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Bonding Together Makes Us Happy! |
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Covalent Bond Energies and Chemical Reactions |
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-The energy to break a bond can vary due to the sensitivity to its environment. Ex: Breaking of a C-H bond -The energy to break a C-H bond in HCBr3 is 380 kJ/mol whereas the energy to break a C-H bond in HCF3 is 430 kJ/mol. Single bond: One pair of electrons are shared. Ex: HI Double bond: Two pairs of electron are shared. Ex: O2 Triple bond:Three pairs of electrons are shared. Ex: CO -The bond length of molecule becomes shorter when there are more shared electron pairs. This means that a triple bond is shorter in length than a double or single bond. -To break a bond, one requires an endothermic reaction while energy releases when bonds are formed. -The expression, ∆ H=(bonds broken)-(bonds formed), shows that the enthalpy change of a reaction is equal to the energy released subtracted from the energy required. Ex: 2HCl —> H2 + Cl2 1) 2mol(427 kJ/mol) - [2mol(239 kJ/mol) + 2mol(432 kJ/mol)] 2) 854 kJ + (478 kJ + 864 kJ) = -488 kJ - When bonds are broken, energy is added to the system (endothermic). - When bonds are formed, energy is released (exothermic). -Energy related to bonds breaking is always positive. -Energy related to bonds forming is always negative.
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