5 Which of the following are Arrhenius acids? 18.6 Which of the following are Arrhenius bases?

Here are the answers to your chemistry questions:

18.5 Which of the following are Arrhenius acids?
An Arrhenius acid is a substance that produces $H^+$ ions (or $H_3O^+$ ions) when dissolved in water.

• a) $H_2O$: Water can act as an Arrhenius acid because it can dissociate to produce $H^+$ ions ($H_2O \rightleftharpoons H^+ + OH^-$).
• b) $Ca(OH)_2$: Calcium hydroxide is an Arrhenius base, as it produces $OH^-$ ions.
• c) $H_3PO_3$: Phosphorous acid is an Arrhenius acid, as it produces $H^+$ ions ($H_3PO_3 \rightleftharpoons H^+ + H_2PO_3^-$).
• d) $HI$: Hydroiodic acid is an Arrhenius acid, as it produces $H^+$ ions ($HI \to H^+ + I^-$).

The Arrhenius acids are: $\boxed{\text{a) H}_2\text{O, c) H}_3\text{PO}_3\text{, d) HI}}$

18.6 Which of the following are Arrhenius bases?
An Arrhenius base is a substance that produces $OH^-$ ions when dissolved in water.

• a) $CH_3COOH$: Acetic acid is an Arrhenius acid, as it produces $H^+$ ions.
• b) $HOH$ ($H_2O$): Water can act as an Arrhenius base because it can dissociate to produce $OH^-$ ions ($H_2O \rightleftharpoons H^+ + OH^-$).
• c) $CH_3OH$: Methanol is an alcohol and does not dissociate to produce $OH^-$ ions in solution.
• d) $H_2NNH_2$: Hydrazine is a Brønsted-Lowry base (it accepts a proton from water to produce $OH^-$), but it does not contain $OH$ groups to dissociate, so it is not strictly an Arrhenius base.

The Arrhenius base is: $\boxed{\text{b) HOH}}$

18.7 Write the $K_a$ expression for each of the following in water:
The general acid dissociation constant ($K_a$) expression for an acid $HA$ reacting with water is:
$HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$
$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

• a) $HNO_2$ (Nitrous acid)

  $$HNO_2(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + NO_2^-(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][NO_2^-]}{[HNO_2]}}$$

• b) $CH_3COOH$ (Acetic acid)

  $$CH_3COOH(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + CH_3COO^-(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][CH_3COO^-]}{[CH_3COOH]}}$$

• c) $HBrO_2$ (Bromous acid)

  $$HBrO_2(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + BrO_2^-(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][BrO_2^-]}{[HBrO_2]}}$$