-A Project by Afrah Aamer
Contents What is an Acid?............................................................................................................................................ 1 Arrhenius acids.......................................................................................................................................... 1 Brønsted-Lowry acids................................................................................................................................ 2 Lewis acids ................................................................................................................................................ 3 Acid Properties .............................................................................................................................................. 3 The pH concentration ............................................................................................................................... 3 Litmus Paper ............................................................................................................................................. 3 Sour Taste ................................................................................................................................................. 4 Acid-Base Reactions .................................................................................................................................. 4 Organic and Inorganic Acids ..................................................................................................................... 4 Acid Strength................................................................................................................................................. 4 Strong Acids .............................................................................................................................................. 4 Common strong acids ............................................................................................................................... 5 Almost strong acids ................................................................................................................................... 5 Extremely strong acids .............................................................................................................................. 5 Weak Acids ................................................................................................................................................ 5 Uses of Acids ................................................................................................................................................. 6 References .................................................................................................................................................... 7
What is an Acid? According to the IUPAC, an acid is defined as: A molecular entity or chemical species capable of donating a hydron or capable of forming a covalent bond with an electron pair. An acid is any compound that yields hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water. When dissolved in water, acids donate hydrogen ions (H+). Hydrogen ions are hydrogen atoms that have lost an electron and now have just a proton, giving them a positive electrical charge.
Arrhenius acids The Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions (H+) or protons in 1884. An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. The chemists often write H+ (aq) and refer to the hydrogen ion when describing acid-base reactions but the free hydrogen nucleus, a proton, does not exist alone in water, it exists as the hydronium ion, H3O+. Thus, an Arrhenius acid can also be described as a substance that increases the concentration of hydronium ions when added to water. This definition stems from the equilibrium dissociation of water into hydronium and hydroxide (OH−) ions: H2O (l) + H2O (l)
H3O+ (aq) + OH− (aq)
In pure water the majority of molecules are H2O, but the molecules are constantly dissociating and re-associating, and at any time a small number of the molecules (always near 1 in 107) are hydronium and an equal number are hydroxide. Because the numbers are equal, pure water is neutral (not acidic or basic). An Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water, hence decreasing the concentration of hydronium. The constant association and disassociation of H2O molecules forms an equilibrium in which any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide, thus an Arrhenius acid could also be said to be one that decreases hydroxide concentration, with an Arrhenius base increasing it. The reason that pHs of acids are less than 7 is that the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less than 7. Figure 1 Svante Arrhenius
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Brønsted-Lowry acids Acetic acid, CH3COOH, is composed of a methyl group, CH3, bound chemically to a carboxylate group, COOH. The carboxylate group can lose a proton and donate it to a water molecule, H2O, leaving behind an acetate anion CH3COO- and creating a hydronium cation H3O. This is an equilibrium reaction, so the reverse process can also take place. Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion. Red: oxygen, black: carbon, white: hydrogen. While the Arrhenius concept is useful for describing many reactions, it is also quite limited in its scope. In 1923 chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently recognized that acid-base reactions involve the transfer of a proton. A Brønsted-Lowry acid (or simply Brønsted acid) is a species that donates a proton to a Brønsted-Lowry base. Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. CH3COOH + H2O
CH3COO−+ H3O+
CH3COOH + NH3
CH3COO−+ NH+4 Both theories easily describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia (NH3), but cannot be described using the Arrhenius definition of an acid because the reaction does not produce hydronium.
Brønsted-Lowry theory can also be used to describe molecular compounds, whereas Arrhenius acids must be ionic compounds. Hydrogen chloride (HCl) and ammonia combine under several different conditions to form ammonium chloride, NH4Cl. In aqueous solution HCl behaves as hydrochloric acid and exists as hydronium and chloride ions. The following reactions illustrate the limitations of Arrhenius's definition: Figure 2 Acetic Acid
H3O+ (aq) + Cl− (aq) + NH3 → Cl− (aq) + NH4+ (aq) + H2O HCl (benzene) + NH3 (benzene) → NH4Cl(s) HCl (g) + NH3 (g) → NH4Cl(s) As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute. The next two reactions do not involve the formation of ions but are still proton transfer reactions. In the second reaction hydrogen chloride
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and ammonia (dissolved in benzene) react to form solid ammonium chloride in a benzene solvent and in the third gaseous HCl and NH3 combine to form the solid.
