Chemistry For Advanced Level Students

The Element Lithium

2011-08-19T02:04:00.000-07:00

Lithium

Atomic Number: 3

Atomic Weight: 6.941

Melting Point: 453.65 K (180.50°C or 356.90°F)

Boiling Point: 1615 K (1342°C or 2448°F)

Density: 0.534 grams per cubic centimeter

Phase at Room Temperature: Solid

Element Classification: Metal

Period Number: 2

Group Number: 1

Group Name: Alkali Metal

History

Lithium was discovered in the mineral petalite (LiAl(Si₂O₅)₂) by Johann August Arfvedson in 1817. It was first isolated by William Thomas Brande and Sir Humphrey Davy through the electrolysis of lithium oxide (Li₂O). Today, larger amounts of the metal are obtained through the electrolysis of lithium chloride (LiCl). Lithium is not found free in nature and makes up only 0.0007% of the earth's crust.

Brief description

Lithium is a Group 1 (IA) element containing just a single valence electron (1s22s1). Group 1 elements are called "alkali metals". Lithium is a solid only about half as dense as water and lithium metal is the least dense metal. A freshly cut chunk of lithium is silvery, but tarnishes in a minute or so in air to give a grey surface. Its chemistry is dominated by its tendency to lose an electron to form Li+. It is the first element within the second period. Lithium is mixed (alloyed) with aluminium and magnesium for light-weight alloys, and is also used in batteries, some greases, some glasses, and in medicine.

The main lithium compound is the lithium hydroxide. It’s a white powder; the manufactured material is monohydrate lithium hydroxide. The carbonate can be used in the pottery industry and in medicine as an antidepressant. The bromine and the lithium chloride both form concentrated brine, which have the property of absorbing the humidity in a wide interval of temperature; these brines are used in the manufactured air conditioning systems.

The main industrial use of lithium is in lithium stearatum form, as lubricant grease’s thickener. Other important applications of lithium compounds are in pottery, specifically in porcelain glaze; as an additive to extend the life and performance of alkaline storage batteries and in autogenously welding and brass welding.

Alloys of the metal with aluminum, cadmium, copper, and manganese are used to make high performance aircraft parts. Metallic lithium will react with nitrogen, oxygen, and water vapor in air. Consequently, the lithium surface becomes coated with a mixture of lithium hydroxide (LiOH), lithium carbonate (Li2CO3), and lithium nitride (Li3N). Lithium hydroxide represents a potentially significant hazard because it is extremely corrosive. Special attention should be given to water organisms.

The Element Helium

2011-08-19T01:47:00.000-07:00

Helium

Atomic Number: 2

Atomic Weight: 4.002602

Melting Point: 0.95 K (-272.2°C or -458.0°F)

Boiling Point: 4.22 K (-268.93°C or -452.07°F)

Density: 0.0001785 grams per cubic centimeter

Phase at Room Temperature: Gas

Element Classification: Non-metal

Period Number: 1

Group Number: 18

Group Name: Noble Gas

History

Helium, the second most abundant element in the universe, was discovered on the sun before it was found on the earth. Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum while studying a total solar eclipse in 1868. Sir Norman Lockyer, an English astronomer, realized that this line, with a wavelength of 587.49 nanometers, could not be produced by any element known at the time. It was hypothesized that a new element on the sun was responsible for this mysterious yellow emission. This unknown element was named helium by Lockyer.

Uses of Helium

The uses of helium depend either on its small atomic mass, or on its chemical inertness. Many applications involve both properties. The uses of argon depend on its inertness. Both helium and argon are available in industrial quantities, and are relatively cheap. The rarer inert gases Ne, Kr and Xe are used in very small amounts in electrical discharges, together with He and A. Xe, as we have remarked, is useful as an anaesthetic.

Helium and argon are used in welding to shield the hot metal from the atmosphere, especially in the case of reactive metals. Classic arc welding uses consumable electrodes coated in a flux. In the heat of the arc, the flux evolves protective gases and forms a liquid slag to protect the metal. In gas metal arc welding (GMAW), helium or argon is blown on the weld, protecting it without producing a slag and excluding oxygen or hydrogen. This seems to have been developed in World War II for welding magnesium, but has now been applied much more generally. It is generally considered necessary for good welds in titanium, as well as for aluminium and stainess steel, where it eliminates weld porosity. In gas tungsten arc welding (GTAW), non-consumable tungsten electrodes are used. Helium is often used as an inert atmosphere in growing semiconductor crystals and for similar processes.

The major use of argon is to fill incandescent lamp bulbs. Although the gas cools the filament somewhat, evaporation of tungsten is discouraged and the life of the lamps is significantly extended.

The small atomic mass leads to large thermal velocities, and this implies rapid diffusion and easy heat transfer. Hydrogen leads in these properties, of course, but helium is not far behind. Helium is used for leak detection in vacuum systems. The gas is blown around likely leak sources, and will diffuse much more rapidly than air through even the smallest opening. The helium is easily detected by an ionization gauge or other means, signalling the presence of a leak. The rapid diffusion makes helium a good carrier gas for gas chromatography. Helium is also used as a driver gas in hypersonic wind tunnels. The high thermal conductivity, and its zero neutron capture cross section, make helium a good coolant in gas-cooled nuclear reactors, though its low density works against it.

