Chemistry 1020Lecture 19Notes
Common bases include metal hydroxides, which usually dissociate completely into the metal cation and the hydroxide ion if the hydroxide is soluble. Soluble metal hydroxides include the alkali metal hydroxides along with Ca(OH)2, Sr(OH)2, and Ba(OH)2. For example:
Ca(OH)2 ---> Ca2+ + 2 OH-
While other hydroxides are not very soluble, the hydroxide ions in them are still capable of reacting with acids. For the example the relatively insoluble Mg(OH)2 can still react with acids in the stomach (where the pH is about 2) to neutralize excess acid according to the equation:
Mg(OH)2 + 2 H+ ---> 2 H2O + Mg2+
Mg(OH)2 is the main ingredient in milk of magnesia.
Metal oxides react with water to form metal hydroxides. Therefore they are bases. For example, consider the reaction of lime (CaO) with water:
CaO + H2O ---> Ca(OH)2
Recall that lime is mentioned in your text as a potential material to be used to neutralize the acidity in some lakes.
Lime can be formed by the thermal decomposition of limestone (CaCO3) as follows:
CaCO3 ---> CO2 + CaO
Hence limestone has some basic character, and can react with acids in the process already discussed by which acid rain can lead to the deterioration of marble statues:
CaCO3 + 2 H+ ---> Ca2+ + CO2 + H2O
This reaction could be considered the result of CO32- reacting with the protons to form carbonic acid (H2CO3), which decomposes into CO2 and H2O:
CO32- + 2 H+ ---> H2CO3 ---> CO2 + H2O
Incidentally, it is the reverse of this process by which lime is used in the production of mortar and in production of cement. The lime reacts with water to form calcium hydroxide, which can slowly react with CO2 from the atmosphere to form CaCO3 producing the cement properties of the mortar:
CaO + H2O ---> Ca(OH)2
Ca(OH)2 + CO2 ---> CaCO3 + H2O
Oxides of non-metals, on the other hand, react with water to form acids. A case in point already discussed is the reverse of the above reaction:
CO2 + H2O ---> H2CO3
H2CO3 is a weak acid which dissociates only slightly into protons. But the CO2 in the air is sufficient to cause your tap water to be slightly acidic, and carbonation of water can increase the concentration of this acid in solution.
H2CO3 (or CO2 if there is water around) can react with bases, however, in a step-wise fashion:
H2CO3 + OH- ---> H2O + HCO3- (bicarbonate ion)
HCO3- + OH-+ ---> H2O + CO32- (carbonate ion)
Note in the first step above, that bicarbonate is the conjugate base of carbonic acid, and in the reverse of the first step it serves as a base which can accept protons. This is the basis of using sodium bicarbonate to neutralize acid spills, because:
HCO3- + H+ ---> H2CO2 ---> CO2 + H2O
The fizzing you get when you add acid to sodium bicarbonate is from the CO2 produced.
In the second step above, though, HCO3- is serving as an acid to neutralize the base OH-.
During the earlier discussion of acid rain, we have already indicated the culprits as non-metal oxides of sulfur and nitrogen (SOx and NOx). These react with water to form acids as well.
In the case of sulfur, the reactions are fairly simple:
SO2 + H2O ---> H2SO3 (sulfurous acid, a weak acid)
SO3 + H2O ---> H2SO4 (sulfuric acid, a strong acid)
The reaction of nitrogen oxides is more complex:
N2O3 + H2O ---> 2 HNO2 (nitrous acid, a weak acid)
N2O5 + H2O ---> 2 HNO3 (nitric acid, a strong acid)
The formation of these two oxides from the more common oxides we have already mentioned, NO and NO2 is a more complicated chemical reaction involving gain and loss of electrons (an oxidation-reduction reaction).
The book also mentions the reaction of NO2 as a pollutant from automobile exhausts reacting directly with another pollutant, the hydroxide radical:
NO2 + OH ---> HNO3
From this brief discussion, you can see that many chemical reactions can be described in terms of the simple concept of an acid-base reaction, even though the role of protons gets a bit obscured, especially when we talk about the reactions of metal and non-metal oxides.
The concept of acid strength is very important to life. Living cells can stay alive only if the [H+] is carefully controlled within rather narrow limits. For example, the pH of your blood is 7.4. If it varies more than about a half unit above or below this figure, it will kill you. That is the reason that acidification of lakes and streams is deleterious to aquatic life, and that acid rain can lead to destruction of plants, such as the waldsterben (forest death) that has been observed in recent years in some of the forests of Europe.
Chapter 8Nuclear Chemistry
A major goal of the alchemists was to convert base metals, such as lead, into gold. This change is called the transmutation of elements. We now know from our understanding of chemistry that there is no chemical process converting one element into another. Chemical reactions relate to the electron configurations about nuclei, and the interactions of these electrons. Elemental identity resides in the nucleus, specifically in the number of protons in the nucleus.
