Acids & Bases
Acids in water solutions exhibit the following common properties: they taste sour; turn litmus paper red; and react with certain metals, such as zinc, to yield hydrogen gas.
Bases in water solutions exhibit these common properties: they taste bitter; turn litmus paper blue; and feel slippery.
When a water solution of acid is mixed with a water solution of base, water and a salt are formed; this process, called neutralization, is complete only if the resulting solution has neither acidic nor basic properties.
Acids and Bases can be classified as organic or inorganic.
Some of the more common organic acids are: citric acid, carbonic acid, hydrogen cyanide, salicylic acid, lactic acid, and tartaric acid.
Some of the common inorganic acids are: hydrogen sulfide, phosphoric acid, hydrogen chloride, and sulfuric acid.
Some examples of organic bases are: pyridine and ethylamine.
Some common inorganic bases are: sodium hydroxide, sodium carbonate, sodium bicarbonate, calcium hydroxide, and calcium carbonate.
Acids, such as hydrochloric acid, and bases, such as potassium hydroxide, that have a great tendency to dissociate in water are completely ionized in solution; they are called strong acids or strong bases.
Strong acids are acids that very nearly completely dissociate when you put them into water. . Strong acids in solution produce a high concentration of hydrogen ions. Most every molecule of the acid HA that’s placed into water breaks up completely to form H+ and A– ions. Some common strong acids include HCl, HBr, HI, HNO3, and HClO4
Acids, such as acetic acid, and bases, such as ammonia, that are reluctant to dissociate in water are only partially ionized in solution; they are called weak acids or weak bases.
Weak acids are acids that stay mostly undissociated in water. As a result, their dissociation is an equilibrium with the following general form:
- HA ⇔ H+ + A–
The equilibrium constant for this expression is given the symbol Ka, which stands for acid-dissociation constant. Some common weak acids include acetic acid and formic acid.
Strong bases in solution produce a high concentration of hydroxide ions and a correspondingly low concentration of hydrogen ions.
The hydrogen ion concentration is often expressed in terms of its negative logarithm, or pH. Strong acids and strong bases make very good electrolytes i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes. (electrolysis, passage of an electric current through a conducting solution or molten salt that is decomposed in the process).
pH: range of numbers expressing the relative acidity or alkalinity of a solution. In general, pH values range from 0 to 14. The pH of a neutral solution, i.e., one which is neither acidic nor alkaline, is 7. Acidic solutions have pH values below 7; alkaline, or basic, solutions have pH values above 7.
A pH value provides a measure of the hydrogen ion concentration of a solution. In pure water the concentration of hydrogen ions is equal to 0.0000001, or 10-7, moles per liter. (A mole is the amount of a substance, expressed in grams, that is equal to the molecular weight, or formula weight, of the substance.)
When an acid is added to pure water, the hydrogen ion concentration increases above this level.
When an alkaline substance, or base, is added to pure water, the hydrogen ion concentration decreases below this level. The equation for finding pH requires that you have the concentration of H+ ions. There are a very small number of H+ ions in basic solutions. These H+ ions are formed when water dissociates in the following way: H2O ↔ H+ + OH– . As a result, whenever water is present (as it must be in an aqueous solution), there will be a few H+ and OH– ions
Once the concentration is determined, the pH value is found by taking the exponent used in expressing this concentration and reversing its sign. This is expressed as pH=-log10 [H+]. For example, if the hydrogen ion concentration of a solution is 10-4, or 0.0001, moles per liter, the pH is 4.
It’s possible to discover the pH of a solution by using compounds known as “indicators.” Indicators are compounds that change colors as the pH of the solution changes. Probably the two most commonly used indicators are litmus (red in acid and blue in base) and phenolphthalein (pronounced “fee-no-thay-leen,” colorless in acid, pink in base).
The electrolytic process requires that an electrolyte, an ionized solution or molten metallic salt, complete an electric circuit between two electrodes. When the electrodes are connected to a source of direct current one, called the cathode, becomes negatively (-) charged while the other, called the anode, becomes positively (+) charged. The positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode, through the wiring and an electric generator, and then back to the cathode constitutes the current in the external circuit.
For example, when electrodes are dipped into a solution of hydrogen chloride (a compound of hydrogen and chlorine) and a current is passed through it, hydrogen gas bubbles off at the cathode and chlorine at the anode. This occurs because hydrogen chloride dissociates into hydrogen ions (hydrogen atoms that have lost an electron) and chloride ions (chlorine atoms that have gained an electron) when dissolved in water. When the electrodes are connected to a source of direct current, the hydrogen ions are attracted to the cathode, where they each gain an electron, becoming hydrogen atoms again. Hydrogen atoms pair off into hydrogen molecules that bubble off as hydrogen gas. Similarly, chlorine ions are attracted to the anode, where they each give up an electron, become chlorine atoms, join in pairs, and bubble off as chlorine gas.
