Enthalpy of Formation

One particularly useful type of process for us to consider is the formation of a substance in its standard state from its elements in their standard states. Standard state is the state in which the substance is most stable. For example, the standard state of elemental oxygen is O2(g) not O(g) or O3(g). Thus, the equation for the formation reaction of water is

The enthalpy change for this reaction can be called the enthalpy of formation of water,

Values of Δ H for many reactions can be determined experimentally; for other reactions, measuring Δ H is impossible because the process is too slow or for some other reason. Values of Δ Hf are useful because we can calculate the Δ Hf for any reaction by subtracting the sum of the Δ Hf values of the reactants of the reaction from the sum of the Δ Hf values of the products of the reaction:

The enthalpy change for any reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants.

The Δ Hf of an element in its standard state is zero because converting an element in its standard state to the element in its standard state does not involve any change. That “reaction” will have a Δ H of zero.

Hess’s Law

Since enthalpy is a state function, the change in enthalpy in going from some initial state to some fi nal state is independent of the pathway. This means that in going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This principle is called Hess’s law. For example, we can calculate the Δ H for the reaction of carbon with oxygen gas to yield carbon dioxide from the values for the reaction of carbon with oxygen to yield carbon monoxide and that of carbon monoxide plus oxygen to yield carbon dioxide:

Note that we have not used enthalpies of formation explicitly in this process. Enthalpy changes of any type of reaction will do. Using Hess’s law is straightforward if the equations given are in a form in which simply adding them yields the equation desired. More often, however, to get the desired equation, we must multiply or divide a given equation by a small integer. When we do so, we must also multiply or divide the corresponding Δ H value by that same integer. Sometimes the given equation must be reversed, whereupon the sign of the corresponding Δ H must be changed. Sometimes, both of these processes are necessary. We decide which of these steps to take by looking at the desired equation to see where we want each reactant and product, and how many moles of each we want. To use Hess’s law to compute enthalpy changes for reactions, it is important to understand two characteristics of Δ H for a reaction:

1. If a reaction is reversed, the sign of Δ H is also reversed.

2. The magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. If the coeffi cients in a balanced reaction are multiplied by an integer, the value of Δ H is multiplied by the same integer.

Both these rules follow in a straightforward way from the properties of enthalpy changes. The fi rst rule can be explained by recalling that the sign of Δ H indicates the direction of the heat fl ow at constant pressure. If the direction of the reaction is reversed, the direction of the heat fl ow also will be reversed.

Сурак

Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass.^{[9][note 1]} Non-remnant stars are mainly composed of hydrogen in its plasma state. The most common isotope of hydrogen, termed protium (name rarely used, symbol ¹H), has a single proton and zero neutrons. Hydrogen gas (dihydrogen or molecular hydrogen)^[15] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.^[16] The enthalpy of combustionfor hydrogen is − 286 kJ/mol: ^[17]

2 H₂(g) + O₂(g) → 2 H₂O(l) + 572 kJ (286 kJ/mol)^{[note 2]}

Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures may be ignited by spark, heat or sunlight. The hydrogenautoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).^[18] Purehydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames.^[19] The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible orange flames were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.

H₂ reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine andfluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerousacids.

52.Tell about oxygen in English. Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element and oxidizing agent that readily forms compounds (notably oxides) with most elements.^[3] By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium.^[4] AtSTP, two atoms of the element bind to form dioxygen, a diatomic gas that is colorless, odorless, and tasteless, with the formula O2. Oxygen was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley inWiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygen was coined in 1777 by Antoine Lavoisier, ^[10] whose experiments with oxygen helped to discredit the then-popular phlogiston theory ofcombustion and corrosion. Its name derives from the Greek roots ὀ ξ ύ ς oxys, " acid", literally " sharp", referring to the sour taste of acids and -γ ε ν ή ς -genes, " producer", literally " begetter", because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. See also: Liquid oxygen and solid oxygen.Oxygen is more soluble in water than nitrogen is. Water in equilibrium with air contains approximately 1 molecule of dissolved O2 for every 2 molecules of N2, compared to an atmospheric ratio of approximately 1: 4. The solubility of oxygen in water is temperature-dependent, and about twice as much (14.6 mg·L^{− 1}) dissolves at 0 °C than at 20 °C (7.6 mg·L^{− 1}).^[27][28] At 25 °C and 1 standard atmosphere (101.3 kPa) of air, freshwater contains about 6.04 milliliters (mL) of oxygen per liter, whereas seawater contains about 4.95 mL per liter.

