We need to know the acidity characteristics of some representative organic/inorganic molecules. Please learn/memorize the following structures and their pKa’s…
There is an extensive discussion of acids and bases and trends in acidity with structure in the regular chem 231 tutorial section named
2_acids_bases_Ka_delta_G_trends
Acid-Base Problems
Write the Ka expression and reaction with water for the following acids:
1) H2S 2) NH4+
3) H3PO4 (only the first deprotonation) 4) NH3
Kb expressions are the base forms of Ka expressions, where a base deprotonates water to make hydroxide. Write the Kb expressions for the following bases:
5) NH3 6) F
For the pairs of molecules below, predict which is the stronger acid, and justify your choice with an explanation.
9) 10) H3PO4 H2PO4
11) 12) CH4 HC(CN)3
13) 14)
15) 16)
For questions 17 through 20, you’ll need to know the following information… write the reaction and the expression for the equilibrium constant and determine the equilibrium constant.
17) The reaction of (phenoxide) with H2S.
18) The reaction of fluoride (F) with (acetic acid).
19) The reaction of NH4+ (ammonium) with .
20) The reaction of HS with HF.
21) This question requires some thought. Shown below is imidazole:
Under acidic conditions, imidazole undergoes protonation to the conjugate acid. Determine which nitrogen receives the H+. Give the structures of the two conjugate acids and determine which is the most stable.
Answers
1) H2S + H2O HS + H3O+
2) NH4+ + H2O NH3 + H3O+
3) H3PO4 + H2O H2PO4 + H3O+
4) NH3 + H2O NH2 + H3O+
5) NH3 + H2O NH4+ + HO
6) F + H2O HF + HO
9) Now, we can’t use atom electronegativity since the charge is on carbon after deprotonation in both cases; also, there is no resonance in either case. Ethylene, the molecule on the right, will have a lone pair in an sp2 orbital; acetylene, the molecule on the right, will have a lone pair in an sp orbital. An sp orbital is smaller (50% s for sp versus 33% s for sp2) so the electrons are more stabilized since they are closer to the nucleus. So acetylene (pKa = ~25) will be more acidic than ethylene (pKa = ~44).
In general, negative charges are better stabilized in orbitals that have more s character (meaning a higher % of s in the hybrid orbital).
10) Intuitively, it will be easier to deprotonate a neutral molecule compared to an anion, if the structures are similar. Here they are since the anion on the right is the conjugate base of the molecule on the left, so the molecule on the left is deprotonated once to give the anion on the right. Indeed, H3PO4 has a pKa of 2.2 and H2PO4 has a pKa of 7.2.
11) In both cases, the negative charge is located on an O after deprotonation, and we can stabilize that negative charge via resonance. There are three possible resonance forms in the anion formed from deprotonation of HNO3, and two possible resonance forms in the anion formed from HONO. So HNO3 (pKa = -1.4) is a stronger acid than HONO (pKa = 3.3).
So the more we can delocalize a negative charge via resonance (i.e. more resonance stcuctures), the more stable the anion is.
12) The negative charge on methane is stuck after the deprotonation. :CH3 has a negative charge localized on the C with no possibility for resonance. Deprotonating HC(CN)3 gives
. There are two additional resonance structures not shown that put the negative charge on the other two N atoms. As a result, HC(CN)3 has a pKa of 1.0 while methane has a pKa of ~49.
13) The nitro group (NO2) has a plus charge on the N adjacent to the ring, so it’s logical to assume it will pull electron density from the ring after deprotonation, making the least acidic molecule the regular (non-nitro phenol). Between the other molecules, we need to deprotonate and see if we can use the NO2 group to stabilize the positive charge. This can
Period 1 Experiment #23 May 6, 2014 Properties of Acids and Bases Objectives: ● To investigate the properties of acids and bases ● To learn the names and formulas of common laboratory acids and bases Procedure: The procedure followed in this lab is as outlined on the first page of the provided lab handout, “Properties of Acids and Bases.” Data: Table I: Properties of Common Laboratory Acids Name Hydrochloric Formula Effect on Blue Litmus HCl Acid Turned pink Effect on…
According to Robert Boyle, all acids have a sour taste and are corrosive. Acids change the color of certain vegetable dyes, such as Litmus, from blue to red. Acids lose their acidity when they are combined with alkalies. Boyle also stated that alkalies feel slippery, they change the color of Litmus from red to blue. Alkalies become less alkaline when they are combined with acids. Boyle defined Alkalies as substances that consume or neutralize acids. Acids lose their sour taste and ability…
Common Foods and the pH Scale Background: Acids and Bases The Bronsted-Lowry definition of acids is that acids are compounds that give off H+ ions when they react with another compound. Likewise, this definition says that bases are compounds that accept H+ ions from other compounds. The Arrhenius definition of acids says that they’re compounds that give off H+ ions in water and that bases are compounds that give off OH- ions in water. These definitions are the same. Basically…
by what we have already done. --Longfellow Acid-base indicators are generally weak acids (hence we typically add them in small amounts) which are distinguished by the fact that the molecular form of the acid is a different color than the conjugate base. For the hypothetical indicator HIn we might write: HIn + H2O ' H3O+ + Incolor 1 color 2 The Common Ion Effect and Buffer Solutions From our discussion in class you are already aware that acids and bases can be divided into two large groups: weak…
made up of 1) Pentose Sugar 2) Nitrogenous Base 3) Phosphate GROUP – The bases may be purines (2 rings; adenine and guanine) or pyrimidines (1 ring; cytosine and thymine or uracil) – Bases bond using hydrogen bonds – The sugar-phosphate backbone may be generalized by 5' (five prime) and 3' (three prime); ensures nucleotides are connected and read in the correct way DNA – Controls cellular activities; carries a genetic code – encoded in sequence of bases strung together – instructions used in building…
electrons (-) found in the electron cloud Atomic number (protons) vs. Atomic mass (protons + neutrons) and radioactive isotopes (differ in number of neutrons, ex. Carbon 14) CARBON-12 CARBON-13 CARBON-14 PROTONS 6 6 6 NEUTRONS 6 7 8 ELECTRONS 6 6 6 ATOMIC MASS 12 13 14 Energy shells: valence shell. Chemical reactivity. Electronegativity of elements Atoms attraction for shared electrons Ionic vs. covalent bonds. (Na+Cl-) and (C2H6) Ionic bonds –…
4 5 6 7 8 9 10 11 12 13 14 [H3O+] 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 0.1 0.01 0.001 0.0001 0.00001 etc Calculating the pH of Strong acids {Calculate the pH of strong acids given hydrogen ion concentration.} When a strong acid, such as hydrochloric acid is added to water, it fully ionises, leaving no HCl molecules in the solution. Thus the concentration of hydronium ions in the solution will be equal to the initial hydrochloric acid concentration…
substrate, forming an enzyme-substrate complex. This interaction strains chemical bonds in the substrate in a way that makes a particular chemical reaction more likely to occur. When the reaction occurs, the enzyme is released in its original form. 6. What are coenzymes and cofactors? Why are they important? Coenzymes are small organic molecules which are cofactors which help the active site attain its appropriate shape or helps bind the enzyme to its substrate. Other cofactors are ions such as…
Chapter 4 Reactions in Aqueous Solutions Summary of the Chapter • General properties of aqueous solutions • Precipitation reactions: soluble reactants could yield an insoluble product • Acid, bases and neutralization reactions: reactions in which H+ are transferred from one reactant to another • Concentrations in solutions: Molarity • Solution stoichiometry and chemical analysis: calculation of amounts and concentrations General Properties of Solutions • Solutions are defined as homogeneous…