CHAPTER 5
CLASSIFICATION OF ORGANISMS
1. Mention the classificationsystem of five kingdoms proposed by R.H Whiattaker with example
Selasa, 17 November 2009
Minggu, 09 November 2008
ACID AND BASE
I. INTRODUCTION
Acids and Bases, two classes of chemical compounds that display generally opposite characteristics. Acids taste sour, turn litmus (a pink dye derived from lichens) red, and often react with some metals to produce hydrogen gas. Bases taste bitter, turn litmus blue, and feel slippery. When aqueous (water) solutions of an acid and a base are combined, a neutralization reaction occurs. This reaction is characteristically very rapid and generally produces water and a salt. For example, sulfuric acid and sodium hydroxide, NaOH, yield water and sodium sulfate:
H2SO4 + 2NaOH⇄2H2O + Na2SO4Microsoft
II. EARLY THEORIES
Modern understanding of acids and bases began with the discovery in 1834 by the English physicist Michael Faraday that acids, bases, and salts are electrolytes. That is, when they are dissolved in water, they produce a solution that contains charged particles, or ions, and can conduct an electric current Ionization. In 1884 the Swedish chemist Svante Arrhenius (and later Wilhelm Ostwald, a German chemist) proposed that an acid be defined as a hydrogen-containing compound that, when dissolved in water, produces a concentration of hydrogen ions, or protons, greater than that of pure water. Similarly, Arrhenius proposed that a base be defined as a substance that, when dissolved in water, produces an excess of hydroxyl ions, OH-. The neutralization reaction then becomes:
H+ + OH-⇄H2O
A number of criticisms of the Arrhenius-Ostwald theory have been made. First, acids are restricted to hydrogen-containing species and bases to hydroxyl-containing species. Second, the theory applies to aqueous solutions exclusively, whereas many acid-base reactions are known to take place in the absence of water.
III. BRONSTED-LOWRY THEORY
A more satisfactory theory was proposed in 1923 by the Danish chemist Johannes Brønsted and independently by Thomas Lowry, a British chemist. Their theory states that an acid is a proton (hydrogen ion, H+) donor and a base a proton acceptor. Although the acid must still contain hydrogen, the Brønsted-Lowry theory does not require an aqueous medium. For example, liquid ammonia, which acts as a base in aqueous solution, can act as an acid in the absence of water by transferring a proton to a base and forming the amide anion (negative ion) NH2-: NH3 + base⇄NH2- + base + H+
The Brønsted-Lowry definition of acids and bases also explains why a strong acid displaces a weak acid from its compounds (and likewise for strong and weak bases). Here acid-base reactions are viewed as a competition for protons. In terms of a general chemical equation, the reaction of Acid (1) with Base (2) Acid (1) + Base (2)⇄Acid (2) + Base (1)results in the transfer of a proton from Acid (1) to Base (2). In losing the proton, Acid (1) becomes its conjugate base, Base (1). In gaining a proton, Base (2) becomes its conjugate acid, Acid (2). The equilibrium represented by the equation above may be displaced either to the left or to the right, and the actual reaction will take place in the direction that produces the weaker acid-base pair. For example, hydrogen chloride (HCl) is a strong acid in water because it readily transfers a proton to water to form a hydronium ion: HCl + H2O⇄H3O+ + Cl-The equilibrium lies mostly to the right because the conjugate base of HCl, Cl-, is a weak base, and H3O+, the conjugate acid of H2O, is a weak acid.
In contrast, hydrogen fluoride, HF, is a weak acid in water because it does not readily transfer a proton to water: HF + H2O⇄H3O+ + F-This equilibrium lies mostly to the left because H2O is a weaker base than F-, and because HF is a weaker acid (in water) than H3O+. The Brønsted-Lowry theory also explains why water can be amphoteric, that is, why it can serve as either an acid or a base. Water serves as a base in the presence of an acid that is stronger than water (such as HCl), in other words, an acid that has a greater tendency to dissociate than does water: HCl + H2O⇄H3O+ + Cl-Water can also serve as an acid in the presence of a base that is stronger than water (such as ammonia): NH3 + H2O⇄NH4+ + OH
IV. MEASURING ACID OR BASE STRENGTH
The strength of an acid can be measured by the extent to which an acid transfers a proton to water to produce the hydronium ion, H3O+. Conversely, the strength of a base is indicated by the extent to which the base removes a proton from water. A convenient acid-base scale is calculated from the amount of H3O+ that is formed in water solutions of acids or of OH- formed in water solutions of bases. The former is known as the pH scale and the latter as the pOH scale (pH). The value for pH is equal to the negative logarithm of the hydronium ion concentration—and for pOH, of the hydroxyl ion concentration—in an aqueous solution: pH = -log [H3O+]pOH = -log [OH-]Pure water has a pH of 7.0. When an acid is added, the hydronium ion concentration [H3O+] becomes larger than that in pure water, and the pH becomes less than 7.0, depending on the strength of the acid. The pOH of pure water is also 7.0, and in the presence of a base, the pOH drops to values lower than 7.0.
