ACIDS AND BASES. for it cannot be But I am pigeon-liver d and lack gall To make oppression bitter Hamlet

Transcription

1 ACIDS AND BASES for it cannot be But I am pigeon-liver d and lack gall To make oppression bitter Hamlet

2 Learning objectives Describe properties of acids and bases Define acid and base using Arrhenius and Brønsted definitions Identify Brønsted acids and bases in solution equilibria Distinguish between strength and concentration of acids and bases Estimate ph of acids and bases Describe phenomenon of acid rain

3 Thinking about what is an acid Acids are sour Acids attack metals Acids react with bases and form salts Acids turn litmus red

4 Common acids

5 Thinking about what makes a Bases taste bitter Bases are slippery Bases react with acids to form salts Bases turn litmus blue base

6 Common bases

7 Acids and bases cancel: Neutralization Neutralization involves reaction of acid with base: ACID + BASE = SALT + WATER Example in nature: Reaction of carbonic acid (rainwater - CO 2 in H 2 O) with the ocean to give limestone: H 2 CO 3 + Ca(OH) 2 = CaCO 3 + 2H 2 O

8 Arrhenius: it s about water The meaning of acid and base has changed over the years Arrhenius acid is one that generates protons when dissolved in water Arrhenius base is one that generates hydroxide ions when dissolved in water

9 Hydronium ion is active ingredient of acid in water Protons (H + ) do not exist in solution CH 3 CO 2 H + H 2 O = H 3 O + + CH 3 CO 2 - Vinegar in water produces hydronium ions

10 Hydroxide ion is active ingredient of base in water NH 3 + H 2 O = NH OH - Ammonia, a base, dissolves in water and produces hydroxide ions

11 The essence of neutralization Elimination of the components of acid and base by combination to give H 2 O H + + OH - H 2 O ACID BASE

12 Brønsted and Lowry: All about protons Broader definition of acids and bases Reaction NH 3 + HCl = NH 4 Cl has all elements of acid-base neutralization but no H 2 O Brønsted acid donates a proton Brønsted base accepts a proton

13 Brønsted acid HCl + H 2 O = H 3 O + + Cl -

14 Brønsted base NH 3 + H 2 O = NH OH - water NH 3 + HCl = NH 4+ Cl - No water

15 Substances can be both acids and bases depends on environment Note that in one instance H 2 O behaves like a base accepting protons, and in another, behaves like an acid donating protons HCl + H 2 O = H 3 O + + Cl - In presence of an acid H 2 O is a base NH 3 + H 2 O = NH OH - In presence of a base H 2 O is an acid

16 The products are themselves acids and bases

17 Equilibrium: solution contains mixture of all components

18 Identifying acids and bases: Follow the protons

19 Salts Products of acid-base neutralization Contain metal cation and nonmetal anion Acid + base = salt + water HCl + NaOH = NaCl + H 2 O HCl + KOH = KCl + H 2 O HNO 3 + KOH = KNO 3 + H 2 O 2HCl + Ca(OH) 2 = CaCl 2 + 2H 2 O HCN + NaOH = NaCN + H 2 O

20 Strong coffee (or concentrated?) Equilibrium: not all acids completely donate protons to water molecules HA + H O = A + H O HA + H 2 O A - + H 3 O + Strength: Degree of ionization Concentration: Number of moles per unit volume

21 Strong and weak Strong acid (HCl) Fully ionized equilibrium to right All H + and Cl - Corrosive HA + H O = A - + H O Weak acid (Acetic) Weakly ionized Equilibrium to left Mostly CH 3 COOH Edible HA + H O = A - + H O + 2 3

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23 Changing concentration does not change strength Strength refers to degree of ionization: Strong is completely ionized (100 %) Weak is partly ionized (1 % - 1:10 6 ) HA + H O = A - + H O Concentration refers to number of moles per unit volume An acid (or base) can be strong and concentrated, weak and concentrated, strong and dilute, weak and dilute

24 Ionization of water Even in pure water some molecules are ionized Concentrations of OH - and H 3 O + are equal Concentration H O + H O = OH - + H O [H 3 O + ] = [OH - ]

25 Equilibrium constant: in all aqueous solutions, product of concentrations is constant [H 3 O + ][OH - ] = constant Add acid or base alters balance of [H 3 O + ] and [OH - ] Increasing [H 3 O + ] decreases [OH - ] (acidic conditions) Increasing [OH - ] decreases [H 3 O + ] (basic conditions)

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27 The ph scale reduces large range of numbers to small In water [H 3 O + ][OH - ] = Range of [H 3 O + ]: ph = - log 10 [H 3 O + ] 10 M (conc acid) M (conc base) Range of ph: -1 (conc acid) to +15 (conc base) Low ph = acid; high ph = basic ph = 7 = neutral

28 ph scale and common substances

29 Relating ph to [H 3 O + ] For ph, take exponent of [H 3 O + ], change sign Acid HCl(aq): [H 3 O + ] = 1 x 10-1 M, ph = 1 Pure H 2 O: [H 3 O + ] = 1 x 10-7 M, ph = 7 Base: NH 3 (aq): [H 3 O + ] = 1 x M, ph = 11 Note: change of 1 unit in ph is factor of ten in [H 3 O + ] When [H 3 O + ] is 1 x 10 x, ph is whole number

30 Estimating ph When [H 3 O + ] is not 1 x 10 x M ph is not whole number Estimating ph is often more useful than doing exact calculations Smaller ph value means larger H + concentration Estimating ph

31 Acidity and the environment Rain is naturally weakly acidic because of CO 2 Alkaline rocks limestone neutralize the acid

32 Granite substrate does not neutralize the acid

33 Ocean water is naturally alkaline

34 Acid Rain Acid rain is polluted by acid in the atmosphere. Two common pollutants acidify rain: sulphur dioxide (SO 2 ) and nitrogen oxides (NO X ) Following information from The Green Lane TM, Environment Canada's World Wide Web site -

35 What s the big deal? Damage to aquatic life Damage to buildings Damage to forests Damage to air quality

36 Source of the problem Sulphur dioxide (SO 2 ) byproduct of industrial processes and burning fossil fuels. Ore smelting coal-fired power generators natural gas processing

37 Where do NO X emissions come Main source of NO X is combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electricalutility boilers and engines. NO X emissions were 2.5 million tonnes in U.S. NO X emissions for 2000 were 21 million tonnes. from?

38 Legislative success with acid rain Eastern Canada Acid Rain program committed Canada to cap SO 2 emissions at 2.3 million tonnes by % reduction from 1980 levels Targets achieved or exceeded By 2001, emissions were 63% reduction from 1980 levels.

39 Would acid rain remain a problem without further controls? Yes. That is why The Canada-Wide Acid Rain Strategy for Post-2000 calls for further emission reductions in both Canada and the United States. In total, without further controls, almost 800,000 km 2 in southeastern Canada-an area the size of France and the United Kingdom combinedwould receive harmful levels of acid rain; that is, levels well above critical load limits for aquatic systems.