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Lesson: Classifying Acids and Bases

Worksheet • 5 Questions

Q1:

Out of P H 3 and N H 3 , which is the stronger acid and why?

  • A P H 3 , phosphorus is larger than nitrogen, with a lower electron density, making it more electrophilic and therefore more acidic.
  • B P H 3 , in binary compounds of hydrogen with nonmetals, the acidity increases down the group.
  • C N H 3 , ammonia is more basic than P H 3 and will therefore be more acidic as well.

Q2:

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

  • AAnion effect
  • BCation effect
  • CHydroxide effect
  • DHydrogen effect

Q3:

Out of H S 2 and H B r , which is the stronger acid and why?

  • A H S 2 , hydrogen sulfide has more hydrogens than hydrogen bromide, so it is more inclined to donate them.
  • B H B r , for binary compounds of hydrogen with nonmetals, in a period, the acidity increases from left to right, and, in a group, it increases top to bottom-bromine is to the right and below sulfur.
  • C H S 2 , sulfur is less electronegative than bromine, so it holds onto the hydrogens less strongly.

Q4:

Of the protonated molecules N H 3 and H O 2 , which is the stronger acid and why?

  • A H O 2 is the stronger acid because oxygen is more electronegative than nitrogen and better able to stabilize the negative charge after dissociation.
  • B H O 2 is the stronger acid because it contains a more polarized arrangement of hydrogen atoms, producing a larger dipole moment that aids dissociation.
  • C N H 3 is the stronger acid because it contains a larger number of hydrogen atoms that can be donated to a base.
  • D H O 2 is the stronger acid because oxygen is more electronegative than nitrogen, so forms more polar bonds that are broken more easily during dissociation.
  • E N H 3 is the stronger acid because nitrogen is less electronegative than oxygen, so forms weaker bonds that are broken more easily during dissociation.

Q5:

Guanidine, a strong Brønsted-Lowry base, reacts with a proton to form the guanidinium cation:

N H H C N N H H H N H H C N + N H H H H
A Brønsted-Lowry base is strong if the conjugate acid is highly stable. Why is guanidine a strong Brønsted-Lowry base?

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