TOPIC 11: Acids and Bases

Transcription

1 TOPIC 11: Acids and Bases ELECTROLYTES are substances that when dissolves in water conduct electricity. They conduct electricity because they will break apart into Ex. NaCl(s)! Na + (aq) + Cl - (aq), and are electrolytes DEFINING ACIDS AND BASES 1. Arrhenius acids and bases An arrhenius acid: substance that when dissolved in water releases o Ex. o Remember: an acid must start with H or end with COOH (Table K) " KH not an acid! The H + ions combine with water molecules to make hydronium ions (H 3 O + ) o H + + H 2 O! H 3 O + A monoprotic acid releases in solution. Ex. HCl, HNO 3 A diprotic acid yields in solution. Ex. H 2 SO 4 An arrhenius base: is a substance that when dissolved in water o Ex. o Remember: An arrhenius base is a metal + OH or a polyatomic + OH (Table L) o Alcohols are not bases! Ex. C 2 H 5 OH o According to this definition, ammonia is not a base 2. Bronsted-Lowry Acids and bases (Alternate Acid-base definition) A Bronsted-Lowry acid is any atom or ion that - B-L acids lose H + It is broader than the arrhenius definition. It includes all arrhenius acids plus other atoms/ions like NH 4 + A Bronsted-Lowry base is any atom or ion that - B-L bases gain H + This definition is broader than the arrhenius definition of a base. - Includes bases that don t have OH 1

2 Ex NH 3 + H 2 O #! NH OH - - In the forward reaction, the water molecule starts with H s and ends up with. It lost an H and therefore is the - The ammonia starts with H s and ends up with. It gained an H and therefore is the - In the reverse reaction, is the acid and is the base Ex. HCl + H 2 0 #! Cl - + H Forward reaction: is the acid, is the base - Reverse reaction, is the acid and is the base Conjugate acid: the ion or molecule that formed from the original base Conjugate base: the ion or molecule that formed from the original acid Ex. NH 3 + H 2 O NH OH - Acid: Base: Conjugate base: Conjugate acid: + : are a conjugate acid-base pair + : are a conjugate acid-base pair Ex. HCl + H 2 O H 3 O + + Cl - Acid: Base: Conjugate base: Conjugate acid: + : are a conjugate acid-base pair + : are a conjugate acid-base pair Properties of Acids and Bases Acids Bases Found on Found on Begin with or end with Have a sour taste (lemon, vinegar) and a slippery feel React with metals to produce H 2 (g) (single replacement) ex. Na + HCl! NaCl + H 2 (g) Reacts with a base to produce Ex. HCl + NaOH! H 2 O + NaCl Electrolytes: conduct electricity in water ph is less than 7 bitter taste and soapy feel Reacts with an acid to produce salt and water Electrolytes : conduct electricity in water ph is greater than 7 2

3 Amphoteric substances Also called Amphiprotic substances can act as either an acid or a base ex. HSO 4 - ex. Water o H 2 O + H 2 SO 4! H 3 O + + HSO 4 - o H 2 O + NH 3! OH - + NH 4 + Salts Contain: 1. Metal + nonmetal 2. Metal + a polyatomic ion (except OH) 3. nonmetal + polyatomic ion 4. poly + poly Ex. LiCl, NaCl, K 2 SO 4, MgCl 2 Salts are neutral, they have a ph of 7 electrolytes IDENTIFY THE FOLLOWING AS AN ACID (H- OR COOH, Table k), BASE (-OH, table L)), SALT (IONIC) OR OTHER 1. NaOH 2. C 2 H 5 OH 3. HF 4. KF 5. C 2 H 3 COOH 6. NH 3 7. C 6 H 12 O 6 8. H 2 SO 4 9. K 2 SO 4 Practice Regents Problems 1) Which formula represents an electrolyte? A) CH 3 OH B) CH 3 COOH C) CH 3 OCH 3 D) C 2 H 5 CHO 2) In the reaction NO - 2 (aq) + H 2 O(l)! HNO 2 (aq) + OH - (aq), the NO - 2 (aq) acts as A) a Bronsted base B) an Arrhenius base C) a Bronsted acid D) an Arrhenius acid 3) Which substance is an electrolyte? A) H 2 O B) CH 3 OH C) KOH D) C 6 H 12 O 6 4) Which compound is an electrolyte? A) C 6 H 12 O 6 B) CCl 4 C) CH 3 OH D) CaCl 2 3

4 5) When an Arrhenius acid dissolves in water, the only positive ion in the solution is A) K + B) Na + C) Li + D) H + 6) A hydrogen ion, H +, in aqueous solution may also be written as A) H 3 O + B) H 2 O C) OH - D) H 2 O 2 7) Which substance yields hydroxide ion as the only negative ion in aqueous solution? A) C 2 H 4 (OH) 2 B) MgCl 2 C) CH 3 Cl D) Mg(OH) 2 NEUTRALIZATION REACTIONS Occur when: an arrhenius acid reacts with an arrhenius base to produce salt and water Ex. H 2 SO 4 + 2KOH! K 2 SO 4 + 2H 2 O Ex. H + + OH -! H 2 O (net equation) When a solution is neutral, the moles of H + = moles OH - (concentrations are equal) Writing neutralization reactions: HNO 3 + KOH! Acid + base! water + salt take the H+ from the acid and the OH- from the base to form water then combine the other ions to form the salt. o Criss-cross the oxidation numbers to get the subscripts. make sure the equation is balanced Ex. NaOH + HCl! Ex. H 3 PO 4 + NaOH! TITRATIONS The purpose of a titration is to determine the concentration of an acid or base by performing a neutralization reaction A titration is the process of adding an acid/base OF KNOWN concentration to an acid/base that you don t know the concentration of, until you have a neutral solution. The indicator is used to indicate when the solution is neutral Uses the formula: Ex. What is the concentration of a HCl solution if 50mL of a.25m KOH solution are needed to neutralize 20 ml of the HCl solution? 4

