IB SL Chemistry Mrs. Page 2015-2016 5.3 Bond Enthalpy IB SL Chemistry Mrs. Page 2015-2016

Understandings: Bond forming releases energy and bond breaking requires energy. Average bond enthalpy is the energy needed to break 1 mol of a bond in a gaseous molecule averaged over similar compounds.

Applications and Skills Calculation of the enthalpy changes from known bond enthalpy values and comparison of these with experimentally measured values. Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic. Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.

Bond Enthalpy The energy required to break 1 mole of bonds in a gaseous covalent molecule under standard conditions. Bond breaking is endothermic (takes in energy from surroundings) (+ ∆H) Also known as bond dissociation enthalpy Data booklet Table 11 Average values from experimental values so only approximations

Bonds & Energy You have to be able to draw the structural chemical formula to see all the bonds and add them up properly. Bond length influences bond enthalpies as the longer the bond (larger atomic radii) the weaker the bond (remember the radii increases down a group and decreases across a period) Bond strength is associated with how much energy it takes to break the bond

Bond Enthalpy (Bond Energy) Examples If H-H is bonded, its bond enthalpy is 436 kJmol-1. It is represented by H2(g)  2H(g) 1 mole of H2 molecules is broken apart to give 2 moles of gaseous hydrogen atoms. Example 2: The H-Cl bond enthalpy is 431kJmol-1, which is represented by : HCl(g)  H (g) + Cl(g) ΔHθ = +431 kJ mol-1

Bond Enthalpies can only be calculated for substances in the gaseous state. Example: If we can consider Br2(l) 2Br(g), the ΔHθ = +224kJ mol-1. This does not represent the bond enthalpy of the Br-Br bond, as the reaction can be broken down into two process.

Br2(l) 2Br(g), ΔHθ = +224kJ mol-1 Continued…

Calculating Overall Bond Enthalpy Change for a Reaction Breaking bonds requires energy (Endothermic). This is because energy is needed to break apart the bonds Making bonds releases energy (Exothermic) because the potential energy of the bonds apart becomes kinetic energy as they come together, and they release heat (think of an object falling to Earth) Reactants – products this time!

Overall Enthalpy Change for a Reaction ΔHrxn = ΣBE(bonds broken) – ΣBE(bonds made)

Example Problem Consider the reaction between ethene and bromine to produce 1,2-dibromoethane: C2H4(g) + Br2(g)  C2H4Br2(g) What is the enthalpy change for this reaction? If the species are drawn as structural formulas, then all the bonds can be seen clearly.

Example problem Continued… Now, we imagine the reaction happening with all the bonds in the reactant being broken. **Note: This is NOT how the reaction occurs, it is just a way of allowing us to work out the enthalpy change.

Example problem Continued… Next, new bonds form to make the products.

Example problem Continued… The bond energies can then be added up to work out the enthalpy change: Bond Broken Bond Energy/ kJmol-1 Number of Bonds Total energy/ kJmol-1 C-H 412 4 1648 C=C 612 1 Br-Br 193 Total energy to break all bonds 2453 The total enthalpy change when all the bonds are broken is +2453 kJmol-1. This is positive, as breaking bonds is an endothermic process.

Example problem Continued… Enthalpy released when the bonds are made: Bond Broken Bond Energy/ kJmol-1 Number of Bonds Total energy/ kJmol-1 C-H 412 4 1648 C-C 348 1 C-Br 276 2 552 Total energy to break all bonds 2548 The total enthalpy change when all the bonds are made is -2548 kJmol-1. This is negative, as making bonds is an exothermic process.

Example problem Continued… ΔHrxn = ΣBE(bonds broken) – ΣBE(bonds made) ΔHθ = [(4 x 412) + (1 x 612) + (1 x 193) – [(4 x 412) + (1 x 348) + (2 x 276)] = -95kJmol-1

Bond Enthalpies and Enthalpies of Combustion The complete combustion of propane can be represented by the equation: CH3CH2CH3(g) + 5O2((g)  3CO2(g) + 4H2O(g) Calculate the bond enthalpy from Table 11 of your Data Book

Bond Enthalpies and Enthalpies of Combustion Bonds Broken 8 C-H = 8(414) 5 O=O = 5(498) 2 C-C = 2(346) Total = +6494 Bonds Made 6 C=O = 6(804) 8 O-H = 8(463) Total = -8528 Bond Enthalpy = -2034 kJmol-1

Bond Enthalpies and Enthalpies of Combustion Bond Enthalpy = -2726 ∆Hcө= -5470 Compare this value to the experimentally determined value for the enthalpy change of combustion for octane (Data Book Table 13) ∆Hcө= liquid state and Bond Enthalpy gaseous

You Try Calculate the bond enthalpy for the following reactions: CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g) 4NH3(g)+ 3O2(g) 2N2(g)+ 6H2O(g) [4(414) + 242] – [3(414)+ 324 + 431] = -99 kJmol-1 [12(391)+3(498)] – [2(945) + 12(463)] = -1260 kJmol-1

CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g) This reaction is overall exothermic because the final product is lower in energy than the reactants.

OZONE, O3 Ozone plays an important role in our atmosphere. There are two types of ozone, One that is critical for life on Earth found in Earth’s stratosphere. This ozone protects us from solar radiation by absorbing UV rays. Loss of this ozone is frequently referred to as a “hole in the ozone layer.” The other type of ozone “bad ozone” is found closer to Earth’s surface in the troposphere. This type of ozone is created due to the burning of fossil fuels and act as a pollutant causing damage to living tissues.

OZONE PRODUCTION Statospheric (“good”) ozone is created in the atmosphere. Formation requires two steps: UV radiation to splits an O2 molecule into 2 O atoms The two oxygen atoms then combine with an oxygen molecule (O2) to form ozone http://www.esrl.noaa.gov/

OZONE DEPLETION http://butane.chem.uiuc.edu/ Stratospheric ozone is also destroyed in the atmosphere in a process called photolysis (breakdown by light). Ozone in the stratosphere protects life on Earth from UV radiation. UV radiation is absorbed by ozone, breaking it down into O2(g) and an oxygen atom Later we will learn how human interactions increase the rate at which ozone in depleted, creating holes in the ozone layer.

OZONE PHOTOLYSIS The destruction of ozone is an endothermic reaction that requires energy (UV light) to occur Reactant more stable than products Potential energy

Extra Practice http://www.matter.org.uk/Schools/Content/Reactions/BE_enthalpyH2O.html Try this site to practice at home