Transmutation was observed, however, in the phenomenon of radioactive decay. This phenomenon was first discovered by the Frenchman Henri Becquerel, who was carrying out a study of the interaction of ultraviolet radiation with matter. He was studying a phenomenon whereby when some materials are bombarded with one form of electromagnetic radiation, they will omit radiation of another wavelength. You may be familiar with flourescence, where you can view certain minerals under UV light, and see them emit light of a visible wavelength. A similar process is known as phosphorescence, where the emission continues for a while even after the stimulating radiation is turned off.
X-rays had just been discovered. These were mysterious forms of electromagnetic radiation that could pass through most opaque substances, and scientists and the public alike were fascinated with the phenomenon. Imagine a ray beam that lets you see through peoples clothing! There was potentially a great future in lead underwear!
Becquerel was attempting to produce X-rays by bombarding various minerals with UV light. His most convenient source of UV was the sun. So his experimental procedure involved putting X-ray film inside an opaque envelope, setting a mineral on top of the envelope, exposing the mineral to the sun, then later developing the film to see if X-rays had been produced which penetrated the envelope and produced an image on the film.
One cloudy day in Paris, February 26, 1896, he put his equipment aside without the exposure to the sun. For some reason, he developed the film anyway, and discovered an image of a mineral on the film! Something was being produced that penetrated the opaque envelope! The mineral source contained the element uranium.
This was indeed a serendipitous discovery. But serendipity favors the prepared mind, and he recognized this as a new phenomenon. Subsequent studies by him and others concluded that the uranium atoms in the mineral were actually undergoing a process of radioactive decay in which small energetic particles were being ejected from the atoms. He shared the 1903 Nobel Prize in Physics, along with his student, Marie Curie, and her husband, Pierre Curie, who in the meantime had gone on to isolate other new elements from the mineral sources--polonium and radium. (Marie Curie went on to win the 1911 Nobel Prize in Chemistry).
Subsequent studies of radioactive decay identified three types of "rays" produced. Their properties are summarized in the following table:
The alpha particle turns out to be composed of two neutrons and two protons which are ejected from a large, unstable nucleus of a heavy atom. (Review the discussion on protons and neutrons and the atomic nucleus earlier). This was the radiation Becquerel was detecting, and when the alpha particle picks up two electrons from its environment, it forms a helium atom. The beta particle turns out to be simply an electron that has been emitted from a nucleus. The gamma particle is just a very high energy (and short wavelength) form of electromagnetic radiation. (Review the discussion on electromagnetic radiation earlier).
Such radioactive decay reactions can be described by writing nuclear reactions. Following is the nuclear reaction describing the alpha particle decay of uranium. (For nuclear reactions, one must specify the isotope of the element undergoing the change.)
Recall our symbolism for representing isotopes which was discussed in Lecture 4. In the case above, both the mass numbers (the numbers in the upper left) and the atomic numbers (the numbers to the lower left) must balance on both sides of the equation.
Mass number balance: 238 = 234 + 4
Atomic number balance: 92 = 90 + 2
When an element undergoes beta decay, it is emitting an electron from the nucleus. An example would be the decay of carbon-14, the radioactive isotope that is used in dating carbon specimens for archeologists:
Notice again the mass and atomic number balances:
Mass number balance: 14 = 14 + 0
Atomic number balance: 6 = 7 + (-1)
Another particle, the neutron, with a mass number of 1 but no charge was discovered later. It was more difficult to detect because it was lacking a charge.
Physicists began to use these particles in experiments to probe the nature of matter. Ernest Rutherford developed his planetary model of the atom from studying how alpha particles acted when directed at gold foil. Most of the particles went straight through, indicating much of the gold atom is empty space. A few bounced back or "scattered", indicating they had collided with a very small, heavy object--the nucleus of the gold atom.
Because both the nucleus and the alpha particle are positively charged, when they get close they tend to repel. Under some conditions, though, for lighter elements, the nucleus can "absorb" the alpha particle and produce an artificial transmutation of elements. Rutherford was the first to accomplish a transmutation when he bombarded nitrogen gas with alpha particles to produce the nuclear reaction:
For larger, highly positive nuclei, it is easier to get neutrons to penetrate the nucleus During the thirties, several laboratories were attempting to create new elements, elements heavier than uranium. (While uranium is radioactive, its decay is very slow. It takes 4.5 x 109 years for half of it to decay, about equivalent to the estimated age of the earth. Those heavier than uranium decay faster, and since have long since decayed, and therefore are not found naturally). When a uranium nucleus takes up a neutron, it creates an unstable intermediate nucleus which gains stability by emitting beta particles, thereby leading to elements with higher atomic numbers: