(Ws # 5) Worksheet 1.9-1.12 Mechanisms: Key 1. OCl - + H2O HOCl + OH - HOCl + I - HOI + Cl - HOI + OH - H2O + OI - i) The net chemical equation is: OCl - + I - + Cl - +OI - ii) The reaction intermediates are: HOCl HOI OH - iii) The catalyst is: H2O 2. Br2 2Br fast Br + OCl2 BrOCl + Cl slow Br + Cl BrCl fast i) The net chemical equation is: Br2 + OCl2 BrOCl + BrCl ii) The reaction intermediates are: Cl & Br iii) The catalyst is: None iv) The rate determining step is 2 v) If the concentration of Br2 is increased will the rate of the reaction increase? Explain your answer. No because it is not in the rate determining step. vi) If the concentration of OCl2 is increased will the rate of the reaction increase? Explain your answer. Yes because, OCl2 is in the rate determining step. 3. The mechanism for the catalytic decomposition of formic acid is shown below. step 1 HCOOH + H + [HCOOHH] + step 2 [HCOOHH] + [HCO] + + HOH step 3 [HCO] + CO + H +
i) The catalyst is H + Crosses out from left to right ii) The rate determining step is Two Highest Ea iii) ΔH = +10 kj From start to end iiv) The forward activation energy is 40 kj iv) The reverse activation energy is 30 kj Reactants to the highest point Products to the highest point v) The enthalpy of [HCOOHH] + is 160 kj After one hump vi) Is the reaction exothermic or endothermic? Endo Uphill vii) Which chemical formula has the greatest potential energy? (HCO) + + HOH Highest point on graph viii) Which chemical formula has the greatest kinetic energy? HCOOH + H + Lowest point on graph ix) Does this reaction absorb or release kinetic energy? Absorb because it is endothermic (uphill) 4. Define and remember the following definitions. mechanism A sequence of steps that determines the overall reaction. activation energy The minimum energy required in a successful collision. rate determining step The slowest step in a reaction mechanism. catalyst A substance that increases the rate of a chemical reaction by providing a alternate mechanism with lower activation energy. reaction intermediate A chemical species produced in a reaction mechanism and then consumed in a later step. endothermic exothermic activated complex A reaction that absorbs energy A reaction that produces energy A unstable reaction intermediate with high potential energy and low kinetic energy.
ΔH reaction rate The change in enthalpy or heat content for a reaction. The change in a reactant or product per unit of time. 5. The catalyzed decomposition of acetaldehyde has an overall reaction of: CH3CHO CH4 + CO. Determine step 2 of the reaction mechanism. A proposed mechanism is: step 1 CH3CHO + I2 CH3I + HI + CO step 2 HI + CH3I I2 + CH4 This is the only step 2 that will give the overall reaction below. overall CH3CHO CH4 + CO 6. The following reaction has an overall reaction of: 2Ce 4+ + Tl + 2Ce 3+ + Tl 3+ Determine step 2 of the reaction mechanism. A proposed mechanism is: step 1 Ce 4+ + Mn +2 Ce 3+ + Mn 3+ step 2 Ce 4+ + Mn 3+ Ce 3+ + Mn 4+ This is the only step 2 that will give the overall reaction below step 3 Mn 4+ + Tl + Tl 3+ + Mn 2+ overall 2Ce 4+ + Tl + 2Ce 3+ + Tl 3+ 7. A reaction has a overall equation of: Br2 + OCl2 BrOCl + BrCl. Determine step 3 of the mechanism. step 1 Br2 2Br 3 that will give the overall reaction below step 2 Br + OCl2 BrOCl + Cl step 3 Br + Cl BrCl This is the only step
overall Br2 + OCl2 BrOCl + BrCl List two intermediates: Br Cl 8. Complete the following mechanism. step 1 NO + Pt NOPt needed for next step step 2 NOPt + NO O2Pt + N2 O2Pt needed for next step and N2 needed to be a product step 3 O2Pt O2 + Pt overall 2NO N2 + O2 Identify the catalyst Pt Crosses out from left to right Identify the two intermediates NOPt O2Pt Crosses out from right to left 9. Draw a collision energy distribution diagram for a reaction where the y-axis is fraction of collisions and the x axis is collision energy. Draw the Ea line showing about 10% of the collisions having sufficient energy. Draw the Ea line for the catalyzed reaction where 20% have sufficient energy.
10. Shade in the area of the collision energy distribution diagram showing those collisions that have the required energy to be successful at the low temperature shown below. Draw the curve that represents the distribution at a higher temperature with a different color. Shade in the area representing the successful collisions at the higher temperature with a new color.