) H 2 sugar( aq)

6 Strong and Weak Electrolytes Strong electrolyte A substance which completely dissociates (1) or ionizes (2) in water. EN=2.1 H2O + NaCl( s) Na ( aq) + Cl ( aq) (1) EN=0.9 + HCl + H 2O H 3O + Cl (2) H + ions in water = hydronium ion (H 3 O + ) 3H 2 O*H or H 7 O + 3

7 Weak electrolyte: A substance which partially ionizes when dissolved in water. 2 + CH 3CO2H + H 2O CH 3CO H 3O Acetate-anion Hydronium-cation + < 5% ionization

8 Problem: How many moles of each free ion are formed when 75.0 ml of 0.56 M scandium bromide is dissolved in water? ScBr 3 (s) H 2 O 3 (s) Sc +3 (aq) Converting from volume to moles: (aq) + 3 Br - (aq aq) Moles of ScBr 1 L 10 3 ml 0.56 mol ScBr 3 1 L ScBr 3 = 75.0 ml x x = mol Moles of Br - = mol ScBr 3 3 mol Br - 1 mol ScBr 3 x = mol Br mol Sc +3 are also present.

9 Arrhenius Acid - substance which contain and releases hydrogen cations=protons (H + ) in water Examples: Hydrochloric Acid HCl Nitric Acid HNO 3 Acetic Acid CH 3 CO 2 H Sulfuric Acid H 2 SO 4 Notice that sulfuric acid can provide two H + s Diprotic acid, the other acids can provide only one H + Monoprotic acid.

10 Arrhenius Bases substances, which produces hydroxide ions on ionization in water NH 3 + H 2 O NH 4+ + OH - Ammonia Ammonium Hydroxide anion H 2 O NaOH Na + + OH -

11 The limitations on Arrhenius definitions 1. Specific to one particular solvent-water. 2. The need for Hydroxide bases

12 Brønsted/Lowry theory Acids: proton donors - HCl, HF, H 2 CO 3 Bases: proton acceptors - OH -, NH 3, N 2 H 4 This theory focuses attention on the ability of acids and bases to participate in the transfer of proton from donor molecule or ion to the acceptor molecule or ion.

13 These definitions are not as restrictive as Arrhenius definitions. 1. No need for water although it can be present, it need not be. 2. Bases do not have to be Hydroxide compounds. 3. The theory makes no distinction between elements and compounds However, one restriction still remaining is the need for a protic acid.

14 Compounds that do not contain OH- ions may also be bases CaO (s)+h 2 O (l) Ca(OH) 2 (aq) Ca 2+ + O 2- +H 2 O (l) Ca OH - (aq) O 2- +H 2 O (l) 2OH - (aq) net ionic eq. The strong ability of O 2- anion to attract protons accounts for the observation that aqueous solutions of ionic oxides do not exist: all soluble oxides immediately form hydroxides when they dissolve.

15 This theory introduced the concept of conjugate acid- base pairs A conjugate base is the remainder of the original acid, after it donates it s hydrogen ion A conjugate acid is the particle formed when the original base gains a hydrogen ion

16 H 2 O + NH 3 (aq) NH + 4 (aq) + OH - (aq) acid base acid base NH 3 is a base and NH 4 + is its conjugate acid. Water is an acid and OH- is its conjugate base. HCO 3 - (aq) + H 2 O (l) H 3 O + (aq) + CO 3 2- (aq) Acid(1) Base (2) Conj. Acid (2) Conj. Base (1) +H + -H + Conjugates differ by by a proton EVERY acid has a conjugate base and vice-versa. versa.

17 Lewis Acids and Bases Gilbert Lewis defined these in an even less restrictive manner: Acid- acceptor of a lone pair electrons Base- donor of a lone pair electrons In this set of definitions there is no longer a need for a protic acid. In other words only electron exchange must occur. Arrhenius, Brønsted/Lowry and Lewis definition sets are NOT contradictory.

18 e pair acceptor Lone pairs e pair donor + HCl acid, H + donor H 2 O base, H + acceptor e pair donor e pair acceptor + Cl H 3 O NH 3 base, H + acceptor H 2 O acid, H + donor NH 4 + OH A Proton donor is the same as an electron acceptor. A Proton acceptor is the same as an electron donor.

19 This definition is more broad than the Bronsted-Lowery definition. Obviously, H + is an acid and OH - is a base under either definition, since for an proton to bind to a base, it must accept a pair of electrons However, the Lewis definition extends beyond just the proton. For example, many metal ions can act as Lewis acids when they form complex ions Note, that there are no protons in this reaction, but it is still an acid/base reaction.

20 Strong and Weak Acids and Bases The strength of acids and bases are concerned with the ionization (or dissociation) of the substance, not its chemical reactivity Strong acids and bases are strong electrolytes. Weak acids and bases are weak electrolytes Example: Hydrofluoric acid (HF) is a weak acid, but it is very chemically reactive. - this substance can t be stored in glass bottles because it reacts with glass (silicon dioxide)

21 Binary acids are acids where the proton is bonded to any element other than oxygen

22 Oxyacids contain three elements (oxygen, hydrogen, and something else). They are ternary compounds with the general formula: H x E y O z They have a E-O-H group that supplies the proton: HNO 3 nitric acid, HC 2 H 3 O 2 acetic acid, H 2 SO 4 sulfuric acid H 2 CO 3 carbonic acid, H 2 CrO 4 chromic acid HClO HClO 2 HClO 3 HClO 4 The larger the number of non-hydrogenated oxygens, the stronger the acid

23 Naming of Acids 12/7/

24 Common Strong Bases Lithium Hydroxide Sodium Hydroxide Potassium Hydroxide Rubidium Hydroxide Cesium Hydroxide LiOH NaOH KOH RbOH CsOH Calcium Hydroxide Ca(OH) 2 Strontium Hydroxide Sr(OH) 2 Barium Hydroxide Ba(OH) 2 Unlike weak acids, weak bases (Ammonia and organic bases) DO NOT dissociate. They undergo hydrolysis reactions. Weak bases only partially undergo hydrolysis to produce hydroxide. The conjugate acids of weak bases are weak acids! Weak bases also react completely if titrated with a - - strong acid! B + H O BH + OH 2 H 2 O + NH 3 (aq) NH 4+ (aq) + OH - (aq)

25 Strong Bases Strong Acids 12/7/

26 Neutralization Reaction -Reaction between an acid and a base, which produces water and a salt The generalized reaction between an Acid and a Base is: HX (aq) + MOH (aq) MX (aq) + H 2 O (l) Acid + Base Salt + Water HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) Salt typically ionic compound whose cation comes from a base and anion from an acid. Net Ionic Equation H + (aq) + OH - (aq) H 2 O(l)

27 An Aqueous Strong Acid-Strong Base Reaction on the Atomic Scale Acid + Base Salt + H 2 O