Weak Acid

Weak acid A weak acid is an acid that dissociates incompletely and does not release all of its hydrogens in a solution i.e it does not completely donate all of its protons. These acids have higher pKa compared to strong acids, which release all of their hydrogens when dissolved in water. While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid (H(CHB11Cl11), which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve, among other things, glass and all metals except iridium.

Explanat ion Weak acids do not ionize in a solution to a significant extent; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains. Weak acids in water dissociate as

The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (Ka):

The greater the value of Ka, the more the formation of H+ is favored, and the lower the pH of the solution. The Ka of weak acids varies between 1.8×10-16 and 55.5. Acids with a Ka less than 1.8×10-16 are weaker acids than water.

Examples The vast majority of acids are weak acids. Organic acids are a large subset of weak acids. However, there are some mineral acids in this field. •

acetic acid

Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. Pure, water-free acetic acid (glacial acetic acid) is a colourless liquid that absorbs water from the environment (hygroscopy), and freezes at 16.7 °C (62 °F) to a colourless crystalline solid. It is a weak acid, in that it is only partially dissociated acid in aqueous solution. Acetic acid is one of the simplest carboxylic acids. It is an important chemical reagent and industrial chemical, used in the production of polyethylene terephthalate mainly used in soft drink bottles; cellulose acetate, mainly for photographic film; and polyvinyl acetate for wood glue, as well as synthetic fibres and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry acetic acid is used under the food additive code E260 as an acidity regulator. The global demand of acetic acid is around 6.5 million tonnes per year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the remainder is manufactured from petrochemical feedstocks or from biological sources. • • •

citric acid Citric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of almost all living things. It also serves as an environmentally benign cleaning agent and acts as an antioxidant and a lubricant.

Citric acid exists in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (1.44 and 1.38 grams per ounce of the juices, respectively[3]). The concentrations of citric acid in citrus fruits range from .005 mol/L for oranges and grapefruits to .030 mol/L in lemons and limes. These values will vary depending on the circumstances in which the fruit was grown. • • •

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boric acid phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a mineral (inorganic) acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. The term phosphoric acid can also refer to a chemical or reagent consisting of phosphoric acids, usually orthophosphoric acid. hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and dangerous to handle, it is technically a weak acid.[1] Hydrogen fluoride, often in the aqueous form as hydrofluoric acid, is a valued source of fluorine, being the precursor to numerous pharmaceuticals (e.g., Prozac), diverse polymers (e.g., Teflon), and most other synthetic materials that contain fluorine. Hydrofluoric acid is best known to the public for its ability to dissolve glass by reacting with SiO2 (silicon dioxide), the major component of most glass. This property has been known since the 17th century, even before hydrofluoric acid had been prepared in large quantities by Scheele in 1771.[2] This dissolution process can be described as follows: SiO2(s) + 4 HF(aq) → SiF4(g) + 2 H2O(l) SiO2(s) + 6HF(aq) → H2[SiF6](aq) + 2H2O(l)

Because of its high reactivity toward glass, hydrofluoric acid must be stored

(for small quantities) in polyethylene or Teflon containers. It is also unique in its ability to dissolve many metal and semimetal oxides. It is corrosive, as explained below.