1 CHM 101 Project #3 Name___________________ Sinex Reaction Dynamics: The Energetics Let’s consider a simple reaction using the “molecules in the box” approach and the energy involved in chemical bonds. Are bonds breaking or forming for the boxes below? The example above illustrates the dissociation of a diatomic molecule into atoms and the process requires energy to be added. energy + Cl2 (g) Æ 2Cl (g) The energy needed to break a mole of Cl-Cl bonds is the bond energy. The bond energies for diatomic halogen molecules with single bonds are given in the table below. Draw a Lewis dot structure of one of the halogens to verify the single bond. Molecules Bond Energy F2154 kJ/mole Cl2239 Br2193 I2149 Zumdahl and Zumdahl (2003) Chemistry, 6th ed. Which halogen molecule is easiest to dissociate? Why? If you placed 2.0 moles of fluorine atoms in a container, you would find, after a period of time, 1.0 mole of F2 molecules. Would energy be required or released in this reverse reaction, where bonds are formed? Explain.When energy is absorbed in a process or reaction, it is referred to as endothermic process or reaction. While when energy is released, it is an exothermic process or reaction. We can use average bond energies to estimate the amount of energy absorbed or released in a reaction. Process Endothermic or exothermic Bond breaking Bond forming Is the dimerization of NO2, endothermic or exothermic? Why? 2NO2 (g) Æ N2O4 (g) + Draw a Lewis dot structure of N2O4. What type of nitrogen-nitrogen bond forms? Now let’s consider a more involved reaction as given below. With the help of Lewis dot structures, what types of bonds and how many are broken and formed? List them and using your textbook, look up their bond energies. Add up the total energy for the bonds broken and compare to the total for the bonds formed. H2 (g) + Cl2 (g) Æ 2HCl (g) Which side of the reaction (reactants or products) involves the larger amount of energy? Is this reaction endothermic or exothermic? Why? 2Is the synthesis of ammonia from its elements an endothermic or exothermic reaction? With the help of Lewis dot structures, what types of bonds and how many are broken and formed? List them and using your textbook, look up their bond energies. N2 (g) + 3H2 (g) Æ 2NH3 (g) This is a simple exercise in bookkeeping all the bonds broken and formed. The heat of reaction, ΔH, is calculated by the equation below, where BE is the bond energy. ∑∑−= formed) (bonds BEbroken) (bonds BE∆H What is the sign of ΔH for an endothermic reaction? What is the sign of ΔH for an exothermic reaction? Go to http://academic.pgcc.edu/~ssinex/chm101.html and find the “Bond Energy Calculator,” an interactive Excel spreadsheet that will do the calculations with the bookkeeping input and energies. Remember this is estimating ΔH. Using Lewis dot structures to determine bond type (single, double, or triple) and average bond energies from your textbook, calculate the value of ΔH and determine if the following reactions are endothermic or exothermic. CH4 (g) + 2O2 (g) Æ CO2 (g) + 2H2O (g) H2O (g) Æ 2H (g) + O (g) CH4 (g) + 2Cl2 (g) + 2F2 (g) Æ CF2Cl2 (g) + 2HF (g) + 2HCl (g) 3Explain these two general observations: 1. Acid-base neutralization reactions tend to be exothermic. H+ + OH- Æ H2O 2. Weak acid dissociation reactions, such as HCN shown below, tend to be endothermic. HCN Æ H+ + CN- Consider the gas-phase reaction for the addition of H2 to ethene, C2H4, given below. Do all the bonds need to be broken and then formed? Describe the actual bond breaking and forming needed for this reaction. 4 H C C C2H4 H2 C2H6 H2C= CH2 H-H H3C- CH3 This reaction is the basis for hydrogenation (adding H2 across double bonds) of vegetable oils (liquids) to form Crisco vegetable shortening and margarines