BIOB111_CHBIO - Tutorial activity for Session 6: General topics for the week: Acids and Bases ph and buffer Interactive animations are used to reinforce the understanding Instructions- interactive animations: Use the following link to learn the names and formulas for acids and bases http://chemistry.about.com/od/acidsbases/a/strengthacids.htm (link verified 16 May 2011) General tutorial questions: 1. Define the terms acids, bases, ph and buffers. Term Definition a. acid (i) Maintains a constant ph by neutralizing any added acid or any added base b. base (ii) Releases hydroxide ions in solution c. buffer (iii) Releases hydrogen ions in solution (iv) Is made up of a weak acid and the salt of its conjugate base (v) Would cause ph to drop in a nonbuffered solution (vi) Has a ph above 7.0 (vii) Would feel slippery and would taste bitter (viii) Proton donor a. matches with: b. matches with: c. matches with: 2. State if the following examples of body fluids are acidic, basic, or neutral, given each of these phs? a) Saliva, ph 6.8 b) Urine, ph 5.9 c) Pancreatic juice, ph 8.0 d) Bile, ph 8.4 e) Arterial blood, ph 7.35 to 7.45 Last Updated on 6-Feb-14 Page 1 of 8
f) Concentrated NaOH, ph 13 g) Concentrated HCl, ph 1 3. According to the Brønsted-Lowry definition, is an acid a proton donor or a proton acceptor? a) Proton donor b) Proton acceptor 4. According to the Brønsted-Lowry definition, is an base a proton donor or a proton acceptor? a) Proton donor b) Proton acceptor 5. What is the difference between strong acids and weak acids? a) Strong acids dissociate only partially into ions in solution b) Weak acids dissociate completely into ions in solution c) Strong acids dissociate completely into ions in solution d) Weak acids do not form ions in solution 6. Which of the following statements about weak acids is correct? a) Weak acids always contain C atoms b) The percentage dissociation for weak acids is usually in the range of 40-60 % c) Weak acid molecules only lose a small percentage of their acidic hydrogen atoms d) More than one correct response 7. In the following reaction, carbonic acid dissociates only slightly in water. Is it a strong acid or a weak acid? H 2CO 3 (aq) + H 2O (l) H 3O + (aq) + HCO 3 (aq) a) Strong acid b) Weak acid (reversible equilibrium, only partially dissociates in ionic solution) 8. What is one of the conjugate acid-base pairs when hydrofluoric acid (HF) dissolves in H 2O? Last Updated on 6-Feb-14 Page 2 of 8
HF (aq) + H 2O (l) H 3O + (aq) + F - (aq) a) H 2O and F b) HF and H 3O + c) HF and F 9. Does the equilibrium below favour the reactants or products? HF (aq) + H 2O (l) H 3O + (aq) + F - (aq) a) Reactants favoured (HF and H 2O) because HF is a weak acid b) Products favoured (hydronium ion and F ion) because HF is a strong acid 10. Select from the response list the compound or ion that best fits the description: conjugate base of H 3PO 4. a) H 3PO 4 b) H 2PO 4 2 c) HPO 4 3 d) PO 4 11. Which one of the following would make a buffer solution? a) HCl and NaCl b) H 3PO 4 c) H 2CO 3 and HCO 3- weak acid and salt of its conjugate base d) NaCl and Na 2CO 3 Conceptual multiple choice questions: 12. Concept: Acid-base reactions Context: Acids and bases react together in chemical reactions referred to as acid-base reactions. Acids are compounds that donate H + to a base, whereas bases are compounds that accept H + from an acid. Question: Which of the following correctly describes how an acid-base reaction proceeds? Last Updated on 6-Feb-14 Page 3 of 8
A. An acid donates a H + to a base, which means the reactant acid is converted to a product acid through the chemical reaction B. A base donates a H + to an acid, which means that the reactant base is converted to a product acid that contains one more H + than the original base C. A base donates a H + to an acid, which means the reactant base is converted to a product base through the chemical reaction D. An acid donates a H + to a base, which means that the reactant acid is converted to a product base with one less H + than the original acid 13. Concept: Assessing whether a compound behaves as an acid or base Context: In acid-base reactions H 2O sometimes acts as a base (becoming the hydronium ion (H 3O + ) and sometimes like an acid (becoming the hydroxide ion (OH - ). Question: For the chemical reaction below, does H 2O function as an acid or