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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


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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.


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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.


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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


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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)?


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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


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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.


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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.htm


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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.

isl entalphy

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