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Energy in Chemistry

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Energy in Chemistry
« on: September 08, 2014, 10:33:20 AM »
Energy in Chemistry During any chemical reaction, an energy change occurs.  Reactions release heat energy, or absorb heat energy from the surroundings. The  ones that release heat energy to the surroundings are called Exothermic Reactions; the ones that absorb energy from the surroundings are called Endothermic  Reactions. In exothermic reactions, the reactants are higher in energy than the  products, in endothermic reactions; the reactants are lower in energy than the  products. What makes a reaction exothermic or endothermic: To start a reaction, a certain amount of energy is given to  the reactants; this is called the Energy of activation because it activates the  reaction. This energy is used to break the bonds between the atoms or molecules  of the reactants, then the reactants rearrange and bond again, this bond  formation releases energy. If the energy given to activate the reaction is less  than the energy released during the bond formation, then the reaction gave out more energy than it took, so it is exothermic. If the energy given to activate the  reaction is more than the energy released during bond formation, then the  reaction took more energy than it gave out, so it is endothermic. The total energy change is called the enthalpy change or  simply energy changed (ΔH). When the bonds are broken, the reaction is endothermic the ΔH value is positive, when bonds are formed, it is exothermic  and the ΔH value is negative, the overall enthalpy change is:
Energy in - Energy out = ΔH
Endothermic reactions always have a positive ΔH and exothermic reactions always have a negative ΔH. Energy is measured in joules (J) or Kilojoules (KJ).   Reaction and Energy Graph: The energy change can be represented by a graph, the energy  is on the y axis and the reaction progress is on the x axis. Endothermic and exothermic reactions have different graphs. Exothermic Reactions: In exothermic reactions, the reactants have more energy then  the products, that is why small amounts of energy is required to activate the,  and that is what makes is exothermic. The reactants having more energy than the  products, make the amount of energy at the beginning of the reaction higher  than at the end, the energy in between is given off creating a curve in the  graph.

 Endothermic Reactions: It is the opposite here, in endothermic reactions, the  reactants have lower energy in them than the products, this makes them less stable, needing more energy to activate the reactions, that is why they are  endothermic. Because of that, the beginning of the reaction has less energy  than at end of it; the difference of amount of values is caused by absorbing  energy from the surroundings.

The overall energy change is determined by the amount of  energy needed to activate the reaction and break the bonds and the amount of energy released during bond formation. So to calculate the overall energy  change we have to know the amount of energy needed to break the bonds between  the reactants and the amount of energy released during bond formation.   
 
 
Calculate the enthalpy change when methane (CH4) reacts with  oxygen (O2), given the following information. CH4 + 2O2 → CO2 + 2H2O [/t][/t]
BondBong Energy (KJ/Mol)
C-H435
O=O497
C=O803
H-O464
C-C347
C-O358
Energy needed to break bonds:
 4 (C-H): 435 x 4 = 1740 KJ/Mol
 2 (O=O): 497 x 1 = 994 KJ/Mol
 Total energy in: 1740 + 994 = 2734 KJ/Mol
Energy needed to form bonds:
 2 (C=O): 803 x 2 = 1606 KJ/Mol
 4 (H-O): 464 x 4 = 1856 KJ/Mol
 Total energy out: 1606 + 1856 = 3462 KJ/Mol
 ΔH= 2734 – 3462 = -728 KJ/Mol r
 Therefore reaction is Exothermic[/t]   When the ΔH is negative, the reaction is exothermic because  the negative means that the reactants LOST energy to the surroundings. If it was positive the reaction is endothermic because the positive means that the  reactants GAINED or ABSORBED energy from the surroundings.
 
 Examples of Exothermic Reactions:  Combustion: This is  the reaction of any carbon containing fuel with oxygen producing carbon dioxide  if complete combustion and carbon monoxide if incomplete combustion and large  amounts of energy.
CH4 + 2O2 → CO2 + 2H2O + Energy
Respiration: Burning food (glucose) in living organisms to  produce energy and carbon dioxide.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Hydration: Adding water to salt powder, it is not  dissolving. The powder changes to crystals.
CuSO4 + 5H2O → CuSO4.5H2O + Energy
Displacement: A more reactive metal displaces the less  reactive one.
Zn + CuSO4 → ZnSO4 + Cu + Energy
Neutralization: Adding and acid to an alkali forming salt,  water and energy.
HCl + NaOH → NaCl + H2O

 Examples of Endothermic Reactions:  Photosynthesis: Using carbon  dioxide and water to make glucose and oxygen in the presence of light energy  and chlorophyll.
6CO2 + 6H2O → C6H12O6 + 6O2
Dehydration:  Supplying heat  to hydrated salt crystals evaropates the water of crystallization, leaving the powered crystals.
CuSO4.5H2O + Heat energy → CuSO4 + 5H2O
  Thermal Decomposition: The breaking down of a compound by  heating it.
CaCO3 + Heat Energy → CaO + CO2
[/td][/tr][/table]

 

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