Energy and Chemical Reaction

Enthalpy changes

Enthalpy = H = heat of reaction at constant pressure = Qp
The enthalpy change is the change in energy that accompany chemical changes incident at a constant pressure.

a. Termination of the bond requires energy (= endothermic)
Example: H 2 → 2H - a kJ; DH = + AKJ

b. Bond formation provides energy (= exothermic)
Example: 2H → H 2 + a kJ; DH =-a kJ

The term used in the enthalpy change:
1. Standard Enthalpy Pembentakan (DHF):
     DH animal lays to form 1 mole of compound directly from its elements were measured at 298 K and pressure of 1 atm.
Example: H 2 (g) + 1/2 O 2 (g) → H 2 0 (l); DHF = -285.85 kJ

2. Enthalpy of decomposition:
     DH from the decomposition of 1 mole of the compound directly into its elements (= opposite of DH formation).
Example: H 2 O (l) → H 2 (g) + 1/2 O 2 (g), DH = +285.85 kJ

3. Standard Enthalpy of Combustion (DHC):
     DH to burn 1 mole of compounds with O 2 from the air measured at 298 K and pressure of 1 atm.
Example: CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O (l); DHC = -802 kJ

4. Enthalpy of reaction:
     DH 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 + 3H 2 SO 4 → Al 2 (SO 4) 3 + 3H 2; DH = -1468 kJ

5. Enthalpy of Neutralization:
     DH generated (always exothermic) in acid or base neutralization reaction.
Example: NaOH (aq) + HCl (aq) → NaCl (aq) + H 2 O (l), DH = -890.4 kJ / mol

6. Lavoisier-Laplace law
     "The amount of heat released in the formation of one mole of a substance from the 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:
N 2 (g) + 3H 2 (g) → 2NH 3 (g), DH = - 112 kJ
2NH 3 (g) → N 2 (g) + 3H 2 (g), DH = + 112 kJ
Enthalpy of formation, combustion and decomposition
Thermochemical Data are generally set at a temperature of 25 0 C and a pressure of 1 atm, hereinafter referred to standard conditions. Enthalpy changes were measured at a temperature of 25 0 C and a pressure of 1 atm is called the standard enthalpy change and is expressed with the symbol Δ H 0atau ΔH298. While the changes in enthalpy measurement not refer to the condition of measurement is expressed with the symbol ΔH alone.
Is the molar enthalpy of the reaction enthalpy change associated with the quantity of the substance involved in the reaction. In the known thermochemical various molar enthalpy, such as enthalpy of formation, enthalpy of decomposition, and the enthalpy of combustion.
Enthalpy of Formation
There are a variety of important thermochemical equation associated with the formation of one mole of unsurunsurnya. The enthalpy change associated with this reaction is called the heat of formation or the enthalpy of formation of a given symbol ΔH f. For example, the thermochemical equation for the formation of water and steam at 100 0 C and 1 atm, respectively.



How can we use this equation to get the heat of evaporation of water? What is clear equation (1) we must reverse and then summed with the equation (2). Do not forget to change the sign of ΔH. (If the formation of H 2 O (l) exothermic, as reflected by a negative ΔH f, the reverse process must be endothermic), which means a positive exothermic which means being endothermic.

Exothermic

Exothermic (heat producing)

Endothermic

If we add the equations (1) and (2), we can

And the hot reaction =

Note that the heat of reaction to all the changes with the heat of formation reaction proceeds minus the heat of formation of the reactants. In general it can be written:

Enthalpy change for reaction rates can be affected by the temperature and pressure conditions when measurements. Therefore, the necessary conditions of temperature and pressure must be specified for each data thermochemical.
Enthalpy of Combustion

The reaction of a substance with oxygen reaction called combustion. Substances that are combustible elements carbon, hydrogen, sulfur, and various compounds of these elements. Said to be perfect if the combustion of carbon (c) burned to CO2, hydrogen (H) burned into H2O, sulfur (S) burned to SO2.
Enthalpy change for the combustion of 1 mol of a substance is measured at 298 K, 1 atm is called the standard enthalpy of combustion (standard enthalpy of combustion), which is expressed by Δ Hc 0. Enthalpy of combustion is also expressed in kJ mol -1.
Price enthalpy of combustion of various substances at 298 K, 1 atm are given in Table 3 below.
Table 3. Enthalpy of combustion of various substances at 298 K, 1 atm



