Chemical Kinetics CHAPTER 14 Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop

CHAPTER 14 Chemical Kinetics Learning Objectives: Factors Affecting Reaction Rate: Concentration State Surface Area Temperature Catalyst Collision Theory of Reactions and Effective Collisions Determining Reaction Order and Rate Law from Data Integrated Rate Laws Rate Law  Concentration vs Rate Integrated Rate Law  Concentration vs Time Units of Rate Constant and Overall Reaction Order Half Life vs Rate Constant (1st Order) Arrhenius Equation Mechanisms and Rate Laws Catalysts Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

CHAPTER 14 Chemical Kinetics Lecture Road Map: Factors that affect reaction rates Measuring rates of reactions Rate Laws Collision Theory Transition State Theory & Activation Energies Mechanisms Catalysts Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

CHAPTER 14 Chemical Kinetics Factors that Affect Reaction Rates Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Kinetics: Study of factors that govern The Speed at Which Reactions Occur Kinetics: Study of factors that govern How rapidly reactions occur and How reactants change into products Rate of Reaction: Speed with which reaction occurs How quickly reactants disappear and products form Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

A B Kinetics The Speed at Which Reactions Occur Reaction rate is measured by the amount of product produced or reactants consumed per unit time. [B] concentration of products will increase over time [A] concentration of reactants will decrease over time A B Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

1. Chemical nature of reactants Kinetics Factors Affecting Reaction Rates 1. Chemical nature of reactants What elements, compounds, salts are involved? What bonds must be formed, broken? What are fundamental differences in chemical reactivity? Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Factors Affecting Reaction Rates Kinetics Factors Affecting Reaction Rates Ability of reactants to come in contact (Reactants must meet in order to react) The gas or solution phase facilitates this Reactants mix and collide with each other easily Homogeneous reaction All reactants in same phase Occurs rapidly Heterogeneous reaction Reactants in different phases Reactants meet only at interface between phases Surface area determines reaction rate Increase area, increase rate; decrease area, decrease rate Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

3. Concentrations of reactants Kinetics Factors Affecting Reaction Rates 3. Concentrations of reactants Rates of both homogeneous and heterogeneous reactions affected by [X ] Collision rate between A and B increase if we increase [A] or increase [B ]. Often (but not always) reaction rate increases as [X ] increases Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

4. Temperature Kinetics Factors Affecting Reaction Rates Rates are often very sensitive to temperature Raising temperature usually makes reaction faster for two reasons: Faster molecules collide more often and collisions have more energy Most reactions, even exothermic reactions, require energy to occur Rule of thumb: Rate doubles if temperature increases by 10 °C (10 K) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

5. Presence of Catalysts Kinetics Factors Affecting Reaction Rates Substances that increase rates of chemical reactions without being used up Rate-accelerating agents Speed up rate dramatically Rate enhancements of 106 not uncommon Chemicals that participate in mechanism but are regenerated at the end Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

CHAPTER 14 Chemical Kinetics Measuring Reaction Rates Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Rate = ratio with time unit in denominator Rate of Chemical Reaction Rates Measuring Rate of Reaction Rate = ratio with time unit in denominator Rate of Chemical Reaction Change in concentration per unit time. Always with respect to a given reactant or product [reactants] decrease with time [products] increase with time Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Measuring Rate of Reaction Rates Measuring Rate of Reaction Concentration in M units Time in s units Units on rate: [product] increases by 0.50 mol/L per second  rate = 0.50 M/s [reactant] decreases by 0.20 mol/L per second  rate = 0.20 M/s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Always positive whether something is increasing or decreasing in [X ] Rates Rate of Reaction Always positive whether something is increasing or decreasing in [X ] Reactants Reactant consumed So [X ] is negative Need minus sign to make rate positive Products Produced as reaction goes along So [X ] is positive Thus rate already positive Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Measuring Rate of Reaction Rates Measuring Rate of Reaction Coefficients indicate the relative rates at which reactants are consumed and products are formed Related by coefficients in balanced chemical equation Know rate with respect to one product or reactant Can use equation to determine rates with respect to all other products and reactants. A + B  C + D Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) Rates Rate of Reaction: Example C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) O2 reacts 5 times as fast as C3H8 CO2 forms 3 times faster than C3H8 consumed H2O forms 4/5 as fast as O2 consumed Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

