2. In any chemical reaction, bonds in the reactant molecules are broken, and new
ID: 562712 • Letter: 2
Question
2. In any chemical reaction, bonds in the reactant molecules are broken, and new bonds are formed in the product molecules. In the case of covalent bonds, two electrons must be "spin paired" in the newly formed bond. (You may want to go back and read section 10.6 again!) H-H High PE. a) What would the enthalpy change be for breaking a bond (exo, endo, or neither) if we consider the boulders (or H atoms) to be the system? b) Would you expect that forming a bond is an exothermic or endothermic process or neither? c) According to Tro in the eText, "... the interaction energy is usually negative (or stabilizing) when the interacting atomic orbitals contain a total of two electrons that can spin-pair (orient with opposite spins)." What would the sign of AHo be if the reactants both had an unpaired electron in their valence shell, and the product was able to pair those electrons (form an additional bond d) If a reaction occurred where bonds were broken and subsequently formed, and energy was given off (exothermic), would you expect the magnitude (absolute value) of the bond breaking process to be greater or of the bond forming process to be greater? Explain.Explanation / Answer
a) For breaking of bond we need to apply some amount of energy, so energy is given to the system. Hence energy of the system after breaking the bond will be greater than the energy of the system before breakong the bond.
Enthaply change = enthalpy of the system after breaking bond - enthalpy of the system before breaking bond
= positive
Hence enthalpy change for breaking of bond should be positive
b) When bond is formed energy is released as the system is stabilized. So forming a bond is an exothermic process.
c) When electrons paired that indicates formation of chemical bond, so energy released in this process.
Products have lower energy than reactants.
delta H^0 = H^0 of products - H^0 of reactants
= lower energy - higher energy
= negative
sign of delta H^0 should be negative sign (-)
d) For a chemical reaction bonds broken in reactants and bonds formed in products
change in energy or change in enthalpy
= energy for breaking bonds - energy for forming bonds.
Given that the above change is exothermic that means energy is given off. So this is negative value.
Therefore energy for forming bonds should be greater than energy for breaking bonds.
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