Upload
johnson-ea
View
220
Download
0
Embed Size (px)
Citation preview
7/30/2019 ISL Entalphy
1/9
ceppoi. (2009, 10 17). Termodinamik 1. Retrieved March 12, 2013, from Scribd.: perbezaan di antara
haba tindak balas piawai dan haba tindak balas
Yahoo Answer. (2008). Retrieved March 12, 2013, from Yahoo:
http://answers.yahoo.com/question/index?qid=20080917141910AA1EcrY
Standard Heat (Enthalpy) of Formation and Reaction. (n.d.). Retrieved March 12, 2013, from Aus-e-tute:
http://www.ausetute.com.au/heatform.html
Blaber, M. (1996 ). Energy Relations in Chemistry: Thermochemistry. Retrieved March 12, 2013, from
Enthalpies of Formation:
http://www.mikeblaber.org/oldwine/chm1045/notes/Energy/HeatForm/Energy05.htm
agray, S. k. (2011, May 18). Factors Affecting Enthalpy of Reaction. Retrieved March 12, 2013, from
Chemist Desk: http://chemistry-desk.blogspot.com/2011/05/factors-affecting-enthalpy-of-
reaction.html
Enthalpy. (n.d.). Retrieved March 12, 2013, from Thermodynamics:
http://www.chem.tamu.edu/class/majors/tutorialnotefiles/enthalpy.htm
Perubahan entalpi piawai
Perubahan fizikal
Perubahan entalpi piawai yang dihasilkan adalah akibat dari peruba
(Enthalpy)hankeadaan fizikal disebut entalpi peralihan piawai
Hotr
. Contoh
H otr ialahpelakuran ( H olak), peruapan (Howap), penghidratan (Hohid).
Perubahan kimia
Entalpi tindak balas piawai ialah perubahan entalpi apabila reaktan dalamkeadaan
piawai berubah kepada produk dalam keadaan piawai (RujukNota CHM 3010).
Bagi tindak balas am2A + B 3C + DPerubahan entalpi tindak balas,Hot.b=Hom- H om(34)
produk reaktan= {3H om (C) + H om (D)} {2H om (A) + H om (B)}dimana
H om
7/30/2019 ISL Entalphy
2/9
ialah entalpi molar piawai.Entalpi piawai bagi tindak balas masing-masing boleh
digabungkan untukmendapatkan entalpi piawai tindak balas lain iaitu menggunakan
hukumHess. Entalpi piawai pembentukan sesuatu bahan ialah entalpi tindak
balaspiawai bagi pembentukan sebatian dan unsur-unsurnya dalam
keadaanpiawai. (Terdapat beberapa entalpi piawai yang lain-nyatakan).
Perubahan entalpi terhadap suhu
Apabila bahan dipanaskan dari T
1
kepada T2
entalpi berubah dari H(T1)kepada H(T2) di manaH(T2) = H(T1) + 21T T P
dT C
(35)Maka perubahan entalpi tindak balas piawai berubah dari
H ot.b(T1)kepada Ht.b(T2) di mana
2. What is the difference between heat of reaction, enthalpy change and heat of
formation?
Best Answer- Chosen by Voters
For any reaction there is an enthalpy change. The "heat of reaction" is the change in
enthalpy for that reaction.
The "heat of formation" of a compound is the enthalpy change for the reaction where
the compound is made from the elements in their standard states at 25C and 1 atm.
CH4(g) + O2(g) --> CO2(g) + 2H2O(g) DH = -890.4 kJThis is the heat of reaction for the combustion of one mole of methane is -890.4 kJ.
C(graphite) + 2H2(g) --> CH4(g) DH = -75 kJ
This is the heat of formation of methane. One mole of CH4 is made from the elements
in their standard states.
7/30/2019 ISL Entalphy
3/9
3. (Standard Heat (Enthalpy) of Formation and Reaction)Key Concepts
Standard Heat (Enthalpy) of Formation, Hfo, of any compound is the enthalpy
change of the reaction by which it is formed from its elements, reactants and
products all being in a given standard state.