NaClO + 2 HCl → Cl2 + H2O + NaCl Group Problem Clorox bleach is sodium hypochlorite. It should never be mixed with acids, (like vinegar) because it forms chlorine gas: NaClO + 2 HCl → Cl2 + H2O + NaCl If Chlorine gas (Cl2) is formed at a rate of 5.0 x 10-4 mol/Ls what rate is HCl consumed? Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Change of Reaction Rate with Time Rates Change of Reaction Rate with Time Generally reaction rate changes during reaction, it isn’t constant Often initially fast when lots of reactant present Slower and slower as reactants are depleted Why? Rate depends on the concentration of the reactants Reactants being used up, so the concentration of the reactants are decreasing and therefore the rate decreases Measured in 3 ways: instantaneous rate, average rate, initial rate Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Instantaneous & Initial Reaction Rate Rates Instantaneous & Initial Reaction Rate Instantaneous rate Slope of tangent to curve at some specific time Initial rate Determined at time = 0 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Rates Average Rate of Reaction Average Rate: Slope of line connecting starting and ending coordinates for specified time frame Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Example Reporting Different Types of Rates Concentration vs. Time Curve for 0.005M phenolphthalein reacting with 0.61 M NaOH at room temperature Rate at any time t = negative slope (or tangent line) of curve at that point http://chemed.chem.purdue.edu

Rates Example Reporting Different Types of Rates [P] (mol/L) Time (s) 0.005 0.0045 10.5 0.004 22.3 0.0035 35.7 0.003 51.1 0.0025 69.3 0.002 91.6 0.0015 120.4 Initial rate = Average rate between first two data points Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Instantaneous Rate at 51.1 s Rates Example Reporting Different Types of Rates Instantaneous Rate at 51.1 s (90,0.0028) (160,0.0018) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Average Rate between 0 and 120.4 s Rates Example Reporting Different Types of Rates Average Rate between 0 and 120.4 s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Group Problem A reaction was of NO2 decomposition was studied. The concentration of NO2 was found to be 0.0258 M at 5 minutes and at 10 minutes the concentration was 0.0097 M. What is the average rate of the reaction between 5 min and 10 min? A. 310 M/min B. 3.2 × 10–3 M/min C. 2.7 × 10–3 M/min D. 7.1 × 10–3 M/min Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

CHAPTER 14 Chemical Kinetics Rate Laws Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

= k[A]m[B]n Rate Laws Rates Based on All Reactants A + B  C + D Rate Law or Rate expression k is the rate constant Dependent on Temperature & Solvent m and n = exponents found experimentally No necessary connection between stoichiometric coefficients (, ) and rate exponents (m, n) Usually small integers Sometimes simple fractions (½, ¾) or zero = k[A]m[B]n Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

rate=0.045 M–1 s–1 [0.2][0.3] rate=0.0027 M/s  0.003 M/s Rate Laws Rates Based on All Reactants Below is the rate law for the reaction 2A +B → 3C rate= 0.045 M–1s–1 [A][B] If the concentration of A is 0.2 M and that of B is 0.3 M, and the reaction is 1st order (m & n = 1) what will be the reaction rate? rate=0.045 M–1 s–1 [0.2][0.3] rate=0.0027 M/s  0.003 M/s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Exponents specify the order of reaction with respect to each reactant Rate Laws Order of Reactions Rate = k[A]m[B]n Exponents specify the order of reaction with respect to each reactant Order of Reaction m = 1 [A]1 1st order in [A] m = 2 [A]2 2nd order in [A] m = 3 [A]3 3rd order in [A] m = 0 [A]0 0th order in [A] [A]0 = 1  means A doesn't affect rate Overall order of reaction = sum of orders (m and n) of each reactant in rate law Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