53.Carbon (from Latin: carbo " coal") is a chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with ¹²C and ¹³C being stable, while ¹⁴C is radioactive, decaying with a half-life of about 5, 730 years. Carbon is one of the few elements known since antiquity.

There are several allotropes of carbon of which the best known are graphite, diamond, and amorphous carbon. Thephysical properties of carbon vary widely with the allotropic form. For example, graphite is opaque and black, while diamond is highly transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek word " γ ρ ά φ ω " which means " to write"), while diamond is the hardest naturally-occurring material known. Graphite is a very good conductor, while diamond has a very low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphenehave the highest thermal conductivities of all known materials.

All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and other transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates. Carbon forms a vast number of compounds, more than any other element, with almost ten million compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all forms of carbon-based life, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life.

54) Nitrogen is a chemical element with symbol N and atomic number 7. It is the lightest pnictogen and at room temperature, it is a transparent, odorless diatomic gas. Nitrogen is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. On Earth, the element forms about 78% of Earth's atmosphere and as such is the most abundant uncombined element. The element nitrogen was discovered as a separable component of air, by Scottish physicianDaniel Rutherford, in 1772. Nitrogen is a nonmetal, with an electronegativity of 3.04.^[13] It has five electrons in its outer shell and is, therefore, trivalent in most compounds. The triple bond in molecular nitrogen (N
2) is one of the strongest. The resulting difficulty of converting N
2 into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2, have dominated the role of nitrogen in both nature and human economic activities. At atmospheric pressure, molecular nitrogen condenses (liquefies) at 77 K (− 195.79 °C) and freezes at 63 K (− 210.01 °C)^[14] into the beta hexagonal close-packed crystalallotropic form. Below 35.4 K (− 237.6 °C) nitrogen assumes the cubic crystal allotropic form (called the alpha phase).^[15] Liquid nitrogen, a fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen.

55 Sodium /ˈ soʊ diə m/^[4] is a chemical element with symbol Na (from Latin: natrium) and atomic number 11. It is a soft, silver-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is ²³Na. The free metal does not occur in nature, but instead must be prepared from its compounds. It was first isolated byHumphry Davy in 1807 by the electrolysis of sodium hydroxide. Sodium is the sixth most abundant element in the Earth's crust, and exists in numerous minerals such asfeldspars, sodalite and rock salt (NaCl). Many salts of sodium are highly water-soluble. Sodium ions have been leached by the action of water so that sodium and chlorine (Cl) are the most common dissolved elements by weight in the Earth's bodies of oceanic water.Physical.Sodium at standard temperature and pressure is a soft silvery metal, that oxidizes to grayish white unless immersed in oil or inert gas. Sodium can be readily cut with a knife, and is a good conductor of electricity. These properties change dramatically at elevated pressures: at 1.5 Mbar, the color changes from silvery metallic to black; at 1.9 Mbar the material becomes transparent, with a red color; and at 3 Mbar sodium is a clear and transparent solid. All of these high-pressure allotropes are insulators andelectrides.^[5]

⁵⁶⁾ Aluminium (or aluminum; see spelling differences) is a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic, ductile metal. Aluminium is the third most abundant element (after oxygen andsilicon), and the most abundant metal in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals.^[7] The chief ore of aluminium is bauxite. Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.^[9] Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.