Acids, Bases and Salts

Transcription

1 Acids, Bases and Salts Acid-Base Theory Self-Ionization of Water Properties of Acids and Bases The ph Concept Strengths of Acids and Bases Analysis of Acids and Bases Buffers 9-1

2 Acids and bases A very important class of chemicals. Control CO 2 transport in the blood (buffers) Amino acids building blocks of protein (contain both and acid and a base) Acids are one of the 4 things we can taste (sour) Bases feel slippery 9-2

3 Acids and bases Acidic Basic Citrus fruits Baking soda Aspirin Detergents Coca Cola Ammonia cleaners Vinegar Tums and Rolaids Vitamin C Soap 9-3

4 Properties of acids Sour taste. Produce H 3 O + when dissolved in water. Undergo double replacement reactions with solid oxides, hydroxides, carbonates and bicarbonates. 2 HCl + CuO CuCl 2 + H 2 O HCl + NaOH NaCl + H 2 O 2 HCl + CaCO 3 CaCl 2 + CO 2 + H 2 O HCl + NaHCO 3 NaCl + CO 2 + H 2 O 9-10

5 Properties of acids Another property is the ability to react with metals, producing hydrogen gas. Zn + 2 HCl ZnCl 2 + H 2 Some metals react better with acids than others. We can show the reactivity using an activity series 9-11

6 increasing reactivity potassium sodium calcium magnesium aluminum zinc chromium iron nickel tin lead copper silver platinum gold Activity series of metals Reacts violently with cold water Reacts slowly with cold water Reacts very slowly with steam but quite reactive in acid Reacts moderately with high levels of acid Unreactive in acid 9-12

7 Activity series of metals - various metals in HCl Iron Zinc Magnesium 9-13

8 Properties of bases Solutions have a slippery or soapy feel. Will react with acids to produce a salt. Neutralization reaction NaOH + HCl NaCl + H 2 O Neutralization of fatty acids in the skin produce soap. This is why bases feel slippery. 9-14

9 Salts When acids and bases react, they form water and a salt. KOH + HNO 3 KNO 3 + H 2 O (salt) Salts are typically crystalline, ionic materials. Most ionic compounds can actually be considered salts. 9-15

10 Acid and Base Strength Strong acids dissociate completely in water HCl, HBr, HI, HClO 3, HNO 3, HClO 4, H 2 SO 4. Weak acids partially dissociate in water most acids are weak. Strong bases dissociate completely in water NaOH, LiOH, KOH strong bases are metal hydroxides. Weak bases partially dissociate in water. 9-16

11 Dissociation Strong acids and bases considered to dissociate completely. HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) NaOH (aq) + H 2 O (l) Na + (aq) + OH - (aq) Weak acids and bases do not dissociate completely. HC 2 H 3 O 2 (aq) + H 2 O (l) NH 3 (aq) + H 2 O (l) H 3 O + (aq) +C 2 H 3 O 2 - (aq) NH 4 + (aq)+ OH - (aq) 9-17

12 Acid dissociation constant, K a The strength of a weak acid can be expressed as an equilibrium. HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) The strength of a weak acid is related to its equilibrium constant, K a. K a = [A - ][H 3 O + ] [HA] We omit water. It s already included in in the constant. 9-18

13 Dissociation of bases, K b The strength of a weak base can also be expressed as an equilibrium. B (aq) + H 2 O (l) BH + (aq) +OH - (aq) The strength of a weak base is related to its equilibrium constant, K b. K b = [OH - ][BH + ] [B] 9-19

14 K a and K b values For weak acids and bases K a and K b always have values that are smaller than one. Acids with a larger K a are stronger than ones with a smaller K a. Bases witha larger K b are stronger than ones with a smaller K b. Most acids and bases are considered weak. 9-20

15 ph and poh We need to measure and use acids and bases over a very large concentration range. ph and poh are systems to keep track of these very large ranges. ph = -log[h 3 O + ] poh = -log[oh - ] ph + poh =

16 Logarithms As with exponents, this is a way of working with very large and very small numbers log n the abbreviation for the term logarithm n is the base log 10 common logs based on 10 the only type we will be using the 10 is usually omitted We will only be using base 10 logs 9-22

17 Logarithms For whole numbers, the common log and the exponent in scientific notation are the same. log(1000) = = 10 3 logs of other numbers are no integers but the idea is the same. log(1500) = =

18 Logarithms A positive log represents a number greater than one. log(13456) = A negative log represents a number smaller than one. log( ) = You CAN T take the log of a negative value. log( -235 ) = error 9-24

19 value log(value) Logarithms The sign tells us if the number is larger or smaller than one. The number before the decimal point tells us where the decimal point would be in scientific notation. The number after the decimal point tells us what the number is. 9-25

20 Log practice Determine the following with your calculator: log(12345) 4.1 <-- rounded off log(1.5x10 5 ) 5.2 log( ) -4.3 log(2.4x10-6 )

21 ph calculations Determine the following. ph = -log[h + ] ph of 6.7x10-3 M H + = 2.2 ph of 5.2x10-12 M H + = 11.2 [H + ], if the ph is 4.5 = 3.2x10-5 M H

22 poh examples Determine the following. poh = -log[oh - ] = 14 - ph poh of 1.7x10-4 M NaOH poh = 3.8 ph = 10.2 poh of 5.2x10-12 M H + ph = 11.2 poh = 2.8 [OH - ], if the ph is 4.5 poh = 9.5 [OH - ] = 3.2x10-10 M 9-31

23 ph scale A log based scale used to keep track of the large change important to acids and bases M 10-7 M 1 M Very Neutral Very Basic Acidic When you add an acid, the ph gets smaller. When you add a base, the ph gets larger. 9-32

24 ph of some common materials Substance ph 1 M HCl 0.0 Lemon juice 2.3 Coffee 5.0 Pure Water 7.0 Blood Milk of Magnesia M NaOH

25 Neutralization The reaction of an acid with a base to produce a salt and water. HCl + NaOH NaCl + H 2 O We do this when we use antacids. Neutralization can be used to determine the amount of acid or base in a sample. - titrations 9-34

26 Indicator examples Acid-base indicators are weak acids that undergo a color change at a known ph. phenolphthalein methyl red bromthymol blue 9-35

27 Titrations Analytical methods based on measurment of volume. If the concentration of an acid is known, the concentration of the base can be found. If we know the concentration of the base, then we can determine the amount of acid. All that is needed is some calibrated glassware and either an indicator or ph meter. 9-36

28 TItrations Buret - volumetric glasware used for titrations. It allows you to add a known amount of your titrant to the solution you are testing. If a ph meter is used, the equivalence point can be measured. An indicator will give you the endpoint. 9-37

29 Buffers Solutions that resist change to ph when small amounts of acid or base are added Two types weak acid and it s salt weak base and it s salt HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Add OH - Add H + shift to right shift to left Based on LeChatelier s Principle. 9-38

30 Buffers and blood Control of blood ph Oxygen is transported primarily by hemoglobin in the red blood cells. CO 2 transported both in plasma and the red blood cells. CO 2 (aq) + 2 H 2 O H 2 CO 3 (aq) H 3 O + (aq) + HCO 3 - (aq) 9-39

31 Buffers and blood The amount of CO 2 helps control blood ph. Too much CO 2 - Respiratory arrest ph goes down, acid level goes up. acidosis Solution - ventilate and give bicarbonate via IV. Too little CO 2 - Hyperventilation, anxiety ph goes up, acid level goes down. alkalosis Solution - rebreath CO 2 in paper bag to raise level. 9-40