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Acids and Bases ACIDS AND BASES

The name "acid" calls to mind vivid sensory images—of tartness, for instance, if the acid in question is meant for human consumption, as with the citric acid in lemons. On the other hand, the thought of laboratory-and industrial-strength substances with scary-sounding names, such as sulfuric acid or hydrofluoric acid, carries with it other ideas—of acids that are capable of destroying materials, including human flesh. The name "base," by contrast, is not widely known in its chemical sense, and even when the older term of "alkali" is used, the sense-impressions produced by the word tend not to be as vivid as those generated by the thought of "acid." In their industrial applications, bases too can be highly powerful. As with acids, they have many household uses, in substances such as baking soda or oven cleaners. From a taste standpoint, (as anyone who has ever brushed his or her teeth with baking soda knows), bases are bitter rather than sour. How do we know when something is an acid or a base? Acid-base indicators, such as litmus paper and other materials for testing pH, offer a means of judging these qualities in various substances. However, there are larger structural definitions of the two concepts, which evolved in three stages during the late nineteenth and early twentieth centuries, that provide a more solid theoretical underpinning to the understanding of acids and bases. Prior to the development of atomic and molecular theory in the nineteenth century, followed by the discovery of subatomic structures in the late nineteenth and early twentieth centuries, chemists could not do much more than make measurements and observations. Their definitions of substances were purely phenomenological—that is, the result of experimentation and the collection of data. From these observations, they could form general rules, but they lacked any means of "seeing" into the atomic and molecular structures of the chemical world. The phenomenological distinctions between acids and bases, gathered by scientists from ancient times onward, worked well enough for many centuries. The word "acid" comes from the Latin term acidus, or "sour," and from an early period, scientists understood that substances such as vinegar and lemon juice shared a common acidic quality. Eventually, the phenomenological definition of acids became relatively sophisticated, encompassing such details as the fact that acids produce characteristic colors in certain vegetable dyes, such as those used in making litmus paper. In addition, chemists realized that acids dissolve some metals, releasing hydrogen in the process. The word "alkali" comes from the Arabic al-qili, which refers to the ashes of the seawort plant. The latter, which typically grows in marshy areas, was often burned to produce soda ash, used in making soap. In contrast to acids, bases—caffeine, for example—have a bitter taste, and many of them feel slippery to the touch. They also produce characteristic colors in the vegetable dyes of litmus paper, and can be used to promote certain chemical reactions. Note that today chemists use the word "base" instead of "alkali," the reason being that the latter term has a narrower meaning: all alkalies are bases, but not all bases are alkalies. Originally, "alkali" referred only to the ashes of burned plants, such as seawort, that contained either sodium or potassium, and from which the oxides of sodium and potassium could be obtained. Eventually, alkali came to mean the soluble hydroxides of the alkali and alkaline earth metals. This includes sodium hydroxide, the active ingredient in drain and oven cleaners; magnesium hydroxide, used for instance in milk of magnesia; potassium hydroxide, found in soaps and other substances; and other compounds. Broad as this range of substances is, it fails to encompass the wide array of materials known today as bases—compounds which react with acids to form salts and water. The reaction to form salts and water is, in fact, one of the ways that acids and bases can be defined. In an aqueous solution, hydrochloric acid and sodium hydroxide react to form sodium chloride—which, though it is suspended in an aqueous solution, is still common table salt—along with water. The equation for this reaction is HCl(aq ) + NaOH(aq ) →H2O + NaCl(aq ). In other words, the sodium (Na) ion in sodium hydroxide switches places with the hydrogen ion in hydrochloric acid, resulting in the creation of NaCl (salt) along with water. But why does this happen? Useful as this definition regarding the formation of salts and water is, it is still not structural—in other words, it does not delve into the molecular structure and behavior of acids and bases. Credit for the first truly structural definition of the difference goes to the Swedish chemist Svante Arrhenius (1859-1927). It was Arrhenius who, in his doctoral dissertation in 1884, introduced the concept of an ion, an atom possessing an electric charge. His understanding was particularly impressive in light of the fact that it was 13 more years before the discovery of the electron, the subatomic particle responsible for the creation of ions. Atoms have a neutral charge, but when an electron or electrons depart, the atom becomes a positive ion or cation. Similarly, when an electron or electrons join a previously uncharged atom, the result is a negative ion or anion. Not only did the concept of ions greatly influence the future of chemistry, but it also provided Arrhenius with the key necessary to formulate his distinction between acids and bases. Arrhenius observed that molecules of certain compounds break into charged particles when placed in liquid. This led him to the Arrhenius acid-base theory, which defines an acid as any compound that produces hydrogen ions (H+) when dissolved in water, and a base as any compound that produces hydroxide ions (OH−) when dissolved in water. This was a good start, but two aspects of Arrhenius's theory suggested the need for a definition that encompassed more substances. First of all, his theory was limited to reactions in aqueous solutions. Though many acid-base reactions do occur when water is the solvent, this is not always the case. Second, the Arrhenius definition effectively limited acids and bases only to those ionic compounds, such as hydrochloric acid or sodium hydroxide, which produced either hydrogen or hydroxide ions. However, ammonia, or NH3, acts like a base in aqueous solutions, even though it does not produce the hydroxide ion. The same is true of other substances, which behave like acids or bases without conforming to the Arrhenius definition. These shortcomings pointed to the need for a more comprehensive theory, which arrived with the formulation of the Brønsted-Lowry definition by English chemist Thomas Lowry (1874-1936) and Danish chemist J. N. Brønsted (1879-1947). Nonetheless, Arrhenius's theory represented an important first step, and in 1903, he was awarded the Nobel Prize in Chemistry for his work on the dissociation of molecules into ions. The Brønsted-Lowry acid-base theory defines an acid as a proton (H+) donor, and a base as a proton acceptor, in a chemical reaction. Protons are represented by the symbol H+, and in representing acids and bases, the symbols HA and A−, respectively, are used. These symbols indicate that an acid has a proton it is ready to give away, while a base, with its negative charge, is ready to receive the positively charged proton. Though it is used here to represent a proton, it should be pointed out that H+ is also the hydrogen ion—a hydrogen atom that has lost its sole electron and thus acquired a positive charge. It is thus really nothing more than a lone proton, but this is the one and only case in which an atom and a proton are exactly the same thing. In an acid-base reaction, a molecule of acid is "donating" a proton, in the form of a hydrogen ion. This should not be confused with a far more complex process, nuclear fusion, in which an atom gives up a proton to another atom. The most fundamental type of acid-base reaction in Brønsted-Lowry theory can be symbolized thus HA(aq ) + H2O(l ) →H3O+(aq )


