# 5.3 Enthalpy  (Page 2/25)

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As discussed, the relationship between internal energy, heat, and work can be represented as Δ U = q + w . Internal energy is a type of quantity known as a state function    (or state variable), whereas heat and work are not state functions. The value of a state function depends only on the state that a system is in, and not on how that state is reached. If a quantity is not a state function, then its value does depend on how the state is reached. An example of a state function is altitude or elevation. If you stand on the summit of Mt. Kilimanjaro, you are at an altitude of 5895 m, and it does not matter whether you hiked there or parachuted there. The distance you traveled to the top of Kilimanjaro, however, is not a state function. You could climb to the summit by a direct route or by a more roundabout, circuitous path ( [link] ). The distances traveled would differ (distance is not a state function) but the elevation reached would be the same (altitude is a state function).

Chemists ordinarily use a property known as enthalpy ( H )    to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum of a system’s internal energy ( U ) and the mathematical product of its pressure ( P ) and volume ( V ):

$H=U+PV$

Since it is derived from three state functions ( U , P , and V ), enthalpy is also a state function. Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined. For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change (Δ H )    is:

$\text{Δ}H=\text{Δ}U+P\text{Δ}V$

The mathematical product P Δ V represents work ( w ), namely, expansion or pressure-volume work as noted. By their definitions, the arithmetic signs of Δ V and w will always be opposite:

$P\text{Δ}V=\text{−}w$

Substituting this equation and the definition of internal energy into the enthalpy-change equation yields:

$\begin{array}{l}\text{Δ}H=\text{Δ}U+P\text{Δ}V\\ ={q}_{\text{p}}+w-w\\ ={q}_{\text{p}}\end{array}$

where q p is the heat of reaction under conditions of constant pressure.

And so, if a chemical or physical process is carried out at constant pressure with the only work done caused by expansion or contraction, then the heat flow ( q p ) and enthalpy change (Δ H ) for the process are equal.

The heat given off when you operate a Bunsen burner is equal to the enthalpy change of the methane combustion reaction that takes place, since it occurs at the essentially constant pressure of the atmosphere. On the other hand, the heat produced by a reaction measured in a bomb calorimeter ( [link] ) is not equal to Δ H because the closed, constant-volume metal container prevents expansion work from occurring. Chemists usually perform experiments under normal atmospheric conditions, at constant external pressure with q = Δ H , which makes enthalpy the most convenient choice for determining heat.

what does the term carbon-12 scale mean
what are the six types of elements
carbon,hydrogen,nitrogen,oxygen,phosphorus and sulphur....#the CHNOPS element
Amanda
👍
clement
those are the six types of elements I know
Amanda
Yea
clement
thats all
Davy
which table is flammable
I haven't gotten to tables o 😥
Rebecca
pls how can I give an IUPAC nomenclature in organic chemistry
Paul's exclusion principle
what do you mean by d block ,give more details element
d block elements are also called transition metals. It includes elements from group 3-12 and d block elements hold up to 10 electrons in their shell
Amanda
all?
Davy
yea
Amanda
Define chemical formula
they are the mathematical representation of chemicals I guess
Uche
Bohr model of hydrogen atom
what is important of this model
nagesh
what is the main reaction between sodium and chlorine
when a sodium atom is transferred an electron to a chloride atom forming a sodium cation and a chlorde anoin both ions are compltely valence shells and a energetically more stable this reaction is extremely more exothermic producing a bright yellow light abd a great deal of heat energy
sani
well what kind of sodium
coland
sodium chloride
coland
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Dehydrocholic acid is a synthetic bile acid, manufactured by the oxidation of cholic acid. It acts as a hydrocholeretic, increasing bile output to clear increased bile acid load.
coland
Ethylenediaminetetraacetic acid (EDTA), also known by several other names, is an aminopolycarboxylic acid and a colourless, water-soluble solid. Its conjugate base is ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ("s
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Sodium ascorbate is one of a number of mineral salts of ascorbic acid (vitamin C). The molecular formula of this chemical compound is C6H7NaO6. As the sodium salt of ascorbic acid, it is known as a mineral ascorbate. It has not been demonstrated to be more bioavailable than any other form of vitamin
coland
Diatrizoic acid (or its anionic form, diatrizoate), also known as amidotrizoic acid, or 3,5-diacetamido-2,4,6-triiodobenzoic acid, is a radiocontrast agent containing iodine. Trade names include Hypaque, Gastrografin, Iothalmate and Urografin, the latter being a combination of the sodium and meglum
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Sodium acetrizoate (rINN, trade names Urokon, Triurol and Salpix), the sodium salt of acetrizoic acid, is a high-osmolality, water-soluble, iodine-based radiographic contrast medium no longer in clinical use. Acetrizoate was developed by V.H. Wallingford of Mallinckrodt, and introduced in 1950; it
coland
i think some was missed
coland
what chlorine L-Alpha glycerylphosphorylcholine (alpha-GPC, choline alfoscerate) is a natural choline compound found in the brain. It is also a parasympathomimetic acetylcholine precursor which may have potential for the treatment of Alzheimer's disease and other dementias. Alpha-GPC rapidly delive
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Choline Theophyllinate , also known as oxtriphylline, is a cough medicine derived from xanthine that acts as a bronchodilator to open up airways in the lung. Chemically, it is a salt of choline and Theophylline. It classifies as an expectorant. The drug is available under the brand names Choledyl an
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Choline (/ˈkoʊlin/) is a water-soluble vitamin. It is usually grouped within the B-complex vitamins. The term cholines refers to the class of quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation (X− on the right denotes an undefined counteranion). The cation appears in the
coland
thanks
sani
BABARINDE
hydrogen reacting with water
what is matter
matter is anything that has Mass and can occupy space
Moshood
apart from those device there is there any device
how many elements do we have
Modern Electronic Theory
a new way or an improvement in modern electrical products.
Andrew
find the volume of oxygen produced from the electrolysis of acidified water of a current of a 2A was passed through the electrolysis of acidified water for 2 hours
14400J
Ese
wrong question, go through it. oxygen cannot be produced.
Andrew