5 Ex. If 50 ml of 3M HNO 3 completely neutralized 150 ml of KOH, what was the molarity of the KOH solution? Solving titration problems using lab data: 3 We calculate the volume of acid used by: 2 We know the concentration (M) of the substance in the buret. In this case, it is the The acid will be added to the base until the phenolphthalein goes from to 1 The substance in the flask is the one we don t know the concentration (M) of. In this case, we are looking for the concentration of. We do know its. Ex. A student recorded the following buret readings during a titration of a base with an acid: Calculate the molarity of the KOH. For diprotic acids use: Ex. What is the concentration of H 2 SO 4 if 50 ml of a.25m KOH solution are needed to neutralize 20mL of the H 2 SO 4? For bases that produce more than 1 mole of OH (ex. Ca(OH) 2 ) use: ph SCALE AND TABLE M

6 Indicators are used to determine the ph of a substance Table M: shows the color an indicator changes in varying ph values Ex. methyl orange red to yellow This means that: 1. methyl orange will be red in a ph of less than 3.2 (strong acid) 2. yellow in a ph greater than orange in a ph between indicators can be used to narrow down the ph range Ex. Methyl orange turns yellow. This means that: Bromthymol blue turns yellow. This means that: Therefore, the ph is between The easiest way to read table M: the first color listed in the one the indicator would be in an acid. The second color listed is the one it would be in a base. Practice Regents Problems: 1) Which of these ph numbers indicates the highest level of acidity? A) 5 B) 12 C) 8 D) 10 2) Which indicator is yellow in a solution with a ph of 9.8? A) methyl orange B) bromcresol green C) bromthymol blue D) thymol blue 3) Which aqueous solution would turn blue litmus red? A) NaCl(aq) B) HCl(aq) C) NaOH(aq) D) K2CO3(aq) Comparing ph and poh ph Indicates how many H + (H 3 O + ) ions are in solution poh Indicates how many OH - are in solution ions Calculated using ph = -log[h + ] Steps: Put the molarity (concentration) in scientific notation The exponent is the ph Ex. what is the ph of a.001m HCl solution? Calculated using poh = -log[oh - ] Same steps but using [OH - ] Ex. If the ph is 6 what is the concentration of the solution? ph + poh = 14 [H + ][OH - ] = 1 x

7 What a change in ph indicates ph change More acidic or basic? Effect on [H + ] Effect on [OH - ] When the [H + ] = [OH - ] the solution is When the [H + ] > [OH - ] the solution is When the [H + ] < [OH - ] the solution is For each one-unit change in ph, the concentration of H+ (H 3 O + ) ions changes by x Ex. if the ph of a solution DECREASES from 4! 3, the resulting solution is now 10x MORE acidic Ex. if the ph of a solution DECREASES from 6! 3 (3 units), the solution is (the amount of H+ ions increases by 1000x) Ex. if the ph of a solution INCREASES from 4!7, the amount of H+ ions. It becomes 1000x more basic. Ex. If the ph started at 8 and there was a 100-fold DECREASE (1/100) in hydronium ion concentration, what is the new ph? 7

8 Practice Problems Questions 1 through 3 refer to the following: A student was studying the ph differences in samples from two Adirondack streams. The student measured a ph of 4 in stream A and a ph of 6 in stream B. 1) For the situation described, compare the hydronium ion concentration in stream A to the hydronium ion concentration in stream B. 2) What is the color of bromthymol blue in the sample from stream A in the situation described? 3) Identify one compound that could be used to neutralize the sample from stream A in the situation described. 4) When the ph of a solution changes from a ph of 5 to a ph of 3, the hydronium ion concentration is A) 0.01 of the original content B) 100 times the original content C) 0.1 of the original content D) 10 times the original content KEY THINGS TO REMEMBER WHEN TAKING THIS EXAM: 1. Identify if the substance is an acid, base, or salt. Then you will know: a. its properties (conducts electricity, ph range, reacts with metals to form H 2 ) b. what color its turns an indicator (TABLE M) i. first column is the color it would be in an acid ii. second column is the color it would be in a base c. use tables K and L to help you 2. Know definition of Arrhenius acids and bases 3. Be able to identify B-L acids and bases and know definitions 4. Know the names of the 3 ions we are working with a. H 3 O + is a hydronium ion b. H + is a hydrogen ion table E can help with this c. OH - is a hydroxide ion 5. Realize that the [H 3 O + ] and [H + ] mean the same thing (both come from acids) 6. Understand how movement along the ph scale affects the [H 3 O + ] / [H + ] a. ph the [H 3 O + ] / [H + ] b. ph the [H 3 O + ] / [H + ] c. every 1 unit change in ph changes the [ ] by 10x 7. Know how to use the molarity formula for word problems and using lab data 8. Be able to identify a neutralization reaction 9. Be able to determine: a. the ph given the Molarity b. the molarity given the ph 8