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 th 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

4 Basically, Bronsted Lowry Acid: proton donor Bronsted Lowry Base: proton acceptor 4

5 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) 5

6 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 6

7 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 7

8 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 8

9 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 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 9

10 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

11 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: K w = [H + ][OH ] = 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 + ] = 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 + ] 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. 11

12 Therefore, we use the same formula again to find = / = 3.45 x M 12

13 in the calculator: 13

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15 Titration The process in which one solution is added to another solution such that it reacts under conditions in which the added volume may be accurately measured. Titrations are most commonly associated with acid base reaction, but they may involve other types of reactions as well. Indicators are usually used to show the point at which the acid and base amounts are equal (by changing colour.) Titration continued 15

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17 Titration curves Strong base and strong acid 17

18 Weak acid and strong base Strong acid and weak base 18