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In the previous study, we developed detailed means to observe and measure the energy changes in chemical reactions. This ability is valuable all on its own, since managing the flow of energy from one form to another is a vital economic activity. But our work is not half done. In Chemistry, we seek not just to observe and measure, but also to model and to understand conceptually. We can make this point clearly by thinking about the following. Some chemical reactions produce energy, even in spectacular amounts. The detonation of a single gram of trinitrotoluene (TNT) produces about 4.2 kJ of energy. The reaction of a single gram of sodium metal (Na) with water produces about 8 kJ of energy. On the other hand, some reactions absorb energy, often evidenced by a significant cooling of the products or the surroundings of the reaction. For example, the hydration of ten grams of ammonium nitrate (NH 4 NO 3 ) in an instant cold pack absorbs about 3.2 kJ of energy, causing it to be cold enough to treat minor athletic injuries.

How can we account for these great variations in the energies of reactions? Where does the energy come from in an exothermic reaction, and where does it in an endothermic reaction? Could we find a way to predict whether a reaction will be exothermic or endothermic? Answering these questions requires us to develop a model for energy transfer during chemical reactions.


We will build significantly on the results of the previous concept study. We know how to measure energy changes in reactions. A reaction which releases energy into the environment is called an exothermic reaction, and the heat transfer q<0. A reaction which absorbs energy from the environment is called an endothermic reaction, and the heat transfer q>0.

Hess’ Law, developed in the previous concept study, is an extremely important observation. Recall that Hess’ Law tells us that the energy of a reaction is equal to the sum of the energies of a set of reactions which add up to the overall reaction. Stated differently, the energy of a reaction does not depend on what “path” we follow in converting reactant to products, whether it be in a single reaction or a series of reactions. As long as we start with the same reactants and wind up with the same products, the energy of the reaction is the same.

Although this is not an observation or previous conclusion, we’ll add to our foundation a definition of a new quantity, called “enthalpy.” To understand the usefulness of this new quantity, let’s remember that, according to Hess’ Law, if we start with a set of reactants and carry out a series of reactions which recreate the reactants, then the total energy change summed over that series of reactions has to be exactly zero. Using the Law of Conservation of Energy, this makes sense. We would not expect to be able to change the energy of a substance or substances without changing the state of those substances. In fact, for this reason, chemists call the energy of a substance a “state function,” meaning that the energy depends only on what state the substance is in (gas, liquid, solid; temperature; pressure).

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
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Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
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