Burning gasoline is an exothermic process. If gasoline is considered consisting of isooctane, C8H18 (a component of gasoline) determine the amount of heat released in the combustion of 1 liter of gasoline. Given the enthalpy of combustion of isooctane = -5460 kJ mol -1, and the density of isooktan = 0.7 kg L -1 (H = 1 and C = 12).
Answer:
Enthalpy of combustion of isooctane is - 5460 kJ mol -1. The mass of 1 liter of gasoline = 1 liter x 0.7 kg L-1 = 0.7 kg = 700 grams. Isooctane mole = 700 g mol -1 gram/114 = 6.14 mol. So the heat is released in the combustion of 1 liter of gasoline is: 6.14 x 5460 kJ mol = 33524.4 kJ mol -1.
Combustion Perfect and Imperfect

Fuel combustion in vehicle engines or the industry does not burn completely. Complete combustion of hydro-carbon compounds (fossil fuels) to form carbon dioxide and water vapor. While imperfect combustion to form carbon monoxide and water vapor. For example:
a. Complete combustion of isooctane:
C8H18 (l) +12 ½ O2 (g) -> 8 CO2 (g) + 9 H2O (g) ΔH = -5460 kJ

b. Incomplete combustion of isooctane:
C8H18 (l) + 8 ½ O2 (g) -> 8 CO (g) + 9 H2O (g) ΔH = -2924.4 kJ
The impact is not perfect Burning
As seen in the example above, incomplete combustion produces less heat. Thus, imperfect combustion reduces fuel efficiency. Another disadvantage of incomplete combustion produces gases are carbon monoxide (CO), which are toxic. Therefore, incomplete combustion will pollute the air.

Enthalpy of decomposition

Decomposition reaction is the opposite of a reaction formation. Therefore, in accordance with the principle of conservation of energy, equal to the value of the enthalpy of decomposition enthalpy of formation, but opposite sign.
Example:
Given Δ Hf 0 H2O (l) = -286 kJ mol -1, the enthalpy of decomposition of H2O (l) into hydrogen gas and oxygen gas is + 286 kJ mol -1
H2O (l) -> H2 (g) + ½ O2 (g) ΔH = + 286 kJ

Determination of Reaction Enthalpy Change

The enthalpy change (DH) for a reaction can be determined in various ways, namely through experiments, based on data from the enthalpy change of formation DHf0 Hess law, and based on bond energies.

a. Determination DH Through Experiments

Enthalpy change of the reaction can be determined by using a device called a calorimeter (heat meter). In the calorimeter, a substance that will be reacted reaction put into place. The place is surrounded by water of known mass. Heat of reaction liberated absorbed by water and the water temperature will rise. Changes in water temperature is measured with a thermometer. Calorimeter placed in an insulated container filled with water to prevent the escape of heat.

 Figure Calorimeter

Based on this research, to raise the temperature of 1 kg of water at 10C required heat of 4.2 kJ or 1 kcal. Water required to heat 1 gram of 4.2 A or 1 cal. Amount of heat is called the specific heat of water with the symbol c. The amount of heat that is absorbed into the water is calculated by multiplying three factors: the mass of water in the calorimeter (g), changes in water temperature (0C), and the specific heat of water. The formula is written: q = heat released or absorbed

m = mass of water (g)

c = heat capacity of water (J)

Dt = change in temperature (0C)

b. DH Determination Under DHf0

Based on the change in the standard enthalpy of formation of substances that are present in the reaction, the reaction enthalpy change can be calculated by the formula:

 DHr0 = standard reaction enthalpy change enthalpy of formation of some substance changes can be seen in the table below.

 Table: Changes in the enthalpy of formation of some substance (t = 250C)

Enthalpy change of reaction sometimes can not be determined directly but must go through the stages of the reaction. For example, to determine the change in enthalpy of formation of CO2 can be done in various ways.

 In one way, the reaction is a single stage, while a 2 way and 3 way last two stages. Apparently a number of ways, the same enthalpy change is -394 kJ.

