Bond Energy and Calculations Learning Objectives 23 February 2019 Bond Energy and Calculations Understand that bonds are broken and made in chemical reactions Learn how to calculate the energy needed and released Key Words – Exothermic Endothermic Energy level diagram What to remember? ENERGY IS NEEDED TO BREAK A BOND ENERGY IS NEEDED TO MAKE A BOND

Energy Level Diagram Exothermic reactions Energy is given out The products have less energy than the reactants Combustion and neutralisation are exothermic

Energy Level Diagrams Exothermic reactions energy

Energy Level Diagrams Exothermic reactions energy course of reaction

Energy Level Diagrams Exothermic reactions energy reactants course of reaction

Energy Level Diagrams Exothermic reactions energy reactants products course of reaction

Energy Level Diagrams Exothermic reactions energy given out ∆H is negative energy reactants products course of reaction

Energy Level Diagrams Endothermic reactions Energy is taken in The products have more energy than the reactants The energy is taken in from the surroundings

Energy Level Diagrams Endothermic reactions energy

Energy Level Diagrams Endothermic reactions energy course of reaction

Energy Level Diagrams Endothermic reactions energy reactants course of reaction

Energy Level Diagrams Endothermic reactions energy products reactants course of reaction

Energy Level Diagrams Endothermic reactions energy products energy taken in ∆H is positive reactants course of reaction

Endothermic reactions Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings ∆H is negative ∆H is positive Products have less energy than reactants Products have more energy than reactants

∆H How much energy is given out or taken in? Energy is needed to break chemical bonds Energy is given out when bonds are made ∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

Bond energies The energy needed to break a chemical bond Different chemical bonds have different bond energies Chemical bond Bond energy, kJ/mole H―H 436 O=O 496 O―H 463

Working out ∆H Draw an energy level diagram with all the reactants and products on it 2 H-O-H H―H O=O + energy 2H2 + O2 2H2O course of reaction

Bond Bond Energy (kJ mol-1) H-H +436 Cl-Cl +242 C-H +413 H-Cl +431 O-H +464 C-O +358 H-Br +366 Br-Br C-C +193 + 348

Multiple Bonds Bond Energy (kJ mol-1) C=C +614 C C +839 C = N +615 C N +891 S=O +523 C = O +799 C O +1072 O=O +496 N=N +418 N N +941 S = S

Working out ∆H Show the bond energies for all the bonds ∆H = ∑ ∆H(bonds broken) − ∑ ∆H(bonds formed) 436 + O=O energy 436 H O H O course of reaction

Working out ∆H Show the bond energies for all the bonds 436 + 496 energy 436 H O H O course of reaction

Working out ∆H Show the bond energies for all the bonds 436 + 496 energy 436 463 + 463 H O course of reaction

Working out ∆H Show the bond energies for all the bonds 436 + 498 energy 436 463 + 463 463 + 463 course of reaction

Working out ∆H Add the reactants’ bond energies together energy 1368 463 + 463 463 + 463 course of reaction

Working out ∆H Add the products’ bond energies together energy 1368 1852 course of reaction

Working out ∆H ∆H = energy in ― energy out 1368 1852 ― energy 1368 course of reaction

Working out ∆H ∆H = energy in ― energy out 1368 1852 -484 ― energy course of reaction

Working out ∆H ∆H = energy in ― energy out ∆H = -484 energy 1370 1852 course of reaction

Example 1 Hydrogen + Chlorine Hydrogen Chloride H2(g) + Cl2(g) 2HCl(g) H-H + Cl-Cl H-Cl + H-Cl

H H Cl Cl Intermediates H-H and Cl-Cl H-Cl and H-Cl Reactants Products 678 Intermediates 862 431 431 436 242 H-H and Cl-Cl H-Cl and H-Cl Reactants Products 678 – 862 = – 184

Is this endothermic or exothermic? The total energy (∆H) = Energy to make bonds – Energy to break bonds = 678 – 8/62 = – 184 kJ mol-1 The answer is NEGATIVE Is this endothermic or exothermic?

Example 2 Hydrogen Bromide Hydrogen + Bromine 2HBr(g) H2(g) + Br2(g) H-Br + H-Br H-H + Br-Br

H H Br Br Intermediates H-Br and H-Br Reactants H-H and Br-Br Products 732 Intermediates 366 366 H-Br and H-Br 629 Reactants 193 436 732 – 629 = 103 H-H and Br-Br Products

Is this endothermic or exothermic? The total energy (∆H) = Energy to make bonds – Energy to break bonds = 732 – 629 = 103 kJ mol-1 The answer is POSITIVE Is this endothermic or exothermic?

Working out ∆H Summary The energy values have units of kJ/mole In the exam, you will be given the energy values and all the bonds to make or break Energy goes in to break bonds Energy goes out when bonds are made ∆H is energy in – energy out