Energy and Chemical Reactions

A chemical reaction is a process of breaking and bond formation. The process is always accompanied by energy changes. The energy required to break chemical bonds, thus forming free radicals called the bond energy. For complex molecules, the energy required to break the molecule 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 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:

DH = S bond breaking energy - S energy bond formation

     =  S bond energy on the left - S bond energy in the right

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:
D H = reaction C2H4 (g) + H2 (g) ® C2H6 (g)


 H 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



 Enthalpy and Enthalpy Changes

1. System and environment
The system is a chemical reaction that is being observed or studied. While the so-called environment is everything outside the system. Example: if we are reacting metals Mg and HCl in a test tube, the substances contained in the test tube called a system and apart from that is the environment.

In the example above the center of attention is the metal Mg and HCl solution. Thus, metallic Mg and HCl solution called the system, whereas the reaction tube, air temperature, air pressure is the environment. Based on interaction with the environment, the system can be divided into three kinds, namely:

a. Open System, a system that allows the exchange of heat and matter (material) between the environment and the system.
b. Closed system, a system which allows the exchange of heat between the system and its environment, but not an exchange of material.
c. Isolated Systems (sealed), a system that does not allow the exchange of heat between the system and the environment danmateri.


2. Energy and enthalpy
In any chemical reaction is always a change of energy.
Energy units:
1 calorie = 4.184 Joules
1 kJ = 1000 Joules
1 kcal = 1000 calories
1kkal = 4.184 k J

Overall energy possessed by a system in a particular state is called energy (U). Energy in a state function, depends only on the state of the system (temperature, volume, pressure, and number of moles), does not depend on the path through the system. Energy can not be measured but the changes can be measured. If the change occurs at a constant pressure (open system), the energy changes that occur in the so-called enthalpy change.

Chemical reactions are generally carried out in an open system (fixed pressure).
Therefore, in any process that involves a change in volume due to constant pressure, there is the work that accompanies the process although a small but significant.
According to the Law of Thermodynamics I (Law of Conservation of Energy),
H = U + PV
The enthalpy change is expressed by the equation:
H = U + PV
From the equation it can be concluded that if the reaction is carried out at a constant pressure the heat changes that occur will be equal to the enthalpy change for the pressure change 0 (Zero). Thus, the enthalpy equal to the amount of energy stored in a system. So the enthalpy (H) is the energy stored in the form of heat in a system.

3. Enthalpy change
The enthalpy change of a system can be measured if the system is changing.
Enthalpy changes (H):
If a reaction takes place at a constant pressure, the change in enthalpy equal
the heat that must be removed from the system into the environment or otherwise that
temperature of the system back to its original state.
H = qp (qp = heat of reaction at constant pressure)
The magnitude of the enthalpy change is the difference in magnitude of the enthalpy of the system after a change in the magnitude of the enthalpy of the system before the changes at a constant pressure.

H = H final - H initial

The enthalpy change accompanying a reaction is influenced by:
• The number of substances
• physical state of a substance
• Temperature (T)
• Pressure (P)

4. Reaction was exothermic and endothermic reactions
1. Exothermic reaction is a reaction that releases heat or energy from the system to the environment.
Enthalpy reduced system (the reaction products have a lower enthalpy than the original substance).
H final <H initial
H final - H initial <0
H negative value

Example:
The reaction between calcium oxide (quicklime) and Cretaceous shallow water put into the water in a test tube. This reaction takes place is characterized by a mixture of temperature rise (the system). Because the temperature of the system is higher than the environment, the heat will come out of the system into the environment until they become the same temperature.
CaO (s) + H2O (l) Ca (OH) 2 (aq)

2. Endothermic reaction is a reaction that absorbs heat or energy needs from the environment to the system.
Enthalpy system is improved (the reaction has a higher enthalpy than the original substance).
H final> H initial
H final - H initial> 0
H positive value

Example:
The reaction between barium hydroxide crystals oktahidrat with ammonium chloride crystals.
When barium hydroxide crystals oktahidrat, Ba (OH) 2. 8H2O crystals mixed with ammonium chloride (NH4Cl), the reaction takes place immediately marked by a drop in temperature and the formation of a mixture of ammonia gas. Therefore temperature of the mixture (the system) will be lower than the environment, the heat will flow from the environment into the system until they become the same temperature. Ba (OH) 2. 8H2O (s) + 2NH4Cl BaCl2.2H2O (s) + 2NH3 (g) + 8H2O (l)

5. Thermochemical Equations
The equation that describes the reaction with the information about the change in enthalpy (heat). Because enthalpy is extensive properties (value depends on the magnitude and size of the system) then the thermochemical equation also listed the number of moles of a substance expressed reaction coefficient, and the phase state of the substance involved.

Example:
a. In the formation of 1 mol of water from hydrogen gas with oxygen gas at 25 ° C
(298 K), 1 atm, was 286 kJ of heat is released.
Thermochemical equations of the above statement is
The word "released" state that quite exothermic reaction. Therefore, the
H = -286 kJ per mole of water formed.
H2 (g) + O2 (g) H2O () H = -286 kJ
or,
2H2 (g) + O2 (g) 2H2O () H = -572 kJ

b. The reaction of carbon and hydrogen to form 1 mole of C2H2 at temperatures of 25oC
and a pressure of 1 atm requires 226.7 kJ of heat.
Termokimianya equation:
The word "need" states that belong to endothermic reactions.
2 C (s) + H2 (g) C2H2 (g) + H = 226.7 kJ

6. Standard enthalpy change (Ho)
Reaction enthalpy changes measured at 25oC temperature (298 K) and
pressure of 1 atm was agreed as the standard enthalpy change, otherwise
with the symbol Ho
The state standard is needed because measurements at different temperatures and pressures will result in prices that different enthalpy changes. Units used to express the change in enthalpy is kJ. Changes in molar enthalpy is kJ / mol.

Enthalpy change based on the type of chemical changes that occur:
1. Standard enthalpy change of formation (Hf o)
(Hf o = standard enthalpy of formation)
Is the enthalpy change for the formation of 1 mole of a compound from its elements are the most stable, the standard state. Unit change in standard enthalpy of formation according to the International System (SI) is kJ / mol.
Example:
Changes from the standard enthalpy of formation of carbon dioxide (CO2) is -393.5 kJ / mol.
Termokimianya equation:
C (s) + O2 (g) CO2 (g) Hf
o = -393.5 kJ / mol

2. Standard enthalpy change of decomposition (Hd o)
(Hd o = standard enthalpy of decomposition)
Is the enthalpy change for the decomposition of 1 mole of a compound into its elements, the standard state.
Example:
Enthalpy change for the decomposition of H 2 O is +286 kJ / mol.
Termokimianya equation:
H2O () H2 (g) + O2 (g) Hd
o = + 286 kJ / mol

3. Standard enthalpy change of combustion (Hc o)
(Hc o = standard enthalpy of combustion)
Is the change in enthalpy in the combustion of 1 mol element or compound at standard conditions. Combustion is the reaction of a substance with oxygen.