Thermochemical
A.
Exothermic and endothermic reactions
1.
Exothermic reaction
In an exothermic
reaction occurs heat transfer of the system to
environment or to the
reaction heat released.? In an exothermic reaction ΔH price = (-)
Example: C (s) + O2 (g) CO2
(g) + 393.5 kJ; ΔH = -393.5 kJ
Endothermic reaction?? In endothermic reactions occur heat
transfer from the environment to
system or to the reaction
heat is needed.? In endothermic reactions price ΔH = (+)
?? Example: CaCO3 (s) CaO (s) + CO2 (g) - 178.5 kJ; ΔH =
+178.5 kJ
. Enthalpy changes
Enthalpy = H = heat of
reaction at constant pressure = Qp? The change in enthalpy is the energy change
accompanying events
chemical changes at a
constant pressure.
a. Termination of the bond
requires energy (= endothermic)? Example: H2 2H - a kJ; ΔH = + AKJ
b. Bond formation provides
energy (= exothermic)? Example: H2 + 2H a kJ; ΔH =-a kJ
The term used in the enthalpy change:
Standard Enthalpy Pembuntakan (ΔHf):? ΔH animal lays to
form 1 mole of compounds directly from the elements
elements were measured at 298 K and
pressure of 1 atm.
Example: H2 (g) + 1/2 O2 (g) H20
(l); ΔHf = -285.85 kJ
Enthalpy of Decomposition:? ΔH of decomposition of 1 mole
of the compound directly into its elements (= Contrary to ΔH formation).
Example: H2O (l) H2 (g) + 1/2 O2
(g), ΔH = +285.85 kJ
Standard Enthalpy of Combustion (ΔHc):? ΔH to burn 1 mole
compound with O2 from the air measured at 298 K and pressure of 1 atm.
Example: CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l); ΔHc = -802
kJ
Enthalpy of reaction:? ΔH of an equation in which
substances contained in the equation is expressed in units of moles and the
coefficients of the equation is simple round.
Example: + 2AL 3H2SO4 Al2 (SO4) 3 + 3H2; ΔH = -1468 kJ
Enthalpy of Neutralization:? ΔH generated (always
exothermic) on the neutralization of acid or alkaline reaction.
Example: NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l);
ΔH
= -890.4 kJ / mol
Lavoisier-Laplace law? "The amount of heat released in
the formation of one mole of substance
elements unsurya = amount of heat
required to decompose the substance into its constituent elements. "?
Meaning: If the reaction is reversed the sign of the heat that is formed is
also reversed from positive to negative or vice versa
Example:? N2 (g) + 3H2 (g)
2NH3 (g), ΔH = - 112 kJ? 2NH3 (g) N2 (g) + 3H2 (g), ΔH = + 112 kJ
C. Determination of Enthalpy Changes and Hess's Law
1. Determination of Enthalpy Changes
To determine the enthalpy
change in a chemical reaction
commonly used tools such as
the calorimeter, thermometer and
etc., that may be more
sensitive.
Calculation: ΔH reaction = Δ;
ΔHfo products - Δ = ΔHfo reactants
2. Hess's Law
"The amount of heat required
or released in a reaction
does not depend on the course
of chemical reactions but is determined by
initial state and the end.
"
example :
According to Hess's Law: x = y + z
D. Energy-Energy and Chemical Bonding
A
chemical reaction is a process of dissolution and formation
ties.
The process is always accompanied by energy changes. The energy
needed
to break chemical bonds, thus forming
free
radicals called the bond energy. For molecules
complex,
the energy required to break the molecule
thus
forming free atoms called atomization energy.
Atomization energy prices is the amount
of the bond energy of the atoms in the molecule. For covalent molecule
consisting of two atoms such as H2, 02, N2 or HI which has a bond equal to the
energy of atomization energy bond energy of atomization of a compound can be
determined by the help enthalpy of formation of these compounds. Mathematically
it can be described by the equation.
Example:
Given:
bond energy
C - H = 414.5 kJ / mole? C = C = 612.4 kJ / mol? C -
C = 346.9 kJ / mol? H - H = 436.8 kJ / mol?? Asked:
ΔH reaction = C2H4 (g) + H2 (g) C2H6 (g)
Answer:
ΔH reaction = Total bond breaking energy - amount of
energy
bond
formation
=
(4 (C-H) + (C = C) + (H-H)) - (6 (C-H) + (C-C))? = ((C = C) + (H-H)) - (2 (C-H)
+ (C-C))? = (612.4 + 436.8) - (2 x 414.5 + 346.9)? = - 126.7 kJ
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BalasHapusI was there in the article section of the energies and chemical bonds, the energy required to break chemical bonds to form free radicals called the binding energy, while the energy needed to break down molecules to form free atoms called atomization energy. Atomization energy prices is the amount of the bond energy of the atoms in the molecule. For covalent molecule consisting of two atoms such as H2, 02, N2 or HI that has one bond, then atomisasinya energy equal to the binding energy. I ask is, why to have a two-atom molecule has a bond? continued how to molecules that have more than two atoms? atomization energy is equal to the bond energy or larger or smaller?
BalasHapusI think if the atoms form molecules there must be a bond depends atoms that make up some of the atoms, eg H2 memebentuk he would have a different case with phosphorus bond he would form some bonds EXAMPLES P4
BalasHapusI think why to have a two-atom molecule has a bond for compounds that form a stable diatomic or polyatomic. Therefore to Formatting a bond must be attractive forces between the ions in the opposite sign and the style of tie between the two, but it is also when the bond formation is a change in the total energy of the atom.
BalasHapusbut for the next question I can not answer.
thanks pipit.
I think the two atoms have a bond because the chemical bonds that bind atoms correspond to electron bonding where atoms can obtain a stable electron configurations by sharing electrons lend. so that way the atoms form a single bond.
BalasHapusI think because the molecule has two atoms that simple, so its bond energy a bit, but if more than 2 then the atom is complex, its many ties ...
BalasHapus