The American chemist Gilbert N. Lewis has offered another theory of acids and bases that has the further advantage of not requiring the acid to contain hydrogen. This theory states that acids are electron-pair acceptors and bases are electron-pair donors. This theory also has the advantages that it works when solvents other than water are involved and it does not require the formation of a salt or of acid-base conjugate pairs. Thus, ammonia is viewed as a base because it can donate an electron pair to the acid boron trifluoride, for example H3N: + BF3⇄H3N-BF3to form an acid-base association pair.
Acids and Bases, two classes of chemical compounds that display generally opposite characteristics. Acids taste sour, turn litmus (a pink dye derived from lichens) red, and often react with some metals to produce hydrogen gas. Bases taste bitter, turn litmus blue, and feel slippery. When aqueous (water) solutions of an acid and a base are combined, a neutralization reaction occurs. This reaction is characteristically very rapid and generally produces water and a salt. For example, sulfuric acid and sodium hydroxide, NaOH, yield water and sodium sulfate:
H2SO4 + 2NaOH⇄2H2O + Na2SO4Microsoft
II. EARLY THEORIES
Modern understanding of acids and bases began with the discovery in 1834 by the English physicist Michael Faraday that acids, bases, and salts are electrolytes. That is, when they are dissolved in water, they produce a solution that contains charged particles, or ions, and can conduct an electric current Ionization. In 1884 the Swedish chemist Svante Arrhenius (and later Wilhelm Ostwald, a German chemist) proposed that an acid be defined as a hydrogen-containing compound that, when dissolved in water, produces a concentration of hydrogen ions, or protons, greater than that of pure water. Similarly, Arrhenius proposed that a base be defined as a substance that, when dissolved in water, produces an excess of hydroxyl ions, OH-. The neutralization reaction then becomes:
H+ + OH-⇄H2O
A number of criticisms of the Arrhenius-Ostwald theory have been made. First, acids are restricted to hydrogen-containing species and bases to hydroxyl-containing species. Second, the theory applies to aqueous solutions exclusively, whereas many acid-base reactions are known to take place in the absence of water.
III. BRONSTED-LOWRY THEORY
A more satisfactory theory was proposed in 1923 by the Danish chemist Johannes Brønsted and independently by Thomas Lowry, a British chemist. Their theory states that an acid is a proton (hydrogen ion, H+) donor and a base a proton acceptor. Although the acid must still contain hydrogen, the Brønsted-Lowry theory does not require an aqueous medium. For example, liquid ammonia, which acts as a base in aqueous solution, can act as an acid in the absence of water by transferring a proton to a base and forming the amide anion (negative ion) NH2-: NH3 + base⇄NH2- + base + H+
The Brønsted-Lowry definition of acids and bases also explains why a strong acid displaces a weak acid from its compounds (and likewise for strong and weak bases). Here acid-base reactions are viewed as a competition for protons. In terms of a general chemical equation, the reaction of Acid (1) with Base (2) Acid (1) + Base (2)⇄Acid (2) + Base (1)results in the transfer of a proton from Acid (1) to Base (2). In losing the proton, Acid (1) becomes its conjugate base, Base (1). In gaining a proton, Base (2) becomes its conjugate acid, Acid (2). The equilibrium represented by the equation above may be displaced either to the left or to the right, and the actual reaction will take place in the direction that produces the weaker acid-base pair. For example, hydrogen chloride (HCl) is a strong acid in water because it readily transfers a proton to water to form a hydronium ion: HCl + H2O⇄H3O+ + Cl-The equilibrium lies mostly to the right because the conjugate base of HCl, Cl-, is a weak base, and H3O+, the conjugate acid of H2O, is a weak acid.
