Acids And Bases: A Particle View

Some water based solutions sting on a cut, fizz with metal, or turn indicator paper a new color. Those observations happen because dissolved particles change. In particular, acids and bases change the concentration of hydronium and hydroxide ions in water. pH is a logarithmic measure of hydronium concentration, so acidity is ultimately a concentration story about $[H_3O^+]$.

You can often connect a macroscopic sign to a particle level cause.

• Indicator color changes because indicator molecules gain or lose a proton, changing their electron distribution and light absorption.
• Conductivity rises when more ions are present, since moving ions carry charge through the solution.
• Reactivity with some metals occurs when $H_3O^+$ is available to be reduced to $H_2(g)$ while the metal atoms are oxidized to metal ions.

Safety boundary
Taste and direct skin testing are not valid identification tools for acids or bases because corrosive solutions can injure tissue without warning.

Proton transfer in water

In the Brønsted-Lowry model, an acid donates a proton $H^+$ and a base accepts a proton. A proton transfer is not a free proton floating around for long in water. The proton is transferred between molecules, and in water it is carried as hydronium, $H_3O^+$.

A useful first particle picture is to treat water as an active participant. When an acid molecule $HA$ enters water, a water molecule can accept its proton to form $H_3O^+$ and the conjugate base $A^-$. The balanced particle level equation is:

$$HA(aq)+H_2O(l)\rightleftharpoons H_3O^+(aq)+A^-(aq)$$

Atom and charge tally confirms conservation.

• Left atoms: $H$ is $1+2=3$, $A$ is 1, $O$ is 1
• Right atoms: $H$ is 3, $A$ is 1, $O$ is 1
• Left charge: 0
• Right charge: $(+1)+(-1)=0$

Similarly, a base $B$ can accept a proton from water, producing $BH^+$ and $OH^-$. The key idea is that acids and bases are defined by what they do in a proton transfer, and water is often the molecule that receives or supplies that proton.

Visualize the particle model of proton transfer in water.

Three definitions and why they matter

Different definitions highlight different evidence and different kinds of reactions. Arrhenius acid increases $[H_3O^+]$ in water and an Arrhenius base increases $[OH^-]$ in water. This works well for many aqueous cases like $HCl(aq)$ and $NaOH(aq)$, but it misses bases that do not contain $OH^-$.

Brønsted-Lowry is broader because it focuses on proton transfer, even when $OH^-$ is not present in the base formula. For example, $NH_3$ is a Brønsted-Lowry base because it accepts a proton from water.

Lewis acid accepts an electron pair and a Lewis base donates an electron pair, so it extends beyond proton transfer. A molecule like $BF_3$ is a Lewis acid because boron has an empty orbital that can accept a lone pair, even though $BF_3$ contains no hydrogen to donate.

Reveal how several familiar substances fit one definition but not another.