ENERGY AND CHEMICAL REACTION"ENTHALPY" (CH 5)

Enthalpy changeA.PERUBAHAN enthalpy STANDARD (ΔH0)Measurement standard state enthalpy change is at a temperature of 298 K and pressure of 1 atm. The state standard is needed because measurements at different temperatures and pressures will result in different prices enthalpy change pula.Beberapa types of standard enthalpy change, the change in the standard enthalpy of formation (ΔHfo), the standard enthalpy change of decomposition (ΔHdo), and the change in enthalpy standard combustion (ΔHco).a.Perubahan entali establishment of standards (ΔHfo)Standard enthalpy change of formation (standard enthalpy of formation) is the enthalpy change that occurs in the formation of 1 mole of a compound from its elements are most stable at the standard state.Unit peruvbahan standard enthalpy of formation according to the international system (SI) is kilojoul per mole (kJ mol-1).b.Perubahan standard enthalpy of decomposition (ΔHdo)Standard decomposition enthalpy change (standard enthalpy of de composition) refers enthalpy changes that occur in the decomposition of 1 mole of a compound into its elements are most stable at the standard state.Basically, the standard enthalpy change of decomposition as opposed to the standard enthalpy change of formation, then the price would be berlawanya.c.Perubahan standard enthalpy of combustion (ΔHco)Standard combustion enthalpy change (standard entalpy of combustion) is the enthalpy change that occurs in the burning of one mole of a substance completely.Combustion is a reaction of a substance, including the following:1.C (S) + O2 (g) → CO2 (g)2. H2 (g) + ⅟ 2 O2 (g) → H2O (g)3.S (S) + O2 (g) → SO2 (g)B.PENENTUAN enthalpy change1.KALORIMETRIThe enthalpy change is the heat change is measured at constant teknan. Therefore, to determine the enthalpy changes take place in the same way denagn determination done peerubahan heat at constant pressure.The change is a transfer of energy that occurs as a result of perbedaab suhu.jadi, heat pad changes a reaction can be measured by measuring the temperature changes that occur.Measurement of heat changes can be done using a tool called the calorimeterBomb calorimeter (bomb calorimeter) is a specifically designed calorimeter, sehinngga system is really in a state of isolation.Picture: Bomb Calorimeter.In the bomb calorimeter there is a special room called the bomb, crazy filled with oxygen gas at high pressure. Bomb calorimeter in a bertisi immersed in water used to hold the sample combustion reaction.The system in the bomb calorimeter is everything that is in the bomb calorimeter, covering: the reqaksi (bombs), water thermometer, stirrer and others.Combustion reactions that occur in the bomb, it will generate heat and absorbed by the water and the bomb at the same temperature, which are shown by the increase in water temperature. Therefore considered that there is no heat absorbed by the system and in the release into the environment during the reaction, thenqreaksi qbom + q + water = qsisteraor qreaksi qbom + water + q = 0qreaksi = - (q + qbom water)The amount of heat absorbed by the bomb can be calculated by measuring the heat capacity of the bomb.With, Cbom = heat capacity of the bomb (J0C-1 or JK-1)ΔT = change in temperature (° C or K)The amount of heat that causes the temperature change (increase or decrease in temperature) water contained in the calorimeter formulated as:By: m = mass of water in the calorimeter (g)c = specific heat in the calorimeter (J g-1K-1)ΔT = change in temperature (K or ° C)Calorimeter who either have a small heat capacity. That is, the calorimeter is really as an isolated system, so that changes, the heat of reaction in the bombing occurred only affect changes in water temperature or solution in the calorimeter.The reaction takes place in a bomb calorimeter is a reaction that takes place at a constant volume (ΔV = 0), the heat changes that occur in the system will be equal to the change nergi it.ΔU = q + w, where w =-p ΔVPengikuran heat of reaction than the reaction heat of combustion, can be done using a calorimeter at constant pressure. For example, the styrofoam calorimeter which is made from styrofoam cups. Calorimeter of this type generally done to measure the heat of reaction in which the reaction takes place in solution, for example, to measure the heat changes that occur in acid-base neutralization reactions.In a chemical reaction calorimeter takes place at constant pressure (Δp = 0), the heat changes that occur in the system will be equal to the enthalpy change.ΔH = qpIt is therefore considered that there is no heat absorbed by the system and in the release into the environment during the reaction, thenqreaksi + q = q calorimeter systemor qreaksi + q calorimeter = 0qreaksi = - (q + calorimeter qlarutan)If the price of the calorimeter heat capacity is very small, it can be ignored, so that the heat can be considered only changes result in an increase in the calorimeter sahu solution.a, m = mass of the solution in the calorimeter (g)c = specific heat of solution in the calorimeter (J g-1 K-1 or J g-1C-1)ΔT = change in temperature (K or 0C)

C.HUKUM HESS
 a  chemist named GHHess Russian citizens in 1840 researching the truth that is free of chemical reactions on a journey through several stages of heat will affect the reaction.
Hess's Law is a law in physical chemistry for the expansion of Hess in Hess cycle. This law is used to predict the enthalpy change of the law of conservation of energy (expressed as a state function ΔH).
According to Hess's law, because enthalpy is a state function, the enthalpy change of a chemical reaction is the same, although the measures used to obtain different products. In other words, only the initial state and the final effect on the change in enthalpy, not the steps taken to achieve it.
This causes a change in enthalpy of a reaction can be calculated even can not be measured directly. The trick is to perform arithmetic operations on an equation known enthalpy changes. Equations are arranged so that the sum of all the equations will produce the reaction we want. If an equation is multiplied (or divided) by a number, the enthalpy changes must also be multiplied (divided). If the equation is reversed, then the sign of the enthalpy changes must be reversed as well (ie be-ΔH).
In addition, by using the law of Hess, ΔH value can also be determined by the reduction of the enthalpy of formation of the products minus the enthalpy of formation of the reactants. mathematically

For other reactions in general
  
 Hess's Law states that the overall enthalpy change of a process depends only on the initial state and the end of the reaction, and does not depend on the route or the steps in between. By knowing ΔHf (change in enthalpy of formation) of the reactants and products, it can be predicted enthalpy change of any reaction, according to the formula

 ΔHfP-ΔH ΔH = fR

Enthalpy change for a reaction can also be predicted from the change in enthalpy combustion reactants and products, with the formula

         ΔH =-ΔHcP + ΔHcR
The concept of Hess's law can also be extended to calculate the change in the function of other circumstances, such as entropy and free energy. Both of these applications is useful because magnitudes are difficult or can not be measured directly, so the calculations with Hess's law is used as a way to determine