From Yahoo Answers

Question:Consider the following reactions: (a) HNO3 + KOH --> H2O + KNO3 (b) Cr3+ + 6 H2O --> [Cr(H2O)6]3+ (c) OCl- + H2O --> HOCl + OH- How to determine, the nature of HNO3, KOH, Cr3+, OCl- and the H20 in reactions (b) and (c) in the Arrhenius, Bronsted Lowry, Lewis Acid and Bases definitions? And which of the ones would be Arrhenius, Bronsted Lowry, Lewis Acid and Lewis Bases in the above question...?

Answers:Bronsted acid can give off H+ and base can take on H+ Arrhenius acid build in water H+ base builds in water OH- Lewis acid has a lack of electrons a base has free electron pairs. so from that : HNO3 is bronsted and arrehnius acid KOH is bronsted and arrehnius base the reaction is a bronsted reaction Cr3+ is Lewis acid (3+ means has space to take on electrons) H2O Lewis base (the oxygen has two free electron pairs not in a covalent bond.) OCl- is an arrhenius base

Question:what are the tests to recognize acid and bases for elementary and middle classes. the test that i can perform on similar looking solution to differentiate between acid and base. for example litmus paper test.

Answers:litmus paper is a good one. or you can use universal indicator. once you put a drop of the indicator into the solution, acid turns the indicator to reddish color (red, orange, yellow), green is neutral, base turns the indicator to blueish color (blue and purple). or you can get the purple cabbage and boil it, the purple solution can also be used as indicator, it works similar to the universal indicator.