Lewis acids A third concept was proposed in 1923 by Gilbert N. Lewis which includes reactions with acid-base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor. Brønsted acidbase reactions are proton transfer reactions while Lewis acid-base reactions are electron pair transfers. All Brønsted acids are also Lewis acids, but not all Lewis acids are Brønsted acids.
Acid Properties The pH concentration The pH scale is used to measure the amount of H+ ions in a solution. The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution. If [H+] is greater than 1 x 10-7 M it is an acid. 1
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Neutral Figure 3 pH scale
Hydrochloric acid, nitric acid and sulfuric acid are strong acids. Acids have a pH below 7; bases have a pH above. Strong acids have the lowest pH levels (0-4) and strong bases have the highest pH levels (10-14). Neutral solutions have a pH of 7 and they are neither acidic nor basic. Distilled water is neutral, because the H+ and OH- ions are balanced. Acids when dissolved in water, are below seven on the pH scale. Acetic acid is a weak acid. Acids when dissolved in water, conducts electricity.
Litmus Paper Acids changes blue litmus to red Litmus is a natural acid-base indicator extracted from a type of lichen. Blue litmus paper turns red when a solution is acidic. Red litmus paper turns blue at a basic pH of about 5. Human blood has an ideal pH of 7.4; even slight fluctuations can seriously affect our bodies. It is possible to tell whether a solution is acidic by testing it with blue litmus paper. Litmus is one of the substances called indicators. Any solution which changes the color of the litmus from blue to red is sure to contain some kind of acid. Red litmus paper (right) is unaffected by acids
Figure 4 Blue Litmus turns red when placed in acid
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Sour Taste One of the characteristics of acids is that they taste sour. Acids when dissolved in water, have a sour taste. Acids when dissolved in water, react with bases to neutralize their properties. Acids when dissolved in water, react with active metals to liberate hydrogen. Acids are a group of chemicals that are known for their sour taste. Some of the acids are very dangerous. Sulphuric acid, for example, is highly corrosive - it can eat away most metals. And it will quickly burn into flesh and other living matter. Nitric acid and hydrochloric acid are two other acids that will attack most metals. These three strong acids are among the most important of all the chemicals used in industry - sulphuric acid especially.
Acid-Base Reactions Acids, bases and salts form three of the most important classes of chemical compounds. They are connected in the following way. An acid reacts with a base to give a salt and water. This is one of the fundamental rules in chemistry. Hydrochloric acid, for example, combines with sodium hydroxide (base) to form sodium chloride (salt) and water. Sulphuric acid combines with potassium hydroxide (base) to form potassium sulphate (salt) and water. In a similar way nitric acid reacts to produce a nitrate. One way acids and bases differ is in their action towards a litmus solution. Litmus is one of the substances called indicators, which have one color in an acid solution and another color in an alkaline solution. In an acid solution, litmus is red; in an alkaline solution it is blue.
Organic and Inorganic Acids The acids mentioned so far are inorganic compounds. There are also organic. In general, organic acids are very much weaker than inorganic acids, and they are also very much more common. For example, vinegar is a weak solution of an organic acid called acetic acid. The sharp taste in citrus fruits such as lemons is caused by citric acid. When an ant stings us, it injects formic acid into the skin, which causes pain. Lactic acid causes the sourness when milk goes bad. Proteins, which living things need to build up tissue, are made up of amino acids.
Acid Strength The strength of an acid refers to its ability or tendency to lose a proton (H+).
