Acids and Bases
In the most generic definition, an acid can be defined as a substance that donates a proton, and a base as a substance that accepts a proton. When I refer to proton, I mean a hydrogen (H+) ion. In the acid HCl, or Hydrochloric Acid, the proton that is donated is the H. As the hydrogen is only an ion when the HCl is dissolved in water, all acids must be dissolved in order to be active and have the properties of acids.
When a base such as NaOH, or Sodium Hydroxide, dissolves in water, it forms a NA+ ion and an OH- ion. When this base is added to a solution of dissolved Hydrochloric Acid which contains H+ ions and Cl- ions, the H+ and OH- combine to form water, thus neutralizing the acid and base, and the Na+ and Cl- act as spectator ions until the solution is evaporated, at which point NaCl will be formed.
The overal net ionic equation for this reaction is shown below:
When a base such as NaOH, or Sodium Hydroxide, dissolves in water, it forms a NA+ ion and an OH- ion. When this base is added to a solution of dissolved Hydrochloric Acid which contains H+ ions and Cl- ions, the H+ and OH- combine to form water, thus neutralizing the acid and base, and the Na+ and Cl- act as spectator ions until the solution is evaporated, at which point NaCl will be formed.
The overal net ionic equation for this reaction is shown below:
This reaction is called a neutralization or titration. An acid and a base react to form water. First, the hydrogen ion from the HCl combines with a water molecule to form the Hydronium ion, H30. Then, the hydronium combines with an OH- ion, or Hydroxide, to form 2 water molecules. As I mentioned before, the Cl- and Na+ ions are spectators and do not play a part in the neutralization.
Another important topic when referring to acids and bases is pH. pH is the concentration of H+ ions in a solution, measured in molarity. To calculate pH, use the following formula:
Another important topic when referring to acids and bases is pH. pH is the concentration of H+ ions in a solution, measured in molarity. To calculate pH, use the following formula:
So to calculate the pH of a solution, take the negative log of the concentration of hydronium ions in molarity. So if the concentration of hydronium was .36 M, the pH of the solution would be about .44. This indicates a strongly acidic solution, as a pH of 7.0 is completely neutral (aka water).
Another measurement is pOH, which you may guess is the concentration of hydroxide ions (OH-) in a solution. To find this, either subtract the pH from 14 or use the pH equation, just substituting [H3O+] with [OH-]. Oh, by the way, [a] means "concentration of a."
Also, to calculate the concentration of hydronium based upon the given pH, use the following formula:
Another measurement is pOH, which you may guess is the concentration of hydroxide ions (OH-) in a solution. To find this, either subtract the pH from 14 or use the pH equation, just substituting [H3O+] with [OH-]. Oh, by the way, [a] means "concentration of a."
Also, to calculate the concentration of hydronium based upon the given pH, use the following formula:
Weak and Strong Acids
Acids are either classified as strong or weak, depending on how they behave when dissociating in water. Strong acids almost totally donate their hydrogens to form hydronium ions. An example of this is HCl. Hydrochloric acid donates almost all of its protons to form hydronium.
Weak acids only give part of their total H+ ions to form H3O. For example, acetic acid (HC2H3O2) already has its available hydrogen bonded to an oxygen molecule. Because of this, its tendency to release it to bond to another oxygen molecule from water is much less than that of the H in HCl.
Weak acids only give part of their total H+ ions to form H3O. For example, acetic acid (HC2H3O2) already has its available hydrogen bonded to an oxygen molecule. Because of this, its tendency to release it to bond to another oxygen molecule from water is much less than that of the H in HCl.
Chemical Equilibrium
For weak acids, protons will only be donated to a point where the attraction of the water and the attraction of the acetate ions are equal. So the amount of acetic acid will go down until they reach equivalence. If more water is added to the solution, the equilibrium will shift to the side that doesn't have the water. In this case, more hydronium will be formed and less acetic acid will remain.
Chemical Equilibrium also takes effect in all kinds of multi-directional chemical reactions, not just acid-base neutralizations.
Chemical Equilibrium also takes effect in all kinds of multi-directional chemical reactions, not just acid-base neutralizations.
Practice Problems
- What would the pH of a .13 M HCl solution be?
- What would the concentration of hydronium be for a solution with a pH of 4?