ENERGY AND CHEMICAL REACTION

Energy and Enthalpy

Every object in the universe has energy. you remember, what is energy? Simply put, energy is defined as the ability to do work. Energy that an object can be muscle energy, wind energy, chemical energy, heat energy, and form - Other forms of energy. A form of energy can be converted into other forms of energy. Thus, energy can be used for human needs. How is the energy change that?Almighty God created the sun terpentinng energy source for life. Sunlight is absorbed by plants is converted into chemical energy in the form of carbohydrate. The ingredients of foods containing carbohydrates, such as rice, corn, wheat, or cassava, consumed by humans for kelangasungan life. Raw foods are digested by the body and produces heat energy.If a piece of wood burned, the chemical energy which is owned wood energy is converted into heat. What is the amount of heat energy generated from burning wood?The total amount of heat energy contained in the matter called enthalpy and given the symbol H. Enthalpy term derived from the German, meaning womb. The symbol H derived from the heat content, which is defined as the heat content of a substance. Enthalpy of a substance does not change (fixed) for which there is no energy in or out.Enthalpy of a substance can not be measured, but the enthalpy change can be measured. Enthalpy change occurs when a substance having a reaction. The enthalpy change ΔH given notation, Δ symbol derived from the Greek delta, which means change. In a chemical reaction, the reaction enthalpy ΔH called. ΔH states received or heat released by the reaction. In other words, ΔH is the addition or subtraction of energy a substance in the process of change in energy that takes place at a constant pressure.Have you ever noticed ice melting events? In this event there is a change of water from a solid form (ice) to liquid.H2O (s) → H2O (l)In this process, you can not measure the enthalpy of H2O (s) or enthalpy of H2O (l), which can be determined only change in enthalpy or enthalpy of reaction (ΔH).ΔH = H H2O (l) - H H2O (s)

Yield ΔH = H - H initialFor chemical reactions,The reaction ΔH = H - H reagent

Endothermic and exothermic reactions

• endothermic reaction is a reaction that is accompanied by heat transfer from the environment to the system (heat absorbed by the system from its environment), characterized by a decrease in the ambient temperature around the system.
• exothermic reaction is a reaction that is accompanied by the transfer of heat from the system to the environment (heat released by the system to the environment), marked by a rise in the ambient temperature around the system.
• exothermic reaction generally takes place spontaneously, while endothermic reactions are not.
• In the endothermic reaction: DH = Hp - Hr> 0 (is positive)
• In an exothermic reaction: DH = Hp - Hr <0 (the negative) 

exothermic reaction

Exothermic reaction is a chemical reaction that produces heat. In this reaction, there is heat transfer from the system to the environment so that the environment becomes hotter. Exothermic reaction will release energy so that the enthalpy of the system decreases and the enthalpy change is negative.
Combustion reactions, such as burning wood, burning methane, propane combustion, and the reaction between aluminum powder and iron oxide are examples of exothermic reactions. Another example of an exothermic reaction, the reaction between calcium (CaO) and water. The reaction produces calcium hydroxide (Ca (OH) 2) by the following equation.

CaO + H20 ----------> Ca (OH) 2 + Heat

Your hands can feel the heat released by this reaction. Your hands (including Environment) receives heat from the reacting system. 

  

 Endothermic reactionEndothermic reaction is a reaction that requires energy or absorb energy from the environment when the reaction occurs, the reaction generally produces dingin.atau temperature endothermic reaction is a chemical reaction that absorbs heat. Heat is taken from the environment so that the enthalpy increases.In endothermic reactions absorb energy systems. Therefore, the enthalpy of the system will increase, meaning that the enthalpy of the product (HP) is greater than the enthalpy of the reactants (HR). Consequently, the change in enthalpy (ΔH), namely the difference between the enthalpy of the products with the enthalpy of the reactants is positive. So:ΔH = HP - HRActivation EnergyΔH> 0From the graph above, it can be seen that ΔH = HP - HR. Because HR is smaller than HP. Then ΔH> 0Examples of endothermic reaction is a reaction to the melting of ice in a container.Examples of the chemical reaction to melt the ice events are:Sign (+) to 6.02 kJ left arrow indicates that heat is absorbed by 6.02 kJ.REACTION ice meltsThus, the reaction can also be written as follows:In reaction endoderm, heat (q) absorbed so positive enthalpy change (ΔH> 0). The reaction can be illustrated on the chart as well.REACTION EXAMPLE endothermicOther examples of endothermic reactions in everyday life include:- Dissolution of sugar. If the sugar is dissolved, it can be felt there was a bit of a cold when the glass is held
   Cold hands when in contact with alcohol- Assimilation
 - Photosynthesis- Burning kerosene oil stove
 - Blazing campfire when campingThe difference between exothermic and endothermic reactionsA. Exothermic reactionExothermic reaction occurs heat transfer from the system to the environment or to the reaction heat released.
 In an exothermic reaction the price DH = (-)Example: C (s) + O2 (g) → CO2 (g) + 393.5 kJ; DH = -393.5 kJB.Reaksi endothermicEndothermic reaction occurs heat transfer from the environment to the system or to the reaction heat is needed.
 In endothermic reactions price DH = (+)Example: CaCO3 (s) → CaO (s) + CO2 (g) - 178.5 kJ; DH = +178.5 kJSimple understanding of endothermic reactions1. The reaction requires heat2. Temperature C system
 A + B ----> C - 25 kjC 25 kJ of heat absorbed
 r = p - 25 kj
 r <h = "Hp"> il ΔH = +

