Unit 6: Topic 7 - Bond Enthalpies

Question 1

In a chemical reaction, determine how bonds are broken or formed and how these processes affect the energy of the system.

Answer 1

In a chemical reaction, the reactants react to form products. The chemical bonds of the reactants are broken and these bonds are rearranged to form the products. Bond breaking requires energy, and the reverse process, bond forming, releases energy. The total enthalpy change equals the energy required to break the bonds of the reactants minus the energy released when forming the bonds of the products.

Question 2

How can we use the bond energies to determine whether a chemical reaction is exothemic or endothermic?

Answer 2

The change in enthalpy is equal to the heat flow. The bonds of the reactants will be broken and the bonds of the products will be formed. Sum the individual bond energies of the reactants; this is energy required to break all the reactant bonds. Similarly, sum the individual bond energies of the products; this is the energy released when forming the product bonds. The change in energy of the system is then the energies of the bonds broken minus the energies of the bonds formed, which can be used to determine if a chemical reaction is exothermic or endothermic. If this value is negative, the reaction is exothermic. If this value is positive, the reaction is endothermic.

Question 3

In the reaction 2H2O2 -> 2H2O + O2, which bonds are broken and which bonds are released? What is the enthalpy change of this reaction in terms of the reactants and products?

Answer 3

Hydrogen peroxide has the bond structure H-O-O-H: all bonds are single bonds. Therefore, in one hydrogen peroxide, two O-H single bonds and one O-O single bond are broken. In two hydrogen peroxides we double these amounts to four O-H single bonds and two O-O single bonds broken. For the products, H2O has two O-H single bonds and O2 consists of a double bond, so there will be four O-H single bonds formed and one O-O double bond formed. Notice that four O-H single bonds are broken and four O-H single bonds are also formed, so the net energy change from this will be 0. Therefore, the enthalpy change is the energy required to break the single O-O bond minus the energy required to break the double O-O bond.
We can predict that this reaction is exothermic because double bonds are stronger than single bonds. The bond energy of the single O-O bond is less than the bond energy of the double O-O bond so subtracting these values gives a negative value.

Question 4

Question

What is the change in enthalpy of the reaction 2HBr –> H2 + Br2?

H-Br, 370kJ/mol
H-H, 440kJ/mol
Br-Br, 190 kJ/mol Click here

Answer 4

There are two H-Br bonds broken, one H-H bond formed, and one Br-Br bond formed. Therefore, the enthalpy change is 370 + 370 - 440 - 190 = 110 kJ. This reaction is endothermic.