User Interface

Acid-Base
Pond Chemistry
Select an acid. This presets Worksheet values to a particular example, including Ka for this reaction.

CH3CO2H ⇋ CH3CO2- + H+

The current reaction—in this case, the equilibrium between acetic acid and its ions.
Start over. Clear all values to 0, except for Ka. To reset Ka to its original value, select another Scenario, then select the current one again.
[CH3CO2H] Concentration of acid or ion to add to the pond, in moles per liter.

Enter (or edit) the value in the "Initial" column (left input).

To specify scientific notation values, use for example “1e-9” for 1.0 x 10-9.

There's also an input field in the “Final” column (right input). This right hand value shows the resulting acid or ion concentration after the system runs to equilibrium.

Concentration is unlocked. When unlocked, some of this will be consumed or released in acid dissociation. But in certain cases this may not be true, because concentrations are controlled by other equilibria. In those cases, you'd enter the fixed concentration value (e.g., based on the water's pH), and lock the ion concentration.

Click the button to lock acid or ion concentration.

In the "Carbonic Acid" Scenario in this lab, [H+] is always locked, and you can optionally also lock [H2CO3].

Ion concentration is locked. Final concentration not allowed to differ from input value.

Click the button to unlock ion concentration.

In the "Carbonic Acid" Scenario in this lab, [H+] is always locked, and you can optionally also lock [H2CO3].

Run Button. Add the inputs to the pond, and run the system to equilibrium.

This means that the initial acid and ion concentrations in the pond are set as per the inputs. Then the system is solved for Q = Ka.

Shows an animation on the pond picture. After the animation, final values are shown on the Worksheet.

pH The pH of water is defined as pH = - log( [H+] ). Neutral pH is 7.0 at [H+] = 1e-7, with lower values acidic, and higher values basic.
Ka The dissociation constant of the current acid, at around 25°C. There are two reactions, and two Ka values, for Carbonic Acid.
Q Q is the extent to which the acid has dissociated. There are two reactions, and two Q values, for Carbonic Acid.

In the generic reaction HA ⇋ A- + H+, with [HA] meaning the concentration (in M) of acid HA,

Q = [A-] [H+] / [HA]

Show more statistics.
Hide the extra statistics.
A ratio, calculated as the label shows. Divide by zero is displayed as infinity.
αa

αa = [HA] / ( [HA] + [A-] )

Pond Animation The pond animation goes through several stages. There is no user interaction. User input is provided on the Worksheet.

First, 15 labels are assigned to acids and ions, weighted by the relative amounts of the inputs. These labels are splashed into the pond.

Smaller labels represent quantities that are present, but not in large enough quantities to be shown in proportion to the bigger labels.

Some acids may dissociate into ions. Or ions meet up with each other and form acids, if the calculation says so.

Extra acid and ions may appear or disappear as needed to keep the visible quantities more or less proportional to the calculated quantities. This is particularly likely when using a locked [H+] or acid, since locking implies the component is being taken or replenished by other processes.


by Ginger Booth for Shimon Anisfeld, Copyright © 2014 Yale University