 

Equation Thermochemistry

•  Is the equation that includes the enthalpy change (DH).
• DH value is written in the thermochemical equation, adjusted for reaction stoichiometry, the = number of moles of a substance involved in a chemical reaction = reaction coefficient; (phase reactants and reaction products to be written).

    Example:In the formation of 1 mol of water from hydrogen gas with oxygen at 298 K, 1 atm released heat of 285, 5 kJ.Thermochemical Equations:

If the coefficient multiplied by 2, then the price of DH reactions must also be multiplied by 2.

• Some things to consider in writing the thermochemical equation:

1. Reaction coefficient indicates the number of moles of the substance involved in the reaction.
2. When the equation is reversed (to change the location of the reactants with the product) then the value of DH remains the same but opposite sign.
3. If we multiply both sides of the equation by a factor thermochemical DH y then the value should be multiplied by the factor y.
4. When writing thermochemical equation, phase reactants and products must be written.

9). Types of Enthalpy Changes

•  Enthalpy changes were measured at 25 ° C and a pressure of 1 atm (standard state) is called the standard enthalpy change (represented by the sign of DHO or DH298).
• The enthalpy change that does not refer to the condition expressed by the symbol DH measurement alone. 
•  Molar enthalpy = enthalpy change per mole of a substance (kJ / mol).

    The enthalpy change, include:

    a. Standard enthalpy change of formation (DHF o) = heat of formationIs the enthalpy change that occurs in the formation of one mole of a compound from its elements at standard temperature and pressure (25 ° C, 1 atm). Enthalpy can be released or absorbed. Its units are kJ / mol.Standard form of an element is the most stable form of the element in the standard state (298 K, 1 atm).If the change in enthalpy of formation was not measured at the standard state is denoted by DHFExample:Note:

•  DHF free element is zero
• In the enthalpy of formation, the amount of substance produced is 1 mol.
• Formed from its elements in standard form.

    b. Standard enthalpy change of decomposition (DHD o)Is the enthalpy change that occurs in the decomposition of 1 mole of a compound into its constituent elements in their default state.If the measurements are not performed at the standard state, denoted by the DHD. Unit = kJ / mol.Standard enthalpy change of decomposition is the reverse of the standard enthalpy change of formation, its value would be opposite in sign.According to the Marquis de Laplace, "the amount of heat released in the formation of compound constituent elements = amount of heat needed to decomposition of the compound into its constituent elements. "This statement is called the Law of Laplace.Example:Given DHF o H2O (l) = -286 kJ / mol, the enthalpy of decomposition of H2O (l) into hydrogen gas and oxygen gas is +286 kJ / mol.

    c. The standard enthalpy change of combustion (DHC o)Is the enthalpy change that occurs in the burning of one mole of a substance completely on the state standards.If the measurements are not performed at the standard state, it is denoted by the DHC. Unit = kJ / mol.Example:

    d. The standard enthalpy change of neutralization (DHN o)Is the enthalpy change that occurs in 1 mole of acid neutralization by alkali or alkaline by 1 mole of acid in the standard state.If the measurements are not performed at the standard state, then denoted by DHN. Unit = kJ / mol.Example:DHN reaction = -200 kJDHN NaOH = -200 kJ / 2 mol = -100 kJ / molDHN H2SO4 = -200 kJ / 1 mol = -200 kJ / mol

    e. Evaporation enthalpy changes Standards (DHovap)Is the enthalpy change that occurs in the evaporation of one mole of a substance in the liquid phase to the gas phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHvap. Unit = kJ / mol.Example:

    f. Standard enthalpy change of fusion (DHofus)Is the enthalpy change that occurs in the melting / smelting 1 mole of a substance in the solid phase a substance in the liquid phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHfus. Unit = kJ / mol.Example:

    g. Standard Sublimation enthalpy changes (DHosub)Is the enthalpy change that occurs in sublimation 1 mole of a substance in the solid phase a substance in the gas phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHsub. Unit = kJ / mol.Example:

    h. Standard Dissolution enthalpy changes (DHosol)Is the enthalpy change that occurs when one mole of a substance dissolves in a solvent (usually water) at standard conditions.If the measurements are not performed at the standard state, then denoted by DHsol. Unit = kJ / mol.