A scientist, German Hess, had done some research enthalpy change and the result is that the reaction enthalpy change of a reaction does not depend on the course of reaction, whether the reaction takes place one stage or several stages. This discovery is known as Hess's Law, which reads:

 According to Hess's study, the change in enthalpy of a reaction can not be determined with the calorimeters can be determined by calculation. Here is an example of the determination of the enthalpy change calculations.

c. Determination DH Based Energy Association

A chemical reaction caused by chemical bonds breaking and formation of chemical bonds of the new. At the time of the formation of chemical bonds of atoms will be the release of energy, whereas the energy required to break the tie. The amount of energy required to break the tie antaratom in 1 mole of gaseous molecules called bonding energy. The stronger the bond the greater the energy required. Some bond energy prices can be seen in the following table:

Table: Some bond energy prices

 Price of the bond energy can be used to determine the H 􀀨 a reaction.

 With this formula can also be determined average bond energy of a molecule and the energy needed to break a bond or bond dissociation energy of a molecule. Here is an example calculation using the DH bond energy prices.

Thermochemistry is a branch of chemistry that
study the relationship between the reaction with
heat.

THINGS LEARNED
• energy changes that accompany chemical reactions
• A chemical reaction takes place spontaneously
• Chemical reactions in the equilibrium position


1. Exothermic reaction
Is a reaction that takes place when accompanied by the release of heat or heat. The heat of reaction is written with a positive sign.
Example:
          N2 (g) + 3H2 (g) 2NH3 (g) + 26.78 kcal
2. REACTION endothermic
Is a reaction that takes place when needed heat. The heat of reaction written de
with a negative sign
Example:
         2NH3 N2 (g) + 3H2 (g) - 26.78 kcal



I Law of Thermodynamics: Law of conservation of mass and
energy, ie energy can not be created and dimusnah
it.
Mathematically formulated as follows:
1. Whenever there is a change in the energy system, the amount of energy change is determined by two factors:
    a. heat energy absorbed (q)
    b. effort (work) done by the system (w)

For systems that absorb heat → q: positive (+)
For systems that emit heat → q: negative (-)

For systems that do business (working) → w: positive
If efforts are made to system → w: negative

Energy system will increase if: q (+) and w (-)
Energy system will be reduced if: q (-) and w (+)
Applicable:
ΔE = q - w

ΔE = change in energy
  q = heat energy absorbed
  w = work done by system

2. His relationship with the enthalpy (H)
The definition of enthalpy:

H = E + P.V

- If P is fixed, ΔH:
ΔH = H2 - H1
= (E2 + P2. V2) - (E1 + P1.V1)
= (E2 - E1) - (P2.V2 - P1.V1)
= (E2 - E1) + P (V2 - V1)
ΔH = ΔE + P.ΔV
Since ΔE = qp - P.ΔV, then:
ΔH = qp-P.ΔV + P.ΔV
ΔH = qp
So the change in enthalpy = heat changes that occur
In the (P, T fixed)



HK. II. THERMODYNAMICS:
• NOT formulated MATHEMATICAL
• Explained SOME EVENTS ASSOCIATED WITH BOTH HK THERMODYNAMICS
1. Spontaneous Processes and Not Spontaneous
Spontaneous process: a process that can take place by itself and can not return without outside influence. Example:
a. Heat always flows from high temperature to tem
peratur low.
b. Gas flows from high pressure to low pressure
c. Water flows from a high to a low.

The process is not spontaneous: a process can not take place without any outside influence. Example:
heat can not flow from low temperature to a high temperature without any outside influence.

ENTROPY (s)
• In addition to the change in enthalpy, chemical or physical change involves a change in the chaos (disorder) relative of atoms, molecules or ions. Chaos (disorder) of a system called ENTROPY
Example:
• The gas is contained in a 1 L flask has a greater entropy than the same quantity of gas to be placed in a 10 ml flask.
• Sodium Chloride ions in the form of gas has higher entropy than the solid crystalline form.
• Water (liquid) at a temperature of 0oC has higher entropy than the ice at the same time.

The amount of entropy in the universe always increases
The more irregular: S increased.

Statement of Third Law of Thermodynamics:
• A perfect crystal at absolute zero temperature has the perfect order → entropy is zero.
• Entropy of a substance compared to the entropy in the form of a perfect crystal at absolute zero, is called the Absolute Entropy
• The higher the temperature of the substance, the greater the absolute entropy