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
Basically, Arrhenius Acid: H + donor Arrhenius Base: OH donor 2
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. 3
Basically, Bronsted Lowry Acid: proton donor Bronsted Lowry Base: proton acceptor 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) 4
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) 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 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 6
Equilibrium: A state in which opposing forces or influences are balanced. A state of physical balance. In chemical reactions, equilibrium is denoted by ph scale 7
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 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 8
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
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EXAMPLE 1 Determining the Molarity of Acids and Bases in Aqueous Solution: Determine the molarities of H + and OH in a 0.025 M HCl solution at 25 C. Solution: K w = [H + ][OH ] = 1.01 10 14 at 25 C 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 + ] = 0.025 M H + We can calculate the concentration of OH by rearranging the water dissociation constant expression to solve for [OH ] and plugging in 1.01 10 14 for K w and 0.025 for [H + ] to get 4x10 13 for [OH ] Note that the [OH ] is not zero, even in a dilute acid solution. If a base, such as sodium hydroxide, is added to water, the concentration of hydroxide goes up, and the concentration of hydronium ion goes down. A basic solution can be defined as a solution in which the [OH ] > [H + ]. The next example illustrates this relationship. http://preparatorychemistry.com/bishop_ph_equilibrium.htm 11
Therefore, we use the same formula again to find = / = 3 x 10 12 M 12
EXAMPLE 3 ph Calculations: In Example 1, we found that the H + concentration of a 0.025 M HCl solution was 0.025 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 2.9 10 3 NaOH solution was 5.1 10 12 M H +. What is its ph? Solution: ph = log[h + ] = log(5.1 10 12 ) = 11.29 EXAMPLE 5 ph Calculations: What is the [H + ] in a glass of lemon juice with a ph of 2.12? Solution: [H + ] = 10 ph = 10 2.12 = 7.6 10 3 M of H + EXAMPLE 6 ph Calculations: What is the [OH ] in a container of household ammonia at 25 C with a ph of 11.900? Solution: First, we find the concentration of H: [H + ] = 10 ph = 10 11.9 = 1.26 10 12 M of H + Second, we find the OH: [H + ][OH ]= 1x10 14 (1.26x10 12 )[OH ]= 1x10 14 [OH ]= 7.94x10 3 M 13