fuel and enthalpy change

 Combustion reaction is the reaction of a substance with oksigen.Biasanya this kind of reaction is used to produce energy. The fuel is a compound that is produced against biladilakukan combustion heat can be used for various purposes. 
Type of fuel that much we know is a fossil fuel. Fossil fuels are derived from the weathering of the remaining organisms, both plants and animals that would take thousands to millions of years, such as petroleum and coal bara.Namun than today's fossil fuels have also developed other types of fuel, such as alcohol and hydrogen. Liquid hydrogen with liquid oxygen together have been used on the space shuttle as its fuel rocket boosters. Burning hydrogen does not have a negative impact on the environment as a result of its combustion is water. 
The sun is the biggest energy umber earth, but the use of solar energy is not yet commercially available. Today, the commercial use of solar energy is for heating domestic water (solar water heater). Below is the calorific value of different types of fuel commonly known.




Calorific value of the fuel is expressed in units of its common kJ / g, which states how many kJ of heat that can be generated from the combustion of 1 gram of fuel. Example: calorific value of gasoline is 48 kJ / g, which means that every 1 gram of gasoline combustion heat will be generated by 48 kJ. Fuel combustion in vehicle engines or its industry generally do not burn completely. Complete combustion of hydrocarbons (fossil fuels) to form carbon dioxide and water vapor. While its incomplete combustion produces carbon monoxide and steam air.Pembakaran imperfect reduce fuel efficiency, the heat produced will be less than if the substance burn completely. Her other disadvantage is his produced carbon monoxide (CO) which is racun.Nilai calorific fuels can be used to estimate the energy of a fuel price

Hess's Law

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

    \ Delta H ^ \ theta = \ Sigma (\ Delta H_ {f ~ product} ^ \ theta) - \ Sigma (\ Delta H_ {f ~ reactants} ^ \ theta).For other reactions in general

    \ Delta H ^ \ theta = \ Sigma (\ Delta H_ {products} ^ \ theta) - \ Sigma (\ Delta H_ {reactants} ^ \ theta).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 = fREnthalpy change for a reaction can also be predicted from the change in enthalpy pembakaranreaktan and products, with the formula

        ΔH =-ΔHcP + ΔHcRSource 1: wikipedia.comThe sound of Hess's Law:"The amount of heat required or released in a chemical reaction does not depend on the course of the reaction but is determined by the initial state and the end."Hess's Law states that,"Enthalpy of a reaction is not affected by the course of the reaction but it just depends on the initial state and the final state. So to determine the enthalpy of a reaction we can get to take all available avenues "That is to determine the enthalpy of a single reaction then we can combine some of the reaction as a "path" to determine the enthalpy of a single reaction. The final results will be obtained will show the same value.For example:Enthalpy of formation of NO2 can we find the following reaction:N2 (g) + O2 (g) -> 2NO2 (g) = 68 KJ deltaHBy knowing the standard enthalpy of formation of NO2 then we can calculate the amount of the value of enthalpy for the reaction above.Or we could calculate it by using a combination of several reactions (minimum 2 comments and even more) the following:reaksipembentukanNO2With mengethaui magnitude enthalpy of reaction I and II above, we can find the enthalpy of formation of NO2. Of course we had to set a reaction with other reactions that will eventually add up all these reactions will be obtained diingkan reaction.Then what is the use of the law Hess? One of the benefits of Hess's law is that we can calculate the enthalpy of a reaction that is very difficult to measure the laboratory.Things that need to be considered in the application of Hess's Law is:

    We can combine some of the reactions that have been known to obtain enthalpy enthalpy of reaction we were looking for.
    The opposite of a reaction enthalpy resulting changes sign, meaning that if an exothermic reaction runs the reverse reaction is endothermic with enthalpy sign a mutually berlawanan.reaksikebalikan
    If a reaction is multiplied by a number is the enthalpy of the reaction must also be multiplied by the same number. reaction