Strong Acids A strong acid is one that completely ionizes (dissociates) in a solution. Examples of strong acids are:
Hydrochloric acid (HCl) Hydroiodic acid (HI) Hydrobromic acid (HBr) Perchloric acid (HClO4) Nitric acid (HNO3) Sulfuric acid (H2SO4)
Figure 5 Sulphuric Acid
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In aqueous solution each of these essentially ionizes 100%.A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton. For sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first proton: H2SO4 (aq) → H+ (aq) + HSO4− (aq)
Common strong acids
Hydroiodic acid (HI) Hydrobromic acid (HBr) Perchloric acid (HClO4) Hydrochloric acid (HCl) Sulfuric acid (H2SO4) p-Toluenesulfonic acid Methanesulfonic acid
Almost strong acids These do not meet the strict criterion of being more acidic than H3O+, although in very dilute solution they dissociate almost completely, so sometimes they are included as "strong acids". Examples are:
Hydronium ion H3O+ (Hydronium is often used as an approximation of the state of protons in water) Nitric acid HNO3 Chloric acid HClO3
Some chemists include bromic acid (HBrO3) perbromic acid (HBrO4), iodic acid (HIO3, and periodic acid (HIO4) as strong acids, although these are not universally accepted.
Extremely strong acids
Fluoroantimonic acid H[SbF6] Magic acid (FSO3HSbF5) Carborane superacid H(CHB11Cl11) Fluorosulfuric acid (FSO3H) Triflic acid (CF3SO3H)
Weak Acids A weak acid only partially dissociates. At equilibrium both the acid and the conjugate base are present in solution. Most acids are weak acids. A weak acid is an acid that dissociates incompletely, releasing only some of its hydrogen atoms into the solution. Thus, it is less capable than a strong acid of donating protons. These acids have higher pKa than strong acids, which release all of their hydrogen atoms when dissolved in water. Examples of weak acids include acetic acid (CH3COOH), oxalic acid (H2C2O4) and also carbonic acid (H2CO3).
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Uses of Acids There are numerous uses for acids:
Acids are often used to remove rust and other corrosion from metals in a process known as pickling. They may be used as an electrolyte in a wet cell battery, such as sulfuric acid in a car battery. Strong acids, sulfuric acid in particular, are widely used in mineral processing. For example, phosphate minerals react with sulfuric acid to produce phosphoric acid for the production of phosphate fertilizers, and zinc is produced by Figure 6 Use of Sulphuric Acid in Car Battery dissolving zinc oxide into sulfuric acid, purifying the solution and electrowinning. In the chemical industry, acids react in neutralization reactions to produce salts. For example, nitric acid reacts with ammonia to produce ammonium nitrate, a fertilizer. Additionally, carboxylic acids can be esterified with alcohols, to produce esters. Acids are used as additives to drinks and foods, as they alter their taste and serve as preservatives. Phosphoric acid, for example, is a component of cola drinks. Acetic acid is used in day-to-day life as vinegar. Carbonic acid is an important part of some cola drinks and soda. Citric acid is used as a preservative in sauces and Figure 7 Citric Acid is present in Lemons pickles. Tartaric acid is an important component of some commonly used foods like unripened mangoes and tamarind. Natural fruits and vegetables also contain acids. Citric acid is present in oranges, lemon and other citrus fruits. Oxalic acid is present in tomatoes, spinach, and especially in carambola and rhubarb; rhubarb leaves and unripe carambolas are toxic because of high concentrations of oxalic acid. Ascorbic acid (Vitamin C) is an essential vitamin for the human body and is present in such foods as amla, lemon, citrus fruits, and guava. Figure 8 Oxalic Acid is Present in Tomatoes
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Certain acids are used as drugs. Acetylsalicylic acid (Aspirin) is used as a pain killer and for bringing down fevers. Acids play important roles in the human body. The hydrochloric acid present in the stomach aids in digestion by breaking down large and complex food molecules. Amino acids are required for synthesis of proteins required for growth and repair of body tissues. Fatty acids are also required for growth and repair of body tissues. Nucleic acids are important for the manufacturing of DNA and RNA and transmitting of traits to offspring through genes. Carbonic acid is important for maintenance of pH equilibrium in the body.
References http://www.wikipedy.com/acids_interesting_facts_about_acids.htm http://en.wikipedia.org/wiki/Equilibrium_chemistry http://www.iupac.org/ http://www.merriam-webster.com/dictionary/acid http://goldbook.iupac.org/A00071.html
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