Olympiads

Enthalpy and Thermochemical Equations?
Thermochemical equations are just like other balanced equation except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:

H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ

HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ

When you write thermochemical equations, be sure to keep the following points in mind:

1. Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H2O (l) is formed from 1 mol H2 (g) and ½ mol O2.
2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.
3. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats formation notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.

Certain laws or rules apply when using thermochemical equations:

1. ΔH is directly proportional to the quantity of a substance that reacts or is produced by a reaction.

Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example:

H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ

2 H2 (g) + O2 (g) → 2 H2O (l); ΔH = -571.6 kJ
2. ΔH for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction.

For example:

HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ

Hg (l) + ½ O2 (l) → HgO (s); ΔH = -90.7 kJ

This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction.
3. ΔH is independent of the number of steps involved.

This rule is called Hess's Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction.

If Reaction (1) + Reaction (2) = Reaction (3), then ΔH3 = ΔH1 + ΔH2


Latent Heat


When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.

If we measure the temperature of the substance which is initially solid as we heat it we produce a graph like Figure(below).

Temperature change with time. Phase changes are indicated by flat regions where heat energy used to overcome attractive forces between molecules

Starting a point A, the substance is in its solid phase, heating it brings the temperature up to its melting point but the material is still a solid at point B. As it is heated further, the energy from the heat source goes into breaking the bonds holding the atoms in place. This takes place from B to C. At point C all of the solid phase has been transformed into the liquid phase. Once again, as energy is added the energy goes into the kinetic energy of the particles raising the temperature, (C to D). At point D the temperature has reached its boiling point but it is still in the liquid phase. From points D to E thermal energy is overcoming the bonds and the particles have enough kinetic energy to escape from the liquid. The substance is entering the gas phase. Beyond E, further heating under pressure can raise the temperature still further is how a pressure cooker works.

Latent Heat of Fusion and Vaporisation

The energy required to change the phase of a substance is known as a latent heat. The word latent means hidden. When the phase change is from solid to liquid we must use the latent heat of fusion, and when the phase change is from liquid to a gas, we must use the latent heat of vaporisation.

The energy require is Q= m L, where m is the mass of the substance and L is the specific latent heat of fusion or vaporisation which measures the heat energy to change 1 kg of a solid into a liquid.

Table show the Latent Heat !!!!




Olympiads
Every student in grade 12th who wants to get through IIT –JEE bears a dream: To qualify Olympiads. Olympiads in Math, Physics, Chemistry begin at regional (RMO) level and go upto International level (INO). Some websites where you can check out information, questions and other related topics are given below:

• Indian National Math Olympiad
• Indian Physics & Chemistry Olympiads
• International Physics Olympiad
• International Chemistry Olympiad
• International Science Olympiads
• International Math Olympiad (IMO)
• IMO Problems Archive
• Asian Physics Olympiad


Books for Olympiads:

• GEOMETRY

1. Durrel M. A., Modern Geometry Macmillan & Co., London.
2. H. S. M. Coxeter and S. L. Greitzer, Geometry Revisited Mathematical Association of America.
3. S. L. Loney, Plane Trigonometry Macmillan & Co., London.


• NUMBER THEORY

1. I. Niven & H. S. Zuckerman An Introduction to the Theory of Numbers Wiley Eastern Ltd. New Delhi.
2. David Burton Elementary Number Theory Universal Book Stall, New Delhi.
3. G. H. Hardy & Wright An introduction to the theory of numbers, Oxford University Publishers.


• JOURNALS

1. Samasya, journal devoted to problem solving, published by Leelavati trust, Bangalore.
2. Bona Mathematica, published by Bhaskaracharya Prathistana , Pune.

Here are some excellent books on chemistry in general:

* Molecules.
Peter W. Atkins.
Scientific American Library, Freeman Press, 1987. [See this book at Amazon.com]

* Chemistry: Principles and Applications.
Peter W. Atkins.
Longman, 1988. [See this book at Amazon.com]

* The Periodic Kingdom: A Journey into the Land of the Chemical Elements.
Peter W. Atkins.
Basic Books, 1995. [See this book at Amazon.com]

* Designing the Molecular World: Chemistry at the Frontier .
Philip Ball.
Princenton Univ Press, 1994. [See this book at Amazon.com]

* Chemical Principles, 4th Edition.
Richard E. Dickerson, H. B. Gray, M. Y. Darensbourg.
Benajamin/Cummings, 1984. [See this book at Amazon.com]

* Chemistry of the Elements.
Norman N. Greenwood and A. Earnshaw.
Pergamon, 1984. [See this book at Amazon.com]

* General Chemistry.
Linus Pauling.
Dover Books, 1989. [See this book at Amazon.com]

* Chemical Curiosities: Spectacular Experiments and Inspired Quotes.
H. W. Roesky, K. Mockel, Roald Hoffman.
Vch Pub, 1996. [See this book at Amazon.com]

The authoritative data book, which can be found in every self-respecting chemistry and physics laboratory is:

* CRC Handbook of C hemistry and Physics, 78th Edition. (table of contents)
David R. Lide (editor-in-chief) and H.P.R. Frederikse (assoc.ed.)
CRC Press, 1997. [See this book at Amazon.com]

There exist student editions of this book as well. Older editions of the CRC Handbook are also still very useful.