base? Why? H 2CO 3 + H 2O <---> H 3O + + HCO 3 A. H 2O accepts H + from H 2CO 3, and is therefore an acid B. H 2O accepts H + from H 2CO 3, and is therefore a base C. H 2O donates H + to H 2CO 3, and is therefore an acid D. H 2O donates H + to H 2CO 3, and is therefore a base 14. Concept: Assessing whether a compound behaves as an acid or base Context: Assessing whether a compound behaves as an acid or base Context: In acid-base reactions H 2O sometimes acts as a base (becoming the hydronium ion (H 3O + ) and sometimes like an acid (becoming the hydroxide ion (OH - ). Question: For the chemical reaction below, does H 2O function as an acid or base? Why? NH3 + H2O <---> NH4+ + OH A. H 2O accepts H + from NH 3, and is therefore an acid B. H 2O accepts H + from NH 3, and is therefore a base C. H 2O donates H + to NH 3, and is therefore an acid D. H 2O donates H + to NH 3, and is therefore a base 15. Concept: Conjugate acid-base pair Last Updated on 6-Feb-14 Page 4 of 8
Context: Acid-base reactions each have two conjugate acid-base pairs. The difference between the two compounds in a conjugate acid-base pair is one H +. During an acid base reaction, the reactant acid loses a H + to become a conjugate base, whereas the reactant base gains a H + to become a conjugate acid. Question: For the below reaction, which conjugate acid-base pair contains H 2CO 3? H 2CO 3 + H 2O <---> H 3O + + HCO 3 A. H 2CO 3 is an acid and its conjugate base product is H 3O + B. H 2CO - 3 is a base and its conjugate acid product is HCO 3 C. H 2CO 3 is a base and its conjugate acid product is H 3O + D. H 2CO - 3 is an acid and its conjugate base product is HCO 3 16. Concept: Strong Vs. weak acids Context: A compound that has the capacity to give away (donate) H + is called an acid. However, strong acids are better than weak acids at giving away H +. Question: Which of the following is true about strong and weak acids? A. Strong acid compounds readily react with base compounds to give away all of their H +, whereas weak acid compounds rarely react with base compounds resulting in only a few H + being donated B. Both strong and weak acid compounds readily react with base compounds to give away all of their H + C. Weak acid compounds readily react with base compounds to give away all of their H +, whereas strong acid compounds rarely react with base compounds resulting in only a few H + being donated D. Both strong and weak acid compounds rarely react with base compounds resulting in only a few H + being donated 17. Concept: Characteristics of strong acids Context: Acid-base reactions each have two conjugate acid-base pairs. The difference between the two compounds in a conjugate acid-base pair is one H +. During an acid-base reaction, the reactant acid loses a H + to become a conjugate base, whereas the reactant base gains a H + to become a conjugate acid. Last Updated on 6-Feb-14 Page 5 of 8
Question: Which of the following correctly describes a strong acid and its conjugate base? HBr + H 2O Br - + H 3O + A. A strong acid is very reactive and readily gives away its H + to a base, which results in the strong acid becoming a strong conjugate base B. A strong acid is very stable and rarely gives away its H + to a base, but when the strong acid donates its H + it becomes a more stable weak conjugate base C. A strong acid is very stable and rarely gives away its H + to a base, but when the strong acid donates its H + it becomes a strong conjugate base D. A strong acid is very reactive and readily gives away its H + to a base, which results in the strong acid becoming a more stable weak conjugate base 18. Concept: Properties of buffers Context: A buffer contains two components, a weak acid (e.g. H 2CO 3) and its conjugate base (e.g. HCO 3- ). The presence of a buffer can prevent large ph changes when a small amount of acid or base is added into blood. Question: How does a buffer prevent a large ph change when a small amount of acid is added into blood? A. Both the weak acid and the conjugate base of the buffer react with the added acid to prevent a large ph change within the blood B. The weak acid of the buffer neutralises the added acid through an acid-base reaction C. Having the buffer in the blood is enough to prevent a ph change from the added acid, an acid-base reaction is not required D. The conjugate base of the buffer neutralises the added acid through an acid-base reaction 19. Concept: Properties of buffers Context: A buffer contains two components, a weak acid (e.g. H 2CO 3) and its conjugate base (e.g. HCO 3- ). The presence of a buffer can prevent large ph changes when a small amount of acid or base is added into blood. Question: How does a buffer prevent a large ph change when a small amount of base is added into blood? Last Updated on 6-Feb-14 Page 6 of 8
