Ch.15 - Acid and Base EquilibriumWorksheetSee all chapters
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Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
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In the 1920s, Gilbert Lewis proposed a new set of definitions for acids and bases. 

Understanding Lewis Acids and Bases

According to the Lewis definition an acid is an electron pair acceptor. 

Concept #1: Understanding Lewis Acids. 

Transcript

Hey guys, in this new video, we're going to take a look at the last and final type of acids and bases – the Lewis acid and the Lewis base. So, let's take a look. We’re going to say in the 1920s, we had Gilbert Lewis, he proposed his own new definition of acids and bases. He basically did this almost to the same exact time as Bronsted and Lowry. Here he defined a Lewis acid as something that accepts electrons. So we’re going to say a Lewis acid is an electron acceptor. To understand a Lewis acid, we have to basically figure out what are the different types of Lewis acids. The first one, we say if we have H acts as a Lewis acid, when connected to a very electronegative element. The electronegative elements are oxygen, nitrogen, sulfur, phosphorus or the halogens, group 7A. I'm talking about fluorine, chlorine, bromine or iodine.
Think about it. Let’s think of H being connected to some of these elements. Since all of these elements are more electronegative, they're going to be partially negative. Since H is less electronegative, it's going to be partially positive. We tend to think of this type of Lewis acid when we have H single bonded to the one of these electronegative elements. That's the only time we really look at it and if we're talking about the bond between H and of these electronegative elements. Think about it. Electrons are negatively charged. Since they’re negatively charged, they’ll be attracted to something that's positive or in this case, partially positive. That's what that delta positive means. It means it’s partially positive, a little bit positive. In this case, we're going to say H can act as a Lewis acid.
The second type is anytime we have a positively, fully positively charged hydrogen or metals. Here H+ would be an example of a Lewis acid because it accepts electrons. Electrons are negative. It's positive, opposites attract. If we have positive metals, since they’re all positive, they’ll attract negative electrons. Then the last and final type of Lewis acid is anytime you're central element has less than eight valence electrons around it. Remember, this is the octet rule. Our central element, once they have eight electrons, so they could have a similar configuration as the noble gases. But we're going to say group 2A and 3A elements and transition metals when they're in the center tend to have less than eight valence electrons.
For example, we could have boron. It's in group 3A, so it makes three bonds. Remember, for every bond we make, we're sharing those electrons. Here, boron has six electrons around it. It still has room to accept two more. That's why boron trifluoride can act as a Lewis acid because the central element can accept more electrons. We could also have Be connected to two Cls. Here Be has four electrons around it, so it can accept more electrons, another electron pair to get closer to the octet rule. We could have aluminum bromide here. It has six around it as well. Then our transition metal, common ones, zinc. Here it has four electrons around it as well.
This last type Lewis acid is common with groups to 2A, 3A and the transition metals. When they're in the center, they tend to have less than eight electrons and as a result, they serve as Lewis acids. They're going to accept an electron pair.

When hydrogen is connected to an electronegative element such as P, O, N, S or halogens then it gains a partially positive charge, which makes hydrogen act as a Lewis acid. 

Since these metal ions accept negative electron pairs then some Lewis Acids may be positively charged. 

If the central element has less than 8 valence electrons around it then it is more likely to accept an electron pair to follow the octet rule. 

According to the Lewis definition a base is an electron pair donor. 

Concept #2: Understanding Lewis Bases. 

Transcript

If the Lewis acid is an electron pair acceptor, that would have to mean that the Lewis base is an electron pair donor now. What makes Lewis bases? We’re going to say Lewis bases are compounds with lone pairs. See here, all of these, what they have in common is they all have lone pairs. They're all Lewis bases. Then we’re going to say also compounds with, what do all these have in common? They all have in common a negative charge. If you have a negative charge, that means you have excess of extra electrons, so excess of extra lone pairs. Remember, if you're an electron pair donor, you have to have lone pairs and/or a negative charge. Negative charge just means you have extra electrons hanging around. We may not see them but we know they’re there.
This definition is a little bit different than the others and honestly, I think it's the most difficult out of the three because when you get to organic chemistry, organic chemistry is predominantly the chemistry of Lewis acid and bases. That's why this type of theory seems a little bit different from the others because it resides more in the organic chemistry region than general chemistry. But you still need to know it. Remember, a Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.

Compounds that have lone pair(s) on a central element can act as Lewis bases. 

Compounds that possess a negative charge can act as Lewis Bases. 

