Worksheet: Classifying Acids and Bases

In this worksheet, we will practice classifying compounds as Lewis and Brønsted–Lowry acids and bases and predicting their strengths from structural formulas.

Q1:

Out of PH3 and NH3, which is the stronger acid and why?

  • ANH3, because ammonia is more basic than PH3 and will therefore be more acidic as well.
  • BPH3, because in binary compounds of hydrogen with nonmetals, the acidity increases down the group.
  • CPH3, because phosphorus is larger than nitrogen, with a lower electron density, making it more electrophilic and therefore more acidic.

Q2:

Out of HS2 and HBr, which is the stronger acid and why?

  • AHS2 is stronger because hydrogen sulfide has more hydrogens than hydrogen bromide, so it is more inclined to donate them.
  • BHBr is stronger because the acidity of binary compounds of hydrogen with nonmetals increases from left to right in a period, while, in a group, it increases from top to bottom (bromine is to the right and below sulfur).
  • CHS2 is stronger because sulfur is less electronegative than bromine, so it holds onto the hydrogens less strongly.
  • DHS2 is stronger because the acidity of binary compounds of hydrogen with non-metals increases from right to left in a period, while, in a group, it increases from bottom to top (sulfur is to the left and above bromine).
  • EHBr is stronger because bromine is less electronegative than sulfur, so it holds onto the hydrogens more strongly.

Q3:

Guanidine, a strong Brønsted–Lowry base, reacts with a proton to form the guanidinium cation shown.

NHHCNNHHHNHHCN+NHHHH
A Brønsted–Lowry base is strong if the conjugate acid is highly stable. Why is guanidine a strong Brønsted–Lowry base?

  • AThe positive charge of guanidinium ion is stabilized by intramolecular hydrogen bonding.
  • BThe guanidinium ion is stabilized by delocalization of the positive charge over the three nitrogen atoms.
  • CThe additional proton in the guanidinium ion is positioned symmetrically between the nitrogen atoms, forming bonds to all three atoms simultaneously.
  • DThe guanidinium cation is stabilized by tautomerization: the proton is transferred reversibly between the three nitrogen atoms.
  • EAny of the three nitrogen atoms in guanidine can react to form the guanidinium ion.

Q4:

Of the protonated molecules NH3 and HO2, which is the stronger acid and why?

  • AHO2 is the stronger acid because oxygen is more electronegative than nitrogen and better able to stabilize the negative charge after dissociation.
  • BNH3 is the stronger acid because it contains a larger number of hydrogen atoms that can be donated to a base.
  • CHO2 is the stronger acid because oxygen is more electronegative than nitrogen and thus forms more polar bonds that are broken more easily during dissociation.
  • DHO2 is the stronger acid because it contains a more polarized arrangement of hydrogen atoms, producing a larger dipole moment that aids dissociation.
  • ENH3 is the stronger acid because nitrogen is less electronegative than oxygen and thus forms weaker bonds that are broken more easily during dissociation.

Q5:

In general, what is the strongest effect on the acidity of a molecule, even more important than bond strength?

  • AHydroxide effect
  • BHydrogen effect
  • CAnion effect
  • DCation effect

Q6:

Why are amines basic in the Brønsted–Lowry acid–base system?

  • AA proton can bind to one of the attached carbon atoms through the excess electron density in the bonding orbital.
  • BThey can accept lone pairs from electron-rich species.
  • CThey have a conjugated set of orbitals capable of forming dative bonds.
  • DThey have a lone pair of electrons in an sp orbital just sitting there waiting for something to happen, like protonation.

Q7:

Sulfate ions may be produced by the reaction shown of oxide ions with sulfur trioxide. O+SOSO2342

Which species acts as a Lewis base?

  • ASO42
  • BO2
  • CSO3

Which species acts as a Lewis acid?

  • ASO3
  • BSO42
  • CO2

Q8:

B(OH)3 reacts with aqueous base according to the equation shown. B(OH)+OHB(OH)34

Which species acts as a Lewis base?

  • AOH
  • BB(OH)4
  • CB(OH)3

Which species acts as a Lewis acid?

  • AB(OH)4
  • BB(OH)3
  • COH

Q9:

Which of the following is not amphiprotic?

  • AHPO24
  • BHSO4
  • CNH4+
  • DHCO3
  • EHO2

Q10:

Which of the following is not a strong Brønsted–Lowry acid?

  • AHNO3
  • BHBr
  • CHClO4
  • DHSO24
  • EHNO2

Q11:

Which one of the following is amphiprotic?

  • AAsO43
  • BBr
  • CNH4+
  • DCO32
  • EHPO4

Q12:

Aluminum trichloride reacts with chloride ions in solution to form tetrachloroaluminate ions, as shown. AlCl+ClAlCl34

Which species acts as a Lewis base?

  • AAlCl3
  • BCl
  • CAlCl4

Which species acts as a Lewis acid?

  • ACl
  • BAlCl3
  • CAlCl4

Q13:

Which one of the following is capable of acting as a Brønsted–Lowry acid?

  • AS2
  • BCO32
  • CNH3
  • DPO43
  • ECN

Q14:

Fill in the blank: A base is defined by its ability to accept .

  • Aa neutron
  • Ba proton
  • Can electron
  • Da free radical

Q15:

Is an aqueous solution of FeCl3 acidic, basic, or neutral?

  • ABasic
  • BNeutral
  • CAcidic

Q16:

Which one of the following is capable of acting as a Brønsted–Lowry base?

  • ACH4
  • BNH4+
  • CHO2
  • DNe
  • EHO42+

Q17:

Carbon dioxide dissolves in aqueous base to form hydrogen carbonate ions according to the following reaction. CO+OHHCO23

Which species acts as a Lewis base?

  • AOH
  • BCO2
  • CHCO3

Which species acts as a Lewis acid?

  • ACO2
  • BOH
  • CHCO3

Q18:

Triiodide ions are formed by the following reaction of aqueous iodine with iodide ions. I+II23

Which species acts as a Lewis base?

  • AI
  • BI2
  • CI3

Which species acts as a Lewis acid?

  • AI3
  • BI2
  • CI

Q19:

Is an aqueous solution of NHBr4 acidic, basic, or neutral?

  • ABasic
  • BNeutral
  • CAcidic

Q20:

Is an aqueous solution of KClO4 acidic, basic, or neutral?

  • ANeutral
  • BAcidic
  • CBasic

Q21:

Is an aqueous solution of KCO23 acidic, basic, or neutral?

  • ANeutral
  • BBasic
  • CAcidic

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