15. Chemical Kinetics

A key reaction in the upper atmosphere is O31g2 + O1g2 ¡ 2 O21g2 For this process, the energy of activation for the forward reaction, Ea1fwd2, is 19 kJ/mol, and the enthalpy change for the reaction, ΔHrxn, is -392 kJ>mol. What is the energy of activation for the reverse reaction, Ea1reverse2? (LO 14.10) (a) 411 kJ/mol (b) 392 kJ/mol (c) 373 kJ/mol (d) 196 kJ/mol

The following graph shows two different reaction pathways for the same overall reaction at the same temperature. Is each of the following statements true or false? (b) For both paths, the rate of the reverse reaction is slower than the rate of the forward reaction.

Based on the following reaction profile, how many intermediates are formed in the reaction A¡C? How many transition states are there? Which step, A¡B or B¡C, is the faster? For the reaction A¡C, is ΔE positive, negative, or zero? [Section 14.6]

The decomposition of sodium bicarbonate (baking soda), NaHCO31s2, into Na2CO31s2, H2O1l2, and CO21g2 at constant pressure requires the addition of 85 kJ of heat per two moles of NaHCO3. (b) Draw an enthalpy diagram for the reaction.

Consider a reaction that occurs by the following mechanism: A + BC SAC + B AC + D SA + CD The potential energy profile for this reaction is as follows: (d) Is the reaction endothermic or exothermic? Add labels to the diagram that show the values of the energy of reaction ΔE and the activation energy Ea for the overall reaction.

Consider a reaction that occurs by the following mechanism: A + BC SAC + B AC + D SA + CD The potential energy profile for this reaction is as follows: (b) Write structural formulas for all species present at reaction stages 1–5. Identify each species as a reactant, product, catalyst, intermediate, or transition state.

The gas-phase reaction Cl1g2 + HBr1g2¡HCl1g2 + Br1g2 has an overall energy change of -66 kJ. The activation energy for the reaction is 7 kJ. (a) Sketch the energy profile for the reaction, and label Ea and E.

The gas-phase reaction Cl1g2 + HBr1g2¡HCl1g2 + Br1g2 has an overall energy change of -66 kJ. The activation energy for the reaction is 7 kJ. (b) What is the activation energy for the reverse reaction?

The diagram shows the energy of a reaction as the reaction progresses. Label each blank box in the diagram.

a. reactants b. products c. activation energy (Ea) d. enthalpy of reaction (ΔHrxn)

(a) Based on the following reaction profile, how many intermediates are formed in the reaction A¡D?

(c) Which step is the fastest?

Consider the following energy profile. (c) Which step is rate limiting?

Consider the following energy profile. (b) How many intermediates are formed in the reaction?

The values of Ea = 248 kJ>mol and ΔE = 41 kJ>mol have been measured for the reaction H21g2 + CO21g2S H2O1g2 + CO1g2 (b) Considering the geometry of the reactants and products, suggest a plausible structure for the transition state.

Consider three reactions with different values of Ea and ΔE: Reaction 1. Ea = 20 kJ>mol; ΔE = -60 kJ>mol Reaction 2. Ea = 10 kJ>mol; ΔE = -20 kJ>mol Reaction 3. Ea = 40 kJ>mol; ΔE = +15 kJ>mol (c) Which reaction is the most endothermic, and which is the most exothermic?

Consider three reactions with different values of Ea and ΔE: Reaction 1. Ea = 20 kJ>mol; ΔE = -60 kJ>mol Reaction 2. Ea = 10 kJ>mol; ΔE = -20 kJ>mol Reaction 3. Ea = 40 kJ>mol; ΔE = +15 kJ>mol (b) Assuming that all three reactions are carried out at the same temperature and that all three have the same frequency factor A, which reaction is the fastest and which is the slowest?

Consider this energy diagram:

d. Is the overall reaction endothermic or exothermic?

Consider the reaction in which HCl adds across the double bond of ethene: HCl + H2C'CH2¡H3C¬CH2Cl The following mechanism, with the accompanying energy diagram, has been suggested for this reaction: Step 1 HCl + H2C'CH2¡H3C'CH2+ + Cl - Step 2 H3C'CH2+ + Cl - ¡H3C¬CH2Cl

a. Based on the energy diagram, determine which step is rate limiting.

The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride: Reaction 1: CH41g2 + Cl21g2 ¡ CH3Cl1g2 + HCl1g2 This reaction is very slow in the absence of light. However, Cl21g2 can absorb light to form Cl atoms: Reaction 2: Cl21g2 + hv ¡ 2 Cl1g2 Once the Cl atoms are generated, they can catalyze the reaction of CH4 and Cl2, according to the following proposed mechanism: Reaction 3: CH41g2 + Cl1g2 ¡ CH31g2 + HCl1g2 Reaction 4: CH31g2 + Cl21g2 ¡ CH3Cl1g2 + Cl1g2 The enthalpy changes and activation energies for these two reactions are tabulated as follows: Reaction H 1kJ ,mol 2 Ea 1kJ ,mol 2 3 +4 17 4 -109 4 (b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4.