Question:I know the definitions of acids and bases but i dont understand how to tell the difference between acids and bases in the formula or conjugate or weak acids and bases as well

Answers:Hello there, I wrote an article on Wikibooks on the properties of acids and bases, in which I also discuss conjugate acids and bases: http://en.wikibooks.org/wiki/General_Chemistry/Properties_and_Theories_of_Acids_and_Bases However, I do not discuss weak acids and bases in-depth so I will answer your question here. You can differentiate between strong and weak acids and bases based on a pH indicator. Recall that weak acids and bases do not fully dissociate in water, so the concentration of hydronium ions (as determined by a pH indicator) would be around neutral. For example, if you had a unknown acid sample of pH 2, then you would know that it is NOT a weak acid. Hope this helps.

Question:2 Major differences between acids and bases (biology)

Answers:Acids are H+ donors and Bases are H+ acceptors in chemical processes. Example: HCl gives H+ in the solvent water to become H+ and Cl- and is an acid. This is only one of several definitions of acids and bases but in my opinion the easiest to understand. Also, acids have a pH below 7 and bases have a pH above 7. The pH means the molar concentration of [H+] ions, 1 pH meaning 1 * 10 ^ -1 moles of [H+] ions per liter and 2 pH meaning 1 * 10 ^ -2 moles per liter. So, each 1 decrease on the pH scale means 10 times more [H+] ions and each 1 increase means 10 times less [H+] ions. Glad to help.

From Youtube

Chemistry Music Video 25: Acids And Bases :This song differentiates between Arrhenius Acids and Bases and describes titration. All music and lyrics copyright 2005 by Mark Rosengarten. All rights reserved. Lyrics: Acids and bases, in life you have a place Acids have a hydrogen, hydroxide for a base Acids give off H+ when in water they are stirred Forming H3O+, hydroniums the word The more hydroniums there are, the lower pH gets All because the silly acid had to go get wet! Reactive metals like Ca and Fe oxidize When reacted with an acid they can really shrink in size Gaseous hydrogen is formed as well as salt in fact But metals like Au, Ag and Cu wont react! Acids and bases, in life you have a place Acids have a hydrogen, hydroxide for a base Bases yield hydroxide, OH- how we write When dissolved in water, and disappear from sight The more hydroxide ions, the higher pH slopes The more slippery it feels as it turns your skin to soap! Fats are hydrolyzed by bases, turns them into soap Which can be fashioned into bars or dangled from a rope Bases form when active metals H2O are placed Sodium forms sodium hydroxide, a strong base! Acids and bases, in life you have a place Acids have a hydrogen, hydroxide for a base Put the two together and you get some real neat stuff Its called neutralization, I can never get enough It makes water molecules and salt with no regrets Titrations what you call it when you mix them with burets MaVa = MbVb can be used to calculate Concentrations of an acid or a base when you titrate Just ...

Acids and Bases :Acids and Bases Verse 1: The first chemist to inform us, His name was Savant Arrhenius. He said acids give the ion H Plus, Bases give the ion OH Minus. HCl shows us a great example Hydronium forms in stock sample. NaOH is the other story Hydroxide makes the base inventory. Chorus: Acids and bases whats left to say? 3 different theories are together today Arrhenius, Bronsted-Lowry, Lewis All their ideas added to this Acids have a pH less than 7 pH 0 is as strong as heaven the pH of 7 is neutral Above which is base, must be well controlled. Verse 2: There was a problem with the first theory Solved by two chemists Bronsted and Lowry They proposed acids were proton donors And that bases were but proton receptors The result of this was acid-base reactions Always wanting equilibrium action Then the rise of conjugate pairs More acids and bases put out there Bridge: These two things can both be quite caustic If it hits you, you should test diagnostics Dont ever mess with the six strong acids They dissociate completely and will leave you roasted Verse 3: A guy called Lewis in 1963 He made the concept much harder for me Lewis bases donate electron pairs Lewis acids accept them there An adduct forms between the compounds This time the protons dont have to move around This is unlike Bronsted-Lowrys theory It makes the subject far from easy