Example #1: Identify each of the compounds in the following chemical equation.

Practice: Identify the Lewis acids and bases in the following reactions.

a) H+ + OH ⇌ H2O

b) Cl + BCl3 ⇋ BCl4

c) SO3 + H2O ⇌ H2SO4

Practice: Identify each of the following compounds as either a Lewis acid, a Lewis base or neither.

a)  ZnCl2                                                                                                           b)  CN 

 

c)  NH4+                                                                                                           d) Co3+

Which of the following is a Lewis base? A) AlF3 B) H2O C) SiF4 D) C5H12 E) None of the above are Lewis bases.
Which species in the following reaction acts as a Lewis acid?             CuSO4(s) + 4NH3(aq) ⇌ [Cu(NH3)4]2+(aq) + SO42−(aq)       A) Cu2+           B) SO42−          C) [Cu(NH3)4]2+(aq)               D) NH3 E) [Cu(NH3)4]2+(aq) and SO42−
Which of the following are Lewis bases? I)         BCl3 II)        H− III)       H2O IV)       NH3 V)        AlCl3        A) I), II), and IV) B) I), II), and III) C) II), III) and IV) D) I), II), and V) E) III), IV) and V)
Which of the following characteristics is/are  potential identifiers of a Lewis acid? I.  Unfilled octet  II. Double bond present  III. Compounds with metal and hydroxide  A.  Only I       B.  Only II       C.  II and III       D.  I and II       E.  All three
NCl3 would be described as: A) Lewis acid because it can donate electrons B) Lewis acid because it can receive electrons C) Lewis base because it can donate electrons D) Lewis base because it can receive electrons E) This compound is neither acidic or basic
All are examples of Lewis acid-base reactions except: A. Cu2+(aq) + 4NH3(aq)   ⇌   [Cu(NH3)4] 2+(aq) B. HCl(aq) + NH3(aq)   ⇌   NH4Cl(aq) C. H+ (aq) + OH- (aq)   ⇌   H2O(l) D. 2Na(s) + Cl2(g)   ⇌   2NaCl(s)
All are potential Lewis bases except:   A. Ammonia B. Water C. Methane (CH4) D. Cyanide ion (CN-)
Which of the following is the definition of a base in the Lewis system? a. A base is a substance that can accept an H + ion. b. A base is a substance that, when dissolved in water, produces hydroxide ions. c. A base is a substance that contains OH - in its chemical formula d. A base is a substance that that can accept a pair of electrons e. A base is a substance that can donate a pair of electrons.
Which of the following can be a potential Lewis base? a. Ca +2 b. CH4 c. H + d. NH3 e. H3O +
What is the most accurate and complete description of a Lewis Acid? A. proton donor B. proton acceptor C. electron donor D. electron acceptor
In the reaction  SnCl4 + 2Cl− → SnCl62-  identify the Lewis Acid. 1. Cl− 2. SnCl4 3. SnCl62-
True or false: SO3 is a stronger Lewis acid than SO2.
Identify the Lewis acid and Lewis base in the following reactions: (a) CH3OH + H3O+ ⇌ CH3OH2+ + H2O (b) BF3 + NH3 → BF3- + NH3+ (c) Cl2 + AlCl3 → AlCl5  
Draw the product(s) of the following acid-base reaction. Include all lone pairs and formal charges. Use curved arrow(s) to show the electron movement that lead to the formation of the product(s). Finally, label (1) the acid and (2) the base.
The second step in a synthesis of isoamyl acetate, an ester commonly used in the food and cosmetic industries to mimic banana flavor, is represented by the mechanism below. Identify the Lewis acid and the Lewis base.
What feature must any molecule or ion have in order to act as a Lewis acid?
What is a general characteristic of a Lewis acid? Of a Lewis base?
For Cu and CH4, which will behave as a Lewis acid toward OH - in water?a. only Cub. only CH4c. Cu and CH4d. neither Cu nor CH4
Which of the following can act as a Lewis base?Cr3+, SO3, CH3NH2, BeCl2 a. Cr3+, BeCl2 b. SO3 onlyc. CH3NH2, BeCl2 d. SO3, CH3NH2 e. CH3NH2 only
Identify the Lewis acid and Lewis base in each of the following reactions.