By definition, the standard enthalpy (heat) of formation of an element in its
standard state is zero, Hfo = 0.
Standard Molar Enthalpy (Heat) of Formation, Hmo, of a compound is the
enthalpy change that occurs when one mole of the compound in its standard
state is formed from its elements in their standard states.
Standard Enthalpy (Heat) of Reaction, Ho, is the difference between the
standard enthalpies (heats) of formation of the products and the reactants.
Ho(reaction) = the sum of the enthalpy (heat) of formation of products - the sum of
the enthalpy (heat) of formation of reactants:
Ho(reaction) = Hof(products) - H
of(reactants)
To calculate an Enthalpy (Heat) of Reaction:
i. Write the balanced chemical equation for the reaction
Remember to include the state (solid, liquid, gas, or aqueous) for each reactant
and product.
ii. Write the general equation for calculating the enthalpy (heat) of reaction:
Ho(reaction) = H
of(products) - H
of(reactants)
iii. Substitute the values for the enthalpy (heat) of formation of each product and
reactant into the equation.
Remember, if there are 2 moles of a reactant or product, you will need to multiply the
enthalpy term by 2, if molar enthalpies (heats) of formation are used.
iv. Solve the equation to find the enthalpy (heat) of reaction.
7/30/2019 ISL Entalphy
4/9
Standard Enthalpy (Heat) of Formation
Example: Standard Enthalpy (Heat) of Formation of Water
The standard enthalpy (heat) of formation for liquid water at 298K (25o) is -286 kJ mol
-1.
This means that 286 kJ of energy is released when liquid water, H2O(l), is produced from
its elements, hydrogen and oxygen, in their standard states, ie, H2(g) and O2(g).
This reaction is written as:
H2(g) + O2(g) -----> H2O(l) Hfo = -286 kJ mol-1
The standard enthalpy (heat) of formation of water vapour at 298K (25o) is -242 kJ mol-1.
This means that 242 kJ of energy is released when gaseous water (water vapour),
H2O(g), is produced from its elements, hydrogen and oxygen, in their standard states, ie,
H2(g) and O2(g).
This reaction is written as:
H2(g) + O2(g) -----> H2O(g) Hfo
= -242 kJ mol-1
4. Energy Relations in Chemistry: Thermochemistry
Enthalpies of Formation
Enthalpies of Formation
Using Hess's Law we can calculate reaction enthalpies for a variety of reactions usingtables of known enthalpies
Many experimentally determined enthalpies are listed by the type of process
7/30/2019 ISL Entalphy
5/9
DH for converting various liquids to the gas phase are listed in tablesofenthalpies of vapor izat ion
DH formelting solids to liquids are listed in tables ofenthalpies of fusio n DH forfor combusting a substance in oxygen are listed in tables ofenthalpies o f
combust ion
The enthalpy change associated with the formation of a compound from its constituentelements is called the enthalpy of fo rmat ion (DHf)
Conditions which influence enthalpy changes include:
temperature pressure state of reactants and products (s, g, l, aq)
The standard stateof a substance is the form most stable at 298 K (25 C, or
standard "room temperature") and 1 atmosphere (1 atm) of pressure
When a reaction occurs with all reactants and products in their standard states, theenthalpy change is the standard enthalpy of react ion(DH)
Thus, the standard enthalpy o f format ion (DHf)of a compound is the change inenthalpy that accompanies the format ion of 1 moleof that substance from its elements,with all substances in theirstandard s tates
The standard enthalpy of formation for ethanol (C2H5OH) is the enthalpy change for thefollowing reaction
Notes:
Elemental source of oxygen is O2 and not O because O2 is the stable form ofoxygen at 25 C and 1 atm, likewise with H2
Elemental source of carbon is specified as graphite (and not, for example,diamond) because graphite is the lowest energy form of carbon at room temp
and 1 atm Why is the O2 stoichiometry left at "1/2"? The stoichiometry of formation
reactions always indicates the formation of 1 mol of product. Thus, DHfvaluesare reported as kJ / mole of the substance produced
If C(graphite) is the lowest energy form of carbon under standard conditions, then whatis the DHf for C(graphite)?