Helium gained fame as a lifting gas, and we have seen that this spurred the large-scale production and conservation of the gas, in spite of its inherent rarity. We have already remarked that it is nearly as effective as hydrogen in this respect, as 25 is to 27. The net lift of a balloon is the weight of the air displaced, less the weight of the balloon, lifting gas and ballast. All balloons using hydrogen or helium are strongly affected by the rapid diffusion of these gases, which soon leads to a decrease in lift, which is compensated by throwing ballast overboard. Incidentally, the lifting gas is contained in separate bags like the compartments of a ship, so that the compromise of one is not the loss of all. The use of helium in small balloons instead of hydrogen seems to be a frivolous waste of a valuable substance. Of course, the reason it is used is the elimination of the hazards of hydrogen combustion, perhaps more in the gas sources than in the actual balloon. If the predictions of a "hydrogen economy" do indeed come true, we will hear more about the dangers of hydrogen. All the problems of working with helium arise with hydrogen as well, with the added excitement of explosion.

The inert gases are excellent as filling gases for electrical discharges, since they do not react with the electrodes, and tubes containing them have long service lives. Neon and argon are used in glow discharge lamps, where the light comes principally from the negative glow near the cathode. Neon has its typical orange-red color, while argon produces a violet. The electrodes are specially treated to encourage electron emission, so that the striking voltage of the lamps is low. "Neon lights" use the positive column in a long tube as the light source. Helium gives a yellow light, and neon red-orange. Mercury with argon to aid starting gives blue. Colored glass tubing is used to create other colors. The pressure is a few mm Hg, and the voltage drop around 100 V per metre. These lamps are supplied with a high-voltage transformer with large leakage reactance so that the voltage is high for striking, and decreases when current flows. The current may be 25 to 50 mA.

A discharge through a mixture of helium and neon creates a population inversion in the neon that can be used in a laser. The helium is ionized in the discharge, and when the electrons recombine and cascade down in energy, they pile up in the two metastable states 2s³S (19.82 eV) and 2s¹ (20.62 eV) of the electron configuration 1s2s. Fast transitions to the ground state are discouraged by the selection rules, in one case forbidding triplet-singlet transitions and in the other S-S transitions. By good luck, the energies of each of these metastable states is about equal to the energies of two upper levels of laser transitions in neon. The transfer of energy is quite likely, populating these states. One state makes a transition of wavelength 632.8 nm, the familiar He-Ne visible red laser line, to a lower state, while the other makes a transition of 1152.3 nm to the same lower state. This lower state is kept empty by allowed transitions of wavelength 594.5 nm to a still lower state that is depopulated by collisions with the walls of the discharge tube.

All of the inert gases have a large energy gap between the ground state and the first excited state, corresponding to a transition in the far ultraviolet. The visible lines in the spectra are due to transitions between higher states. This explains why the inert gases are transparent, since ordinary visible frequencies are too low to excite them when they are in the ground state. The helium spectrum in a discharge tube shows strong lines at 447.1 nm (blue), 501.6 nm (green), 587.56 nm (yellow) and 667.8 nm (red). This spread of lines creates a whitish light. Neon has a cluster of lines in the red and orange, while argon has groups of lines in the blue and in the red.

First Element Hydrogen

2011-08-19T01:22:00.000-07:00

Hydrogen

Atomic Number: 1

Atomic Weight: 1.00794

Melting Point: 13.81 K (-259.34°C or -434.81°F)

Boiling Point: 20.28 K (-252.87°C or -423.17°F)

Density: 0.00008988 grams per cubic centimeter

Phase at Room Temperature: Gas

Element Classification: Non-metal

Period Number: 1

Group Number: 1

History

Scientists had been producing hydrogen for years before it was recognized as an element. Written records indicate that Robert Boyle produced hydrogen gas as early as 1671 while experimenting with iron and acids. Hydrogen was first recognized as a distinct element by Henry Cavendish in 1766.

Brief description

Hydrogen is the lightest element. It is by far the most abundant element in the universe and makes up about about 90% of the universe by weight. Hydrogen as water (H2O) is absolutely essential to life and it is present in all organic compounds. Hydrogen is the lightest gas. Hydrogen gas was used in lighter-than-air balloons for transport but is far too dangerous because of the fire risk (Hindenburg). It burns in air to form only water as waste product and if hydrogen could be made on sufficient scale from other than fossil fuels then there might be a possibility of a hydrogen economy.

About 10 percent of the weight of living organisms is hydrogen - mainly in water, proteins and fats. Liquid hydrogen has the lowest density of any liquid. Solid, crystalline hydrogen has the lowest density of any crystalline solid. Hydrogen is believed to be one of three elements produced in the Big Bang; the others are helium and lithium. We owe most of the energy on our planet to hydrogen. The Sun's nuclear fires convert hydrogen to helium releasing a large amount of energy in the process. Hydrogen forms both positive and negative ions. It does this more readily than any other element. Hydrogen is the most abundant element in the universe. Hydrogen reacts explosively with the elements oxygen, chlorine and fluorine

how do you measure the charge passed through a solution

2011-08-13T05:25:00.000-07:00

The process of measurement of the charge involved in a process is called coulometry. one of the very accurate methords of coulometry is based on the measurement of the mass of products formed at the anode or the cathode electrolytic cell.

Experimental method and calculations

Example 01

calculate the mass of copper deposited on the cathode when a solution of cuso4 is electrolyzed for 20.0 s with a 2.0 a electric current. [atomic mass of cu = 63.5gmol]

Answer

First I give you a formula that not familiar for advanced level students because it is not in A/L chemistry syllabus it will help you to solved questions easily and quickly in exams.

m = Q/F *1/Z * M = QM/FZ

Q = mFZ/M

Where,

m = mass of the ions discharged
Q = charge passed through the solutions
F = faraday constant
Z = charge number of the ions discharged
M = molar mass of the ion

now we can solved the question using that formula

Q = mFZ/M

m = QM/FZ

Q =It

m = ItM/FZ

m = 2.0 A *20.0 S *63.5 gmol^-1 /96500 C * 2

m = 0.01316 g

I hope that clarifications are helpful for you.