A. The conjugate base of the buffer neutralises the added base through an acid-base reaction B. Both the weak acid and conjugate base of the buffer react with the added base to prevent a large ph change within the blood C. The weak acid of the buffer neutralises the added base through an acid-base reaction D. Having the buffer in the blood is enough to prevent a ph change from the added base, an acid-base reaction is not required 20. Concept: Role of the hydronium (H 3O + ) and the hydroxide ions (OH - ) in ph Context: ph is used to measure how acidic or basic a solution is, with low phs being acidic and high phs being basic. The ph of a solution is determined by assessing the concentrations (amounts) of the hydronium ion (H 3O + ) and the hydroxide ion (OH - ). Question: Which of the following correctly describes the ph of solution A that has a higher concentration of hydronium ions, and solution B that has a higher concentration of hydroxide ions? A. Both solutions A and B have a neutral ph, as the concentration of the H 3O + ions and OH - ions is equal B. The ph of solution A is high due to the basic nature of H 3O + ions, whereas the ph of solution B is low due to the acidic nature of the OH - ions C. Both solutions A and B have an acidic ph, as both OH - ions and H 3O + ions are acidic D. The ph of solution A is low due to the acidity of H 3O + ions, whereas the ph of solution B is high due to the basic nature of the OH - ions 21. Concept: Adjusting the ph of a solution Context: When acid or base is added into a solution, the ph changes. In the absence of a buffer the ph change will be more drastic than in the presence of a buffer, as buffers function to prevent large ph change. Question: For a solution that has a ph of 11, what is the best way to decrease the ph to 5? A. To decrease the ph to 5, add hydronium ions (H 3O + ) in the presence of a buffer Last Updated on 6-Feb-14 Page 7 of 8
B. To decrease the ph to 5, add hydroxide ions (OH - ) in the absence of a buffer C. To decrease the ph to 5, add hydronium ions (H 3O + ) in the absence of a buffer D. To decrease the ph to 5, add hydroxide ions (OH - ) in the presence of a buffer 22. Concept: Adjusting the ph of a solution Context: When acid or base is added into a solution, the ph changes. In the absence of a buffer the ph change will be more drastic than in the presence of a buffer, as buffers function to prevent large ph change. Question: For a solution that has a ph of 2, what is the best way to increase the ph to 12? A. To increase the ph to 12, add hydronium ions (H 3O + ) in the presence of a buffer B. To increase the ph to 12, add hydroxide ions (OH - ) in the absence of a buffer C. To increase the ph to 12, add hydronium ions (H 3O + ) in the absence of a buffer D. To increase the ph to 12, add hydroxide ions (OH - ) in the presence of a buffer 23. Concept: Effect of ketone bodies on blood ph Context: Buffers in our blood (e.g. H 2CO 3/HCO 3- ) assist in maintaining a tight ph range between 7.35-7.45. However, when many ketone bodies accumulate in the blood, the ph of the blood becomes more acidic. Question: Why does an over accumulation of ketone bodies in an individual s blood lead to acidosis? A. The large amount of ketone bodies present means that ketone bodies are available to react with H 3O + to produce the acidic OH - ions, which lowers the ph of the blood B. The large amount of ketone bodies present means that ketone bodies are available to react with H 2O to produce the acidic H 3O + ions, which lowers the ph of the blood C. The large amount of ketone bodies present means that ketone bodies are available to react with OH - to produce the acidic H 3O + ions, which increases the ph of the blood D. The large amount of ketone bodies present means that ketone bodies are available to react with H 2O to produce the acidic OH - ions, which increases the ph of the blood Last Updated on 6-Feb-14 Page 8 of 8