7/30/2019 ISL Entalphy
6/9
By definition, the standard enthalpy of formation of the most stable form ofanyelementis zero because there is no formation reaction needed when theelement is alreadyin its standard state
DHf for C(graphite), H2(g) and O2(g) = 0
Using enthalpies of formation (DHf) to calculate enthalpies of reaction understandard conditions (DHrxn)
We can determine the standard enthalpy change for any reaction (DHrxn) by usingstandard enthalpies of formation (DHf) and Hess's Law
1996 Michael Blaber
5.
Factors Affecting Enthalpy of Reaction
The various factors on whichenthalpyof reaction depend are as follows:
1. Phys ical State of Reactants and Produ cts
The enthalpy of reaction changes with change in physical state. This is
because as the physical state changes, extent heat is evolved.
Example:
H2 (g) + 1/2 O2 (g) ------------> H2O (l) ; DH = 286KJ.
H2 (g) + 1/2 O2 (g) ------------> H2O (g) ; DH = 242 KJ.
2. Quanti ty of Reactants
The change in enthalpy of reaction depends upon the quantity of
reactants used. When the number ofmolesof reactants are doubled, the enthalpy
change also becomes double.
3. Al lotropic Modi f icat ion
For elements existing in different allotropic modifications, the heat of reaction is
different if different allotropic form is involved in reaction.
Example:
C (graphite) + O2 (g) ------------> CO2 (g) ; DH = 393.4 KJ
7/30/2019 ISL Entalphy
7/9
C (diamond) + O2 (g) ------------> CO2 (g) ; DH = 395.4 KJ.
4. Temperature and Pressure
The enthalpy of reaction depends upon the temperature and
pressure of reaction. Therefore, the values are generally expressed under standard
conditions of temperature (298K) and pressure (1 atm.)
6.
Thermodynamics : EnthalpyEnthalpy (H) - The sum of the internal energy of the system plus the product of thepressure of the gas in the system and its volume:
After a series of rearrangements, and if pressure if kept constant, we can arrive at thefollowing equation:
where H is the Hfinal minus Hinitial and q is heat
Enthalpy of Reaction ( H)
enthalpies of the reactants:
In the above reaction, n and m are the coefficients of the products and the reactants inthe balanced equation.
7/30/2019 ISL Entalphy
8/9
Exothermic - Reaction in which asystem RELEASESheat to its surroundings.
H is negative ( H < 0)
Ea is the activation energy which isdiscussed in moredetail in the kinetics unit. (SeeActivationEnergy notes)
Endothermic - Reaction in which asystem ABSORBSheat from its surroundings.
H is positive ( H > 0)
Let's distinguish various phase changes of water as either endothermic or exothermic.
1) The above reaction is EXOTHERMIC because heat is released when liquid waterfreezes to form ice.
2) The above reaction is ENDOTHERMIC because there must be an input of energy inorder for water molecules in the liquid phase to have enough energy to escape into thegas phase.
3) The above reaction is ENDOTHERMIC because there must be an input of energy tobreak the bonds holding water molecules together as ice.
http://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htmhttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htmhttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htmhttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htmhttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htmhttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/activation.htm7/30/2019 ISL Entalphy
9/9
Standard-State Enthalpy of Reaction ( H )
Three factors can affect the enthalpy of reaction:
The effects of changes in these factors can be shown relative to the standard-state
enthalpy of reaction ( H ) which is the change in the enthalpy during a chemicalreaction that begins and ends under standard-state conditions.
Standard-state conditions
partial pressures of any gases involved in the reaction is 0.1 MPa.