THERMOCHEMICAL

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: H2 → 2H - a kJ; DH = + AKJb. Bond formation provides energy (= exothermic)Example: H2 → 2H + a kJ; DH =-a kJThe 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: H2 (g) + 1/2 O2 (g) → H20 (l); DHF = -285.85 kJ2. Enthalpy of decomposition:DH from the decomposition of 1 mole of the compound directly into its elements (= opposite of DH formation).Example: H2O (l) → H2 (g) + 1/2 O2 (g), DH = +285.85 kJ3. Standard Enthalpy of Combustion (DHC):DH to burn 1 mole of compound with O2 from the air measured at 298 K and pressure of 1 atm.Example: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l); DHC = -802 kJ4. 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 3H2SO4 → Al2 (SO4) 3 + 3H2; DH = -1468 kJ5. Enthalpy of Neutralization:DH generated (always exothermic) in acid or base neutralization reaction.Example: NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l), DH = -890.4 kJ / mol6. 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 versaExample:N2 (g) + 3H2 (g) → 2NH3 (g), DH = - 112 kJ2NH3 (g) → N2 (g) + 3H2 (g), DH = + 112 kJ• Enthalpy of Formation, Combustion and DecompositionThermochemical 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 0 or 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 FormationThere 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 CombustionThe 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.• Burning Perfect and ImperfectFuel 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 kJb. Incomplete combustion of isooctane:C8H18 (l) + 8 ½ O2 (g) -> 8 CO (g) + 9 H2O (g) ΔH = -2924.4 kJThe impact is not perfect BurningAs 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 DecompositionDecomposition 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 -1H2O (l) -> H2 (g) + ½ O2 (g) ΔH = + 286 kJ• Energy AssociationA chemical reaction involving the breaking and formation of chemical bonds. Termination of chemical bonds causing substances reagent separated into their constituent atoms. These atoms would be rearranged and regrouped to form chemical bonds in these substances the reaction products. Termination of a bond requires energy. Conversely, a bond formation would release some energy. Energy associated with the termination or formation of chemical bonds is called bond energy.Energy bond dissociation energy to be divided into compound molecules, and lattice energy for ionic compounds. Dissociation energy (D) related to the energy required to break the bond compound molecular / covalent. While the lattice energy associated with the energy released to the formation of ionic bond compound. In berikutknya discussion focused on the dissociation energy.• Institute for Energy Calculating Reaction Enthalpy ChangeAs described above, the chemical reaction will involve energy for breaking the bond between atoms bond formation between the reactants and reaction products atom. The difference between the energy for breaking and bond formation is ΔH. Can be formulated as follows:ΔHreaksi = Σ (bond energy reactants) - Σ (bonding energy of the reaction product)Consider an example of decomposition reaction H2O (endothermic reaction) and formation of CH4 (exothermic reaction) below.1. H2O decomposition reactionH2O can be broken down into gaseous H2 and O2 gases, with the help of the energy gained from the electric current.There are two stages in the reaction as shown in enthalpy diagram in Figure 2: 
1. Termination 2 bond H - O into atoms H and O. energy required is equal to Σ (bond energy reagent)2. The formation of two bonds H - H and 1 O = O bond of atom H and O. the energy released is equal to Σ (bond energy reaction product).From the diagram, we see that Σ (bond energy of the reactants) <Σ (bond energy of the reaction product). Using the formula ΔH above, the obtained reaction ΔH is positive (+) or the reaction is endothermic.
 2.Reaksi formation of CH4Combustion reaction CH4 with O2, CO2 and gas produced H2O



 QUESTION:
mengapa (-DH/mol=+Q),and (+DH/mol= -Q) ?