An electric cell is an electrolytic system in which a chemical reaction causes a current to flow in an external circuit; it essentially reverses electrolysis.
Metal corrosion can take place by electrolysis in an unintentionally created electric cell.
a. a compound that reacts with an acid to form a salt, as ammonia, calcium hydroxide, or certain nitrogen-containing organic compounds.
b. the hydroxide of a metal or of an electropositive element or group.
c. a group or molecule that takes up or accepts protons.
d. a molecule or ion containing an atom with a free pair of electrons that can be donated to an acid; an electron-pair donor.
e. any of the purine and pyrimidine compounds found in nucleic acids: the purines adenine and guanine and the pyrimidines cytosine, thymine, and uracil.
1. a. any of various bases, the hydroxides of the alkali metals and of ammonium, that neutralize acids to form salts and turn red litmus paper blue.
b. any of various other more or less active bases, as calcium hydroxide.
c. (not in technical use) an alkali metal. Any of the group of univalent metals including potassium, sodium, lithium, rubidium, cesium, and francium, whose hydroxides are alkalis. The alkaline-earth metals are beryllium, magnesium, calcium, strontium, barium, and radium. The alkaline-earth metals, especially of calcium, strontium, and barium. They are not readily soluble in water and form solutions less basic than those of alkalies.
d. Obs.any of various other compounds, as the carbonates of sodium and potassium.
2. Agric.a soluble mineral salt or a mixture of soluble salts, present in some soils, esp. in arid regions, and detrimental to the growing of most crops.
- Of or like an alkali.
- Containing an alkali.
- Having the properties of an alkali.
- Having a pH value greater than 7.
- A compound usually having a sour taste and capable of neutralizing alkalis and reddening blue litmus paper, containing hydrogen that can be replaced by a metal or an electropositive group to form a salt, or containing an atom that can accept a pair of electrons from a base. Acids are proton donors that yield hydronium ions in water solution, or electron-pair acceptors that combine with electron-pair donors or bases.
- Belonging or pertaining to acids or the anhydrides of acids.
b. having only a part of the hydrogen of an acid replaced by a metal or its equivalent: an acid phosphate.
c. having a pH value of less than 7.
2. sharp or biting to the taste; tasting like vinegar; sour: acid fruits.
3. Geol.containing much silica.
4. Metall.noting, pertaining to, or made by a process in which the lining of the furnace, or the slag that is present, functions as an acid in high-temperature reactions in taking electrons from oxide ions: usually a siliceous material, as sand or ganister. Cf. basic (def. 3).
- A colorless, pungent, suffocating, highly water-soluble, gaseous compound, NH3, usually produced by the direct combination of nitrogen and hydrogen gases: used chiefly for refrigeration and in the manufacture of commercial chemicals and laboratory reagents.
- NH3 a colorless gas that is about one half as dense as air at ordinary temperatures and pressures. It has a characteristic pungent, penetrating odor. Ammonia forms a minute proportion of the atmosphere; it is found in volcanic gases and as a product of decomposition of animal and vegetable matter.
- Also called ammonia solution, ammonia water, aqua ammoniae, aqua ammonia, aqueous ammonia. this gas dissolved in water; ammonium hydroxide.
A basic compound, NH4OH, existing only in solution, formed by dissolving ammonia gas in water.
Any salt containing the NH4+ ion, formed by the neutralization of ammonium hydroxide by an acid.
- CH3CO2H, colorless liquid that has a characteristic pungent odor, boils at 118°C, and is miscible with water in all proportions; it is a weak organic carboxylic acid. Carboxylic acids are compounds whose molecules contain a carboxyl group that is joined to a hydrogen atom by a single bond to its carbon atom (carboxyl group; a functional group that consists of a carbon atom joined to an oxygen atom by a double bond and to a hydroxyl group, OH, by a single bond).
- Glacial acetic acid is concentrated, 99.5% pure acetic acid; it solidifies at about 17°C to a crystalline mass resembling ice.
- Acetic acid is the major acid in vinegar; as such, it is widely used as a food preservative and condiment.
- For industrial use concentrated acetic acid is prepared from the oxidation of acetaldehyde.
- Acetic acid is also a product in the destructive distillation of wood. It reacts with other chemicals to form numerous compounds of commercial importance. These include cellulose acetate, used in making acetate rayon, nonflammable motion-picture film, lacquers, and plastics; various inorganic salts, e.g., lead, potassium, and copper acetates; and amyl, butyl, ethyl, methyl, and propyl acetates, which are used as solvents, chiefly in certain quick-drying lacquers and cements. Amyl acetate is sometimes called banana oil because it has a characteristic banana odor.