Classify each of the following as a Lewis acid or a Lewis base.
Identify the Lewis acid and Lewis base in each of the following reactions:a. HNO2(aq) + OH-(aq) ⇌ NO2-(aq)+H2O(l)b. FeBr3(s) + Br-(aq) ⇌ FeBr4-(aq)c. Zn2+(aq) + 4NH3 ⇌ Zn(NH3)42+(aq)d. SO2(g) + H2O(l) ⇌ H2SO3(aq)
Classify each of the following as a Lewis acid or a Lewis base.
For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.a. BF3 is the Lewis acid.b. BF3 is the Lewis base.c. CH3OH is the Lewis acid.d. CH3OH is the Lewis base.e. This not a Lewis acid-base reaction.
Classify the following as a Lewis acid or Base:BeCl2, OH-, B(OH)3, CN-
For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.SO2 + OH- ⇌ HSO3-a. SO2 is the Lewis acid.b. SO2 is the Lewis base.c. OH- is the Lewis acid.d. OH- is the Lewis base.e. This not a Lewis acid-base reaction.
Which of the following is NOT a Bronsted Acid but IS a Lewis Acid?a. H2Ob. BCl3c. NH3d. NH4+e. HCl
For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base. 4NH3 + Cu2+ ⇌ Cu(NH3)42+    a. NH3 is the Lewis acid. b. NH3 is the Lewis base. c. Cu2+ is the Lewis acid. d. Cu2+ is the Lewis base. e. This not a Lewis acid-base reaction.
For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base. a. Benzene is the Lewis acid. b. Benzene is the Lewis base. c. Br+ is the Lewis acid. d. Br+ is the Lewis base. e. This not a Lewis acid-base reaction.
Classify each of the following as a Lewis acid or a Lewis base:
Which are Lewis acids and which are Lewis bases?(a) Mg2+ (b) OH− (c) SiF4 (d) BeCl2
Identify the Lewis acid and Lewis base in the reaction:(b) CO2 + H2O ⇌ H2CO3
Identify the Lewis acid and Lewis base in the reaction:(c) F− + BF3 ⇌ BF4−
We have seen an introductory definition of an acid: An acid is a compound that reacts with water and increases the amount of hydronium ion present. In the chapter on acids and bases, we saw two more definitions of acids: a compound that donates a proton (a hydrogen ion, H+) to another compound is called a Brønsted-Lowry acid, and a Lewis acid is any species that can accept a pair of electrons. Explain why the introductory definition is a macroscopic definition, while the Brønsted-Lowry definition and the Lewis definition are microscopic definitions.
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:(c) I− + I2 ⟶ I3−
Identify the Lewis acid and Lewis base in the reaction:(b) H2O + H− ⇌ OH− + H2
Identify the Lewis acid and Lewis base in the reaction:(c) 4CO + Ni ⇌ Ni(CO)4
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:(a) CS2 + SH− ⟶ HCS3−
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:(c) I− + SnI2 ⟶ SnI3−
Classify the following as Arrhenius, Brønsted-Lowry, or Lewis acid-base reactions. A reaction may fit all, two, one, or none of the categories:(b) H2SO4 + NH3 ⇌ HSO4− + NH4+
Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:(d) Al(OH)3 + OH− ⟶ Al(OH)4−
Classify the following as Arrhenius, Brønsted-Lowry, or Lewis acid-base reactions. A reaction may fit all, two, one, or none of the categories:(c) 2HCl ⇌ H2 + Cl2
Using Lewis structures, write balanced equations for the following reactions:(a) HCl(g) + PH3(g) ⟶
Boric acid, H3BO3, is not a Brønsted-Lowry acid but a Lewis acid.(a) Write an equation for its reaction with water.
Boric acid, H3BO3, is not a Brønsted-Lowry acid but a Lewis acid.(c) What is the hybridization on the boron consistent with the shape you have predicted?
Identify the Lewis acid and Lewis base from among the reactants in each of the following equations.Fe3+(aq) + 6H2O(l) ⇌ Fe(H2O)63+(aq)
Identify the Lewis acid and Lewis base from among the reactants in each of the following equations.(CH3)3N(g) + BF3(g) ⇌ (CH3)3NBF3(s)
Consider the following reaction:Identify the Lewis base in the reaction.
Identify the Lewis acid and Lewis base from among the reactants in each of the following equations.AlBr3 + NH3 ⇌ H3NAlBr3