In contrast, hydrogen fluoride, HF, is a weak acid in water because it does not readily transfer a proton to water: HF + H2O⇄H3O+ + F-This equilibrium lies mostly to the left because H2O is a weaker base than F-, and because HF is a weaker acid (in water) than H3O+. The Brønsted-Lowry theory also explains why water can be amphoteric, that is, why it can serve as either an acid or a base. Water serves as a base in the presence of an acid that is stronger than water (such as HCl), in other words, an acid that has a greater tendency to dissociate than does water: HCl + H2O⇄H3O+ + Cl-Water can also serve as an acid in the presence of a base that is stronger than water (such as ammonia): NH3 + H2O⇄NH4+ + OH
IV. MEASURING ACID OR BASE STRENGTH
The strength of an acid can be measured by the extent to which an acid transfers a proton to water to produce the hydronium ion, H3O+. Conversely, the strength of a base is indicated by the extent to which the base removes a proton from water. A convenient acid-base scale is calculated from the amount of H3O+ that is formed in water solutions of acids or of OH- formed in water solutions of bases. The former is known as the pH scale and the latter as the pOH scale (pH). The value for pH is equal to the negative logarithm of the hydronium ion concentration—and for pOH, of the hydroxyl ion concentration—in an aqueous solution: pH = -log [H3O+]pOH = -log [OH-]Pure water has a pH of 7.0. When an acid is added, the hydronium ion concentration [H3O+] becomes larger than that in pure water, and the pH becomes less than 7.0, depending on the strength of the acid. The pOH of pure water is also 7.0, and in the presence of a base, the pOH drops to values lower than 7.0.
The American chemist Gilbert N. Lewis has offered another theory of acids and bases that has the further advantage of not requiring the acid to contain hydrogen. This theory states that acids are electron-pair acceptors and bases are electron-pair donors. This theory also has the advantages that it works when solvents other than water are involved and it does not require the formation of a salt or of acid-base conjugate pairs. Thus, ammonia is viewed as a base because it can donate an electron pair to the acid boron trifluoride, for example H3N: + BF3⇄H3N-BF3to form an acid-base association pair.
Kamis, 30 Oktober 2008
SCIENCE TEST FOR SBI CLASSES
Answer the following questions correctly!
1. Write the definitions of Acid and Base!
2. Write the characterisntics of Acid and Base!
3. What is the meaning of 'pH indicators'? Write 3 examples of it you know!
4. Write the natural substances which can be used as an acid-base indicator!
5. What is the meaning of 'neutralization reaction'?
Good Luck!
1. Write the definitions of Acid and Base!
2. Write the characterisntics of Acid and Base!
3. What is the meaning of 'pH indicators'? Write 3 examples of it you know!
4. Write the natural substances which can be used as an acid-base indicator!
5. What is the meaning of 'neutralization reaction'?
Good Luck!
CHARLES DARWIN
Darwin, Charles Robert (1809-1882), British scientist, who laid the foundation of modern evolutionary theory with his concept of the development of all forms of life through the slow-working process of natural selection. His work was of major influence on the life and earth sciences and on modern thought in general.
Born in Shrewsbury, Shropshire, England, on February 12, 1809, Darwin was the fifth child of a wealthy and sophisticated English family. His maternal grandfather was the successful china and pottery entrepreneur Josiah Wedgwood; his paternal grandfather was the well-known 18th-century physician and savant Erasmus Darwin. After graduating from the elite school at Shrewsbury in 1825, young Darwin went to the University of Edinburgh to study medicine. In 1827 he dropped out of medical school and entered the University of Cambridge, in preparation for becoming a clergyman of the Church of England. There he met two stellar figures: Adam Sedgwick, a geologist, and John Stevens Henslow, a naturalist. Henslow not only helped build Darwin’s self-confidence but also taught his student to be a meticulous and painstaking observer of natural phenomena and collector of specimens. After graduating from Cambridge in 1831, the 22-year-old Darwin was taken aboard the English survey ship HMS Beagle, largely on Henslow’s recommendation, as an unpaid naturalist on a scientific expedition around the world.
Born in Shrewsbury, Shropshire, England, on February 12, 1809, Darwin was the fifth child of a wealthy and sophisticated English family. His maternal grandfather was the successful china and pottery entrepreneur Josiah Wedgwood; his paternal grandfather was the well-known 18th-century physician and savant Erasmus Darwin. After graduating from the elite school at Shrewsbury in 1825, young Darwin went to the University of Edinburgh to study medicine. In 1827 he dropped out of medical school and entered the University of Cambridge, in preparation for becoming a clergyman of the Church of England. There he met two stellar figures: Adam Sedgwick, a geologist, and John Stevens Henslow, a naturalist. Henslow not only helped build Darwin’s self-confidence but also taught his student to be a meticulous and painstaking observer of natural phenomena and collector of specimens. After graduating from Cambridge in 1831, the 22-year-old Darwin was taken aboard the English survey ship HMS Beagle, largely on Henslow’s recommendation, as an unpaid naturalist on a scientific expedition around the world.
Langganan:
Postingan (Atom)
