What is the difference between weak and strong acids?

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The strength of an acid refers to the tendency of a certain acid to lose a proton. One of the main differences between a strong and a weak acid is the degree of dissociation in water. Strong acids completely dissociate in water into its ions; on the other hand, weak acids only undergo partial dissociation, which means that the original acid as well as its conjugate base will be present in solution at equilibrium.

For example: hydrochloric acid (HCl) is a strong acid. This means that in water HCl completely dissociates into H+ and Cl- ions:

`HCl rarr H^+ + Cl^-`

On the other hand, acetic acid is a weak acid. It does not completely dissociate:

`CH_3COOH harr CH_3COO^(-) + H^+`

Notice the difference in arrows used. The dissociation of acetic acid is reversible, and both the acid and its conjugate base are present in solution.

Some of the factors that determine acid strength are the following:

  • Electronegativity. The more electronegative A- is in HA, the stronger the acid (e.g. Cl is electronegative)
  • Atomic Radius. Increasing atomic radius gives a higher tendency to be a strong acid (e.g. HI is a stronger acid than HCl since I has a larger radius).
  • Charge. A more positively charged species is more acidic.
  • Equilibirum. The equilibrium position determines strength. Strong acids have equilibirum position to the far right (towards the products, as they more easily dissociate).
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