The fluoride ion reacts with water to produce HF. Is fluoride acting as a Lewis acid or as a Lewis base when reacting with water?
Chloral (Cl3C—CH=O) forms a monohydrate, chloral hydrate, the sleep-inducing depressant called “knockout drops” in old movies. (a) Write two possible structures for chloral hydrate, one involving hydrogen bonding and one that is a Lewis adduct. 
Esters, RCOOR′, are formed by the reaction of carboxylic acids, RCOOH, and alcohols, R′OH, where R and R′ are hydrocarbon groups. Many esters are responsible for the odors of fruit and, thus, have important uses in the food and cosmetics industries. The first two steps in the mechanism of ester formation areIdentify the Lewis acids and Lewis bases in these two steps.
Classify each of the following as either a Lewis acid or a Lewis base.
Identify the Lewis acid and the Lewis base in the following reaction.a. B(OH)3(aq) + H2O(l) ⇌ B(OH)4-(aq) + H+(aq)
Identify the Lewis acid and the Lewis base in the following reaction.b. Ag+(aq) + 2NH3(aq) ⇌ Ag(NH3)2+(aq)
Identify the Lewis acid and the Lewis base in the following reaction.c. BF3 (g) + F -(aq) ⇌ BF4-(aq)
Identify the Lewis acid and the Lewis base in the following reaction.a. Fe3+(aq) + 6H2O(l) ⇌ Fe(H2O)63+(aq)
Identify the Lewis acid and the Lewis base in the following reaction.c. HgI2(s) + 2I-(aq) ⇌ HgI42-(aq)
Would you expect Fe3+ or Fe2+ to be the stronger Lewis acid? Explain.
Which are Lewis acids and which are Lewis bases?(a) Na+ (b) NH3 (c) CN− (d) BF3
Which of the following can act as a Lewis base? Cr3+, SO3, CH3NH2, BeCl2A. SO3 onlyB. SO3, CH3NH2C. CH3NH2 onlyD. CH3NH2, BeCl2E. Cr3+, BeCl2
Classify each of the following as a Lewis acid or a Lewis base. H-, I- Fe3+ NO2 SnCl4 CH3OH (CH3)3N
Classify each of the following as a Lewis acid or a Lewis base:Fe3+ BCl3 S2-H2OSO3
A large molecule X is classified as a Lewis acid, and another molecule, Y, is a Bronsted-Lowry acid. Which of the following best describes a comparable behavior in aqueous solution?a. Both molecules will tend to acquire a net positive chargeb. Both molecules will release hydroxide ionsc. Only molecule Y will react with the waterd. Both molecules will release hydrogen gase. Both molecules will tend to acquire a net negative charge
In many reactions the addition of AlCl3 produces the same effect as the addition of H+.Predict the result of the reaction between AlCl3 and NH3 in a solvent that does not participate as a reactant.
Classify each of the following as either a Lewis acid or a Lewis base:H+ B(OH)3Cl-P(CH3)3
Identify the Lewis acid and Lewis base in each of the following reactions. HNO2(aq) + OH−(aq) ⇌ NO2−(aq) + H2O(l) SO2(g) + H2O(l) ⇌ H2SO3(aq)
Which of the following is a Lewis acid?A) BBr3B) CCl4C) NH3D) CHBr3E) None of the above are Lewis acids.
Which of the following could act as a Lewis base?a. H2Ob. PCl3c. Lu3+d. FeCl3e. Both (a) and (b) are Lewis bases
For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.Al3+ + 6H2O ⇌ Al(OH)3 + 3H3O+a) Al3+ is the Lewis acid.b) Al3+ is the Lewis base.c) H2O is the Lewis acid.d) H2O is the Lewis base.e) This is not acid-base reaction.
Is SiF4 a lewis base or lewis acid?
Classify each of the following as a Lewis acid or a Lewis base: B(OH) 3OF2Cl-SO3 Na+
Classify each of the following as a Lewis acid or a Lewis base.
Classify each of the following as a Lewis acid or a Lewis base. 
Classify each of the following as either a Lewis acid or a Lewis base.?1. B(OH)32. P(CH3)33. BeCl24. CN-
Of the following list of molecules and ions, which can behave as Lewis bases: CO3-2, NH4+, Na+, OH-?  (i) NH4+ only (ii) NH4+ and OH- (iii) CO3-2 and Na+ (iv) CO3-2 and OH- (v) OH- only
Why is BCl3 a Lewis acid?
Classify each of the following as a Lewis acid or a Lewis base. 
Classify each of the following as a Lewis acid or a Lewis base.BeCl2, NH3, O2-, SO3, Fe2+