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Bond energy & activation energy. Chemical compounds have an internal energy obtained when they were originally formed. To get them to react further may require more energy for the new reaction to take place.
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Bond energy & activation energy Chemical compounds have an internal energy obtained when they were originally formed. To get them to react further may require more energy for the new reaction to take place. Bond energy is the amount of energy required to break the bonds in one mole of a particular covalent bond of a substance in the gaseous state. Bond energy - breaking and making bonds
Bond energy & activation energy Before the reaction, the reactants have energy, but are stable & do not react. Bonds in the reactants need to be broken & energy – called activation energy, is required for this. As new bonds form during the reaction, energy is released. Sometimes the energy released is greater than the activation energy & we call the reaction an exothermic reaction. If less – it is called an endothermic reaction. Heating CuSO4
A + B C + D Energy changes during reactions Exothermic reactions Energy 1 = activation energy 2 = energy liberated 3 = ∆H = heat of reaction 1 2 A + B Reactants 3 C + D Products Course of reaction Exothermic reaction – net energy liberated
A + B C + D Energy changes during reactions Endothermic & exothermic reactions Energy 1 = activation energy 2 = energy liberated 3 = ∆H = heat of reaction 1 C + D 2 A + B 3 Products Reactants Activation energy in reactions Course of reaction - time Endothermic reaction – net energy absorbed
Forces of repulsion. + D 0 E complete change - Ep Forces of attraction. Energy vs Distance in molecules Forming the H2 molecule
Forming a hydrogen molecule Forming the H2 molecule
Cl Bond Cl Cl Cl Cl Cl Cl Cl Breaking & making chemical bonds The Cl2 molecule is stable & requires energy to break the bond. This is a endothermic process. Energy supplied is called the bond energy. Making new bonds is an exothermic process as energy is liberated. Bond energy liberated measures strength of bond.
Bond energy & bond strength The bond strength is an indication of how much energy is required to break a bond or how much is released when the new bond is formed. This table gives an idea of relative sizes of bond energies (measured in kJ.mol-1) H-H 436 H-C 414 C-C 347 O=O 498 Methane is a gas that burns in O2 to form CO2, H2O & liberates energy. Energy must be supplied (match- activation energy) to start the reaction. Establish if this is endothermic or exothermic.
CH4 + 2O2 CO2 + 2H2O + energy Breaking Energy input Making Energy output bonds kJ.mol-1 bonds kJ.mol-1 4 C-H 4 X 414 4 X O-H 4 X 464 bonds = 1656 bonds = - 1856 2 O=O 2 X 498 2 X C=O 2 X 803 bonds = 996 bonds = - 1606 Totals = 2652 = - 3462 As 1 mole of CH4 burns, there is thus a net decrease in energy of -810 kJ.mol-1 & ∴ it is an exothermic reaction with a net loss of energy. Bond energies Net energy change
CH4 + 2O2 CO2 + 2H2O + energy Breaking bonds – activation energy C C C More energy released – bond energy - exothermic H H H H H H H H H H H H H O O O O O O O O O O O O Methane combustion (burning) Separate atoms + Reactants Burning methane Products
Bond energy & bond length in C-C bonds Bond length & bond energies In general, the longer the bond, the less energy required to break the bond.
Factors affecting the bond strength: • Bond energy • Bond length • Size of bonded atoms • Number of bonds (bond order) Energy to break bonds
