What's the difference between a strong and weak acid?

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The difference between strong and weak acids is that strong acids are likely to fully ionize whereas weak acids are not. Although a common misconception, when used in regards to the strength of an acid, the terms “strong” and “weak” do not refer to the terms “concentrated” and “diluted."  

Acids are compounds that donate hydrogen ions. The amount of ionization of an acid has to do with the stability of an acidic compound and its likelihood to dissociate. The amount of dissociation of protons within an acid has to do with the polarity of the covalent bond that connects hydrogen to another atom (the H-A bond). Polarity has to do with the electronegativity differences between the atoms that share a covalent bond. The more electronegative the conjugate base (A-) within an acid, the more polar the H-A bond, the more likely the hydrogen is to dissociate, and the stronger the acid.                                

Because strong acids are more likely to ionize and dissociate, their dissociation reactions are considered to be irreversible. 

Stronger acids have larger acid dissociation constants (Ka) than weak acids.

All acids have a pH between one and seven. However, when of equal concentrations, strong acids have lower pH’s than weak acids.

Examples of strong acids are hydrochloric acid, sulfuric acid, and hydrobromic acid.

Examples of weak acids are acetic acid and formic acid.

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What is the difference between a strong acid and a weak acid?

A strong acid is one that completely ionizes in solution to produce hydrogen ions and weak acid is one that shows less than 100% ionization in solution.

The strength of an acid isn't the same thing as its concentration. A strong acid can be either concentrated or dilute, but a concentrated strong acid will have more hydrogen ions than a weak acid of the same volume and concentration.

The strong acids are:

hydrochloric acid, `HCl`

hydroiodic acid, `HI`

hydrobromic acid, `HBr`

nitric acid, `HNO_3`

sulfuric acid, `H_2SO_4`

perchloric acid, `HClO_4`

Some common weak acids are:

acetic acid, `HC_2H_3O_2`

carbonic acid,` H_2CO_3`

phosphoric acid, `H_3PO_4`

nitrous acid, `HNO_2`

When these weak acids dissolve in water most of the acid molecules stay intact and only a small percentage ionize.

When an acid ionizes in water it produces the hydronium ion, `H_3O^+` , which is also represented as the hydrogen ion `H+` . Here's an example of ionization of HCl shown both ways: 

`HCl + H_2O -gt H_3O^+ + Cl^-`

`HCl -gt H^+ + Cl^-`

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What is strong acid and weak acid?

The pH scale is used to measure how acidic or basic a substance is.  The scale goes from 0 to 14.  A substance with a pH of 0 through 6 is considered an acid, while a substance with a pH of 8 through 14 is considered a base.  A pH of 7 is neutral.  Uncontaminated, pure water is neutral.  The closer the number is to 7, the weaker the substance is.  The farther the number is from 7, the stronger the substance is. 

A substance with a pH of 1 or 2 would be a strong acid.  A substance with a pH of 5 or 6 would be a weak acid.

A substance with a pH of 13 or 15 would be a strong base.  A substance with a pH of 8 or 9 would be a weak base.

An example of a strong acid is vinegar.  The pH of vinegar is 2.4.  An example of a weak acid is citric acid.  The pH of citric acid is 4.8.

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What is the difference between weak and strong acids?

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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