Energy+Changes+in+Chemical+Reactions



=__All__ chemical reactions involve an energy change.= which means involve the transfer of energy, usually in the form of heat, into or out of the reaction mixture. When petrol burns heat is given out. When ammonium nitrate dissolves in water heat is taken in.
 * __ For example __**:

And now let’s find out the details of the heat changes in the chemical reactions!

=Molecules are held by chemical bond=
 * reference:www.davis.k12.ut.us/staffrcarr/chemistry.html

Thus, in order to break the bond energy is needed to added to the bond, this fact is as the same as when you are going to break a rope or a string, you need to add energy on it, the bond is just like the spring. On the opposite, if two atoms want to combine together in order to form a molecule, and not simply coliding and flying apart, these two atoms need to give off the excess energy.

**__ For example: __** Do you remeber the example that our Mr. Standring give us? About Remeo and juliet. Break this couple is vey hard and you need lots of "energy", but when they are allowed to stay together, they will be together automatically, then you save lots of energy. ^ ^~




 * reference: http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Bond_Energies

=Next, there appear two different situations.=
 * 1) If the reactants have weak bonds, while the products have strong bonds than the reaction. Then the reaction will takes less energy to break the bonds than it gives off to form bonds, the reaction will net give off energy which is called exothermic rreaction.
 * 2) If the reactants have strong bonds, but the products have weak bonds. Then the reaction takes more energy to breask the bonds than it gives off to form bonds, the reaction will net input energy.

=Defintion:= > __** For example **__:is a exothermic reaction.(reactans are combining bonds) Note: The coefficients always refer to "moles" when the heat is expressed in KJ of energy, but we don't need to know how to predict the amount of energy in Chemistry 11. *reference: [] 2. An **//__en__//**dothermic reaction absorbs heat from its surroundings.(**//__En__//**: Heat **//__enters__//** the reaction.) For example: is a endothermic reaction.(reactant is breaking bonds) *reference: [] Note:Each type of bond requires a different amount of energy during the breaking process, various reactants change into product, the products can have various energy.
 * 1) An **__//ex//__**othermic reaction gives off heat to its surroundings.( //**__Ex:__** heat **__exits__** from the reactants)//
 * //The reaction like synthesis, combination or combusion of the hydrocarbon can be always considered exothermic, and the reactions like decomposition can be cosidered as endothermmic.//**

PS: Actually we need to draw the energy changes graph which includes an __**activation energy//.//**__
 * reference:http://www.avogadro.co.uk/h_and_s/enthalpy.htm

In chemistry, __**activation energy**__ is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction. The activation energy of a reaction is usually denoted by Ea, and given in units of kilojoules per mole.

=Reacting chemicals and their surroudings.= --In the **//__exothermic__//** reaction, the chemicals give energy to their surroundings, since less energy is used to break the bonds in the reactants than is give off producing the bonds in the reaction. The surroundings absorbs the energy and become **__//warmer//__**. __** For example **__, the respiration of the plants: C12H22O11+12O2=12CO2+11H2O+5638KJ

--In the **//__endothermic__//** reaction, the chemicals absorb energy from their surroundings, since more energy is used to break the bonds in the reactants than is given off producing the bonds in the products. The surroundings lose energy to the reactants, the surroundings feel **//__cooler__//**. __** For example **__, an ice cube absorbsn heat from your hands in order to melt. your hand then feels cooler. Also, the photosynthesis reaction of the plants:12CO2+11H2O+5638KJ=C12H22O11(SUGAR)+12O2
 * reference:http://www.helpchemistry.net/chemistry-tutorials-states-of-matter.html

Melting, evaporation,sublimation are endothermic. Freezing, condensation,resublimation are exothermic.**
 * These are the examples of the endothermic and exothermic reaction.

=Enthalpy= Enthalpy is the heat contained in a system. The total enthalpy of a system cannot be measured directly; the //enthalpy change// of a system is measured instead. Enthalpy change is defined by the following equation: Δ//H// = //H//final (product)− //H//initial (reactant) whereΔ//H// is the //enthalpy change,// //H//final is the final enthalpy of the system, expressed in joules. In a chemical reaction, //H//final is the enthalpy of the products. //H//initial is the initial enthalpy of the system, expressed in joules. In a chemical reaction, //H//initial is the enthalpy of the reactants. Δ//H>0,means HreactantsHproducts, it is an exothermic.//__**
 * __Remeber:

As a result, exothermic and endothermic can be shown like this:
 * endothermic:** A+50KJ=B OR A=B; Δ//H=+50KJ//
 * exothermic:** C=D+80KJ OR C=D; Δ//H**= —**80KJ
 * Remeber that if heat is on the left, then**// **Δ//H is positive. if heat is on the right,// Δ**//**H is negative.**//

=Information references:=

1. textbook 2. Mr. Standring 3. [] ([] and []) 4. http://www.baidu.com/ ([])