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Laws of Thermochemistry
Thermochemical equations are just like other balanced equations 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:
- 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.
- 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.
- 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 of 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.
- 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
- 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.
- 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
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Roads from plastic waste
The debate on the use and abuse of plastics vis-a-vis
environmental protection can go on, without yielding
results until practical steps are initiated at the grassroots
level by everyone who is in a position to do something
about it. The plastic wastes could be used in road
construction and the field tests withstood the stress and
proved that plastic wastes used after proper processing
as an additive would enhance the life of the roads
and also solve environmental problems. The present
write-up highlights the developments in using plastics
waste to make plastic roads.
Plastic is everywhere in today’s lifestyle. It is used for
packaging, protecting, serving, and even disposing
of all kinds of consumer goods. With the industrial
revolution, mass production of goods started and
plastic seemed to be a cheaper and effective raw
material. Today, every vital sector of the economy
starting from agriculture to packaging, automobile,
building construction, communication or infotech has
been virtually revolutionised by the applications of
plastics. Use of this non-biodegradable (according to
recent studies, plastics can stay unchanged for as long
as 4500 years on earth) product is growing rapidly and
the problem is what to do with plastic-waste. Studies
have linked the improper disposal of plastic to problems
Inventions With Impact
Polymers for chemical sensing and a process for alternative fuels reap prizes
Linda Wang
Two researchers whose inventions are making—or have the potential to make—a broad impact on society are being recognized with prestigious awards from the Lemelson-MIT Program.
Lemelson-MIT ProgramDETECTING Swager demonstrates a handheld monitor that checks people, clothing, and automobiles for trace explosives.
Timothy M. Swager, the John D. MacArthur Professor of Chemistry and head of the chemistry department at MIT, is the winner of this year's $500,000 Lemelson-MIT Prize. The award recognizes Swager's development of highly sensitive semiconducting fluorescent polymers that can detect traces of chemicals found in explosives.
Swager's invention is widely used by American soldiers in Iraq to detect explosives. Specifically, it is being used in handheld monitors to check people, clothing, and automobiles for trace TNT.
Swager is now working on electrical resistance-based polymers that can detect changes in the levels of nitric oxide, an important indicator of a person's health. These sensors could be used by doctors, for example, as an early diagnostic of a respiratory infection.
Mascoma Corp.LYND
COMMON REACTION MECHANISMS
A reaction mechanism is a series of smaller reactions that form an overall reaction. Substitutions and elimination reactions are very important in the study of organic chemistry mechanisms.[2]
[change]Unimolecular substitution (SN1)
A unimolecular substitution mechanism occurs in steps. First, an atom called the leaving group breaks away from a molecule, leaving the molecule with a positive electric charge. This intermediate is called a carbocation. Second, a nucleophile forms a chemical bond with the carbocation. If the nucleophile has a neutral charge, a third step is required. The third step uses a reagent to remove the positive charge. The shorthand symbol for unimolecular substitution is SN1.
[change]Bimolecular substitution (SN2)
In a bimolecular substitution, a nucleophile, which is a reagent that forms a chemical bond, replaces another atom attached to a molecule. The original atom is called the leaving group because it leaves the molecule. The bonding of the nucleophile and the departure of the leaving group are concerted and happen at the same time instead of one at a time. The shorthand symbol for a bimolecular substitution is SN2.
[change]Unimolecular elimination (E1)
The first step of a unimolecular elimination reaction is an atom, the leaving group, breaks away from a molecule. The loss of this atom forms an intermediate carbocation. Then a basic reagent attacks a hydrogen atom and forces a double bond. The reagent in an elimination reaction is a base. These are the same steps as in a bimolecular elimination, but they are not concerted and occur one at a time. The shorthand symbol for unimolecular elimination is E1.
[change]Bimolecular elimination (E2)
The reagent in an elimination reaction is a base. The base takes a proton from the leaving group, forcing electrons to form a double chemical bond and break the bond to the leaving group. Like bimolecular substitution, this is a concerted step. The shorthand symbol for bimolecular elimination is E2.
ENERGY
In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. A reaction is said to be exothermic if the reaction releases heat to the surroundings; in the case of endothermic reactions, the reaction absorbs heat from the surroundings.
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Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor e − E / kT - that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation. The activation energy necessary for a chemical reaction can be in the form of heat, light, electricity or mechanical force in the form of ultrasound.[45]
A related concept free energy, which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, inchemical thermodynamics. A reaction is feasible only if the total change in the Gibbs free energy is negative, 
; if it is equal to zero the chemical reaction is said to be at equilibrium.
; if it is equal to zero the chemical reaction is said to be at equilibrium.There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of quantum mechanics, which require quantization of energy of a bound system. The atoms/molecules in a higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive; that is, more amenable to chemical reactions.
The phase of a substance is invariably determined by its energy and the energy of its surroundings. When the intermolecular forces of a substance are such that the energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with water (H2O); a liquid at room temperature because its molecules are bound by hydrogen bonds.[46] Whereas hydrogen sulfide (H2S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker dipole-dipole interactions.
The transfer of energy from one chemical substance to another depends on the size of energy quanta emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the phonons responsible for vibrational and rotational energy levels in a substance have much less energy than photons invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, ultraviolet electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy.
The existence of characteristic energy levels for different chemical substances is useful for their identification by the analysis of spectral lines. Different kinds of spectra are often used in chemical spectroscopy, e.g. IR, microwave, NMR, ESR, etc. Spectroscopy is also used to identify the composition of remote objects - like stars and distant galaxies - by analyzing their radiation spectra.
The term chemical energy is often used to indicate the potential of a chemical substance to undergo a transformation through a chemical reaction or to transform other chemical substances.
The Nobel Prize 2010 in Chemistry announced- "Palladium catalysed cross-coupling in organic synthesis
"The key word here is versatility," Negishi said. "One of our dreams is to be able to synthesize any organic compounds of importance, whether it is medicinally important
compounds ... or important from the point of view of material science. And we believe that our technology or our chemistry will be applicable to a very wide range of compounds, without knowing what they might be."
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The 2010 Nobel Prize in chemistry was awarded Wednesday to three professors for a tool to make carbon-carbon bonds in organic chemistry, the Royal Swedish Academy of Sciences announced.
The professors are Richard Heck of the University of Delaware, Ei-Ichi Negishi of Purdue University and Akira Suzuki of Hokkaido University.
The tool the professors devised is called palladium-catalyzed cross-coupling, according to the committee.
Negishi told reporters by phone from the United States that he was asleep when he heard the news an hour earlier. He said winning the prize was a dream of his, but he didn't know whether he would win.
The tool has applications in a wide range of fields, from agriculture and pharmaceuticals to coatings for electronic components, like chips, the Nobel committee said.
The tool the professors devised is called palladium-catalyzed cross-coupling, according to the committee.
Negishi told reporters by phone from the United States that he was asleep when he heard the news an hour earlier. He said winning the prize was a dream of his, but he didn't know whether he would win.
The tool has applications in a wide range of fields, from agriculture and pharmaceuticals to coatings for electronic components, like chips, the Nobel committee said.
compounds ... or important from the point of view of material science. And we believe that our technology or our chemistry will be applicable to a very wide range of compounds, without knowing what they might be."
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