What is an acid? What is a base?

Transcription

1 What is an acid? What is a base? Properties of an acid Sour taste Turns litmus paper red Conducts electric current Some acids are strong and some are weak Properties of a base Bitter taste Slippery to the touch Turns litmus paper blue Conducts electric current Some bases are strong and some are weak 1

2 Basically, Arrhenius Acid: H + donor Arrhenius Base: OH donor 2

3 Bronsted Lowry Definition of Acids and Bases We will use the Bronsted Lowry definitions for acids and bases: Acids are species that donate a proton (H + ). and bases are species that accept a proton. Acid example: HNO 3 (aq) + H 2 O NO 3 (aq) + H 3 O + (aq) In this example, HNO 3 is an acid and H 2 O is acting as a base. NO 3 is called the conjugate base of the acid HNO 3, and H 3 O + is the conjugate acid of the base H 2 O. Base example: NH 3 (aq) + H 2 O NH 4 + (aq) + OH (aq) In this example, NH 3 is a base and H 2 O is acting as an acid. NH 4 + is the conjugate acid of the base NH 3, and OH is the conjugate base of the acid H 2 O. A compound that can act as either an acid or a base, such as the H 2 O in the above examples, is called amphiprotic. Basically, Bronsted Lowry Acid: proton donor Bronsted Lowry Base: proton acceptor 3

4 Strong acid: Dissociates completely i.e.: breaks down completely into its ions example: Hydrochloric acid HCl H + (aq) + Cl (aq) Notice that the reaction is one way (no reverse reaction) Strong base: Dissociates completely i.e.: breaks down completely into its ions example: Sodium hydroxide NaOH Na + (aq) + OH (aq) Notice that the reaction is one way (no reverse reaction) 4

5 Weak acid: Doesn't dissociates completely i.e.: does not break down completely into its ions example: Acetic acid CH 3 COOH CH 3 COO (aq) + H + (aq) This creates an equilibrium between the acid and its ions 5

6 Weak base: Doesn't dissociates completely i.e.: does not break down completely into its ions example: Ammonia This creates an equilibrium between the base and its ions Equilibrium: A state in which opposing forces or influences are balanced. A state of physical balance. In chemical reactions, equilibrium is denoted by 6

7 ph scale ph: A figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acidic, and those higher than 7 are more basic. ph can be calculated with the formula: log[h + ] [ ]= concentration H + = acid 7

8 ph: Since we agreed that strong acids dissociate completely, then their ph can be calculated using the concentration of the acid. Example: If we had a strong acid, HCl, with a concentration of 0.01M in water, ph can be calculated with the formula: log [H + ], where [H + ] in this case = [HCl] Therefore, ph= log[h + ] = log0.01 = 2 Side track example to discuss C a V a = C b V b 8

9 back to ph: ph calculations: Well, if we know the ph, but we needed to find the concentration of the acid, then the reverse formula is used: [H + ]= 10 ph There, if we knew the ph of an acid to be 3, then: [H + ]= 10 ph = 10 3 = 0.001M Since acids and bases are often dissolving in solution (in H 2 O), there is always 2 acid base reactions taking place in fact: 1 The acid that gives a proton to form its conjugate base OR the base that takes a proton to form its conjugate acid, and 2 Water with an acid, will take the H to form a H 3 O OR water with a base will lose an H to form OH Examples: HNO 2 (aq) + H 2 O(l) H 3 O + (aq) + NO 2 (aq) (CH 3 CH 2 ) 2 NH (aq) + H 2 O OH + (CH 3 CH 2 ) 2 NH 2 + 9

10 The water is always undergoing the following reaction: H 2 O H + + OH The multiplication of the concentrations of the H + and OH produce a total concentration of 1x10 14 Hence, this is called the water's dissociation constant. 10

11 This is why it makes sense to use the value 1x10 14 to calculate the concentrations of the acid and/or the base. EXAMPLE 1 Determining the Molarity of Acids and Bases in Aqueous Solution: Determine the molarities of H + and OH in a M HCl solution at 25 C. Solution: We assume that hydrochloric acid, HCl(aq), like all strong acids, is completely ionized in water. Thus the concentration of H + is equal to the HCl concentration. [H + ] = M H + We can calculate the concentration of OH by rearranging the water dissociation constant expression to solve for [OH ] and plugging in for K w and for [H + ]. K w = [H + ][OH ] = at 25 C = 0.025M x ([OH ]) /0.025M = [OH ] 4 x M = [OH ] Note that the [OH ] is not zero, even in a dilute acid solution. 11

12 Therefore, we use the same formula again to find = / = 3 x M EXAMPLE 3 ph Calculations: In Example 1, we found that the H + concentration of a M HCl solution was M H +. What is its ph? Solution: ph = log[h + ] = log(0.025) = 1.60 EXAMPLE 4 ph Calculations: In Example 2, we found that the H + concentration of a NaOH solution was M H +. What is its ph? Solution: ph = log[h + ] = log( ) =

13 EXAMPLE 5 ph Calculations: What is the [H + ] in a glass of lemon juice with a ph of 2.12? Solution: [H + ] = 10 ph = = M H + EXAMPLE 6 ph Calculations: What is the [OH ] in a container of household ammonia at 25 C with a ph of ? Solution: [H + ] = 10 ph = = M H + Then [H + ] x [OH ] = 1x ( ) x [OH ] = 1x [OH ] = M or 7.9 x 10 3 As you can see, there are different ways to arrive at the same answer. 13

14 The acid ionization constant K a : A measure of an acid's dissociation. Every acid has a specific constant, which tells us the rate at which the acid dissociates in solution. The higher the K a, the stronger the acid. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/l): HA (generic acid) + H 2 O H 3 O + + A K a = [H 3 O + ][A ]/[HA]. An acid dissociation constant, K a, is a quantitative measure of the strength of an acid in solution. The higher the K a the stronger the acid. 14