D.CHO3HE.KOHB.NHC.CHC3OHHCl3F.H.CHHCH3COG.H2HCHEM 5 PAL Worksheet Acids and Bases Fall 2017 Chem 5 PAL Worksheet Acids and Bases Smith text Chapter 8 Many substances in the body are acids and bases. Many diseases alter acid-base chemistry and many drugs and mediations are acids or bases. Therefore, we need to understand the fundamental properties of acids and bases. From the standpoint of medicine, this is one of the most important chapters in the course. Principle: Acids are substances that donate protons in water. Bases are substances that accept protons in water. 1. Identify whether the following molecules or compounds are acids, bases, or neither. A.O3N3Acids Bases Neither 2. A. Write a generic equation for the dissociation of an acid using the formula HX. B. Write a generic equation for the protonation of a base using the formula HY. C. Write a generic equation for the dissociation of an acid R-COOH. D. Write a generic equation for the protonation of a base using the formula R-NH 2. 3. How many protons does the acid H 3 PO 4 have to donate? Write a dissociation reaction equation for each one. Then identify the acid and conjugate base in each equation. 1
4. Draw the conjugate acid or base of the substances in question 1. If the substance is neither, write NA (not applicable). A. B. C. D. E. F. G. H. Principle: Acids and bases vary in strength depending on how easily they donate or accept protons. Weak acids and bases are reversible reactions and therefore have the properties of all reversible reactions, including a drive towards equilibrium. Weak acid strength can be judged by an equilibrium constant, the Ka. 5. In terms of dissociation, what differentiates strong acids from weak acids? 6. The Ka expression for a weak acid, HA, is written as follows: HA H + + A - Ka = [H+][A-] [HA] A. Write a Ka expression for each of the following weak acids: 1) HNO 3 2) H 2 CO 3 (first dissociation) 3) CH 3 CH 2 COOH B. What does a large Ka mean in regard to acid strength? What does a small Ka mean in regard to acid strength? Answer this question in terms of the relative amounts of H+, A- and HA for a weaker acid vs a stronger acid. Specifically refer to the numerator and denominator of the expression in your answer. 2
7. Another way to express Ka is to use pka instead where pka = -logka. These two terms have an inverse relationship (if one is large, the other is small). Given your response above, what does a large pka mean in regard to acid strength? What does a small pka mean? 8. Rank the following acids from strongest to weakest according to their pka s and Ka s. A. H 2 CO 3, Ka = 4.2 x 10-7 CH 3 CO 2 H, Ka = 1.8 x 10-5 HCN, Ka = 4.0 x 10-10 B. HCOOH, pka = 3.75 HCl, pka = -7 CH 3 CH 2 CH 2 COOH, pka = 4.8 9. Which is the strongest acid? A. pka = 5.7 B. Ka = 1.2 x 10-7 C. pka = 13.9 D. Ka = 3.3 x 10-11 Principle: Water is both an acid and a base. 10. Using two water molecules, write a dissociation reaction for water acting as an acid. Label the acid, base, conjugate acid and conjugate base in this reaction. Principle: In pure water, the amount of H + (or H 3 O) and OH - is very small and the concentration of water is essentially constant at 55 M. The Ka of pure water is 1.8 x 10-16 (this reflects that water is a very weak acid/base). 11. A. Write an equilibrium Ka expression for water. 3
B. What is the product of H + and OH - equal to? This is called the ion product of water, Kw. Write an expression for the ion product of water. C. Using your expression from B above, calculate [H + ] or [OH - ] for each of the solutions below. 1) OH - = 2.5 x 10-6 M 2) H + = 8.3 x 10-11 M 3) OH - = 7.8 x 10-12 M Principle: The ph scale is used to determine whether an aqueous solution is acidic or basic. By definition, ph = log[h + ]. In a neutral solution, [H+] = [OH-] = 1 x 10-7 M. 12. A. Calculate the ph of each solution in question 11C and then indicate whether the solution is acidic or basic. B. What is the OH- concentration of a solution with a ph = 5.6? Principle: An acid can neutralize a base and vice versa. 13. Draw a balanced equation for each neutralization reaction A. H 2 SO 4 mixed with KOH B. CH 3 NH 2 mixed with HNO 3 4
DCHEM 5 PAL Worksheet Acids and Bases Fall 2017 C. CaCO 3 mixed with HCl (this is the neutralization action of antacids; indicate in your equation the source of gas in burps). COOH.mixed with NaOH E. NH 3 mixed with CH 3 COOH Principle: An acid and its conjugate base in solution can absorb added H+ or OH-, thus providing a buffer to changes in ph. 14. Explain using words, drawings, and an equation how a buffer can adjust to changes in added acid or base. Assume your buffer is CH 3 COOH and its conjugate base. 15. The blood buffering system is a major mechanism the body uses to control ph. CO 2 + H 2 O H 2 CO 3 H+ + HCO 3 - How will the blood buffering system respond to each of the following actions (shift left or shift right in the equilibrium)? Action Remove CO 2 Add H + Add HCO - 3 Decrease ph Shift Principle: ph controls solubility for acids and bases which exist in different charge states. 5
16. Draw both forms of the following acids or bases. Then use the pka to determine which form will predominate at the given ph. Substance pka Predominant form at ph 3 CH 3 NH 2 10.6 H 2 CO 3 6.8 Predominant form at ph 7 Predominant form at ph 12 O 4.8 OH H N 9.8 6