5Br– + BrO3– + 6H+  3Br2 + 3H2O Rate Laws Order of Reactions: Example 5Br– + BrO3– + 6H+  3Br2 + 3H2O x = 1 y = 1 z = 2 1st order in [BrO3–] 1st order in [Br –] 2nd order in [H+] Overall order = 1 + 1 + 2 = 4 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Order of Reaction & Units for k Rate Laws Order of Reaction & Units for k Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

The following rate law has been observed: Group Problem The following rate law has been observed: Rate = k[H2SeO][I–]3[H+]2. The rate with respect to I– and the overall reaction rate is: A. 6, 2 B. 2, 3 C. 1, 6 D. 3, 6 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

If we know rate and concentrations, can use rate law to calculate k Rate Laws Calculating k If we know rate and concentrations, can use rate law to calculate k From Text Example of decomposition of HI at 508 °C Rate= 2.5 × 10–4 M/s [HI] = 0.0558 M Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Determining Exponents in Rate Law Rate Laws Determining Exponents in Rate Law Experimental Determination of Exponents Method of initial rates If reaction is sufficiently slow or have very fast technique Can measure [A] vs. time at very beginning of reaction Before it slows very much, then Set up series of experiments, where initial concentrations vary Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

3A + 2B  products Rate = k[A]m[B]n Expt. # [A]0, M [B]0, M Rate Laws Determining Rate Law Exponents: Example 3A + 2B  products Rate = k[A]m[B]n Expt. # [A]0, M [B]0, M Initial Rate, M/s 1 0.10 1.2  10–4 2 0.20 4.8  10–4 3 Convenient to set up experiments so The concentration of one species is doubled or tripled And the concentration of all other species are held constant Tells us effect of [varied species] on initial rate Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Determining Rate Law Exponents Rate Laws Determining Rate Law Exponents If reaction is 1st order in [X], Doubling [X]1  21 Doubles the rate If reaction is 2nd order in [X], Doubling [X]2  22 Quadruples the rate If reaction is 0th order in [X], Doubling [X]0  20 Rate doesn't change If reaction is nth order in [X] Doubling [X]n  2n times the initial rate Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Comparing Expt. 1 and 2 2m = 4 or m = 2 Rate Laws Determining Rate Law Exponents: Example Expt. # [A]0, M [B]0, M Initial Rate, M/s 1 0.10 1.2  104 2 0.20 4.8  104 3 Comparing Expt. 1 and 2 Doubling [A] Quadruples rate Reaction 2nd order in A = [A]2 2m = 4 or m = 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Comparing Expt. 2 and 3 2n = 1 or n = 0 Rate Laws Determining Rate Law Exponents: Example Expt. # [A]0, M [B]0, M Initial Rate, M/s 1 0.10 1.2  104 2 0.20 4.8  104 3 Comparing Expt. 2 and 3 Doubling [B] Rate does not change Reaction 0th order in B = [B]0 = 1 2n = 1 or n = 0 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Expt. # [A]0, M [B]0, M Initial Rate, M/s Rate Laws Determining Rate Law Exponents: Example Expt. # [A]0, M [B]0, M Initial Rate, M/s 1 0.10 1.2  10–4 2 0.20 4.8  10–4 3 Conclusion: rate = k[A]2 Can use data from any experiment to determine k Let’s choose first experiment Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Initial Rate of SO3 Formation, M s–1 Rate Laws Determining Rate Law Exponents: Ex 2 2 SO2 + O2  2 SO3 Rate = k[SO2]m[O2]n Expt # [SO2] M [O2] M Initial Rate of SO3 Formation, M s–1 1 0.25 0.30 2.5  103 2 0.50 1.0  102 3 0.75 0.60 4.5  102 4 0.90 3.0  102 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

4 = 2m or m = 2 Rate Laws Determining Rate Law Exponents: Ex 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

3 = 3n or n = 1 Rate Laws Determining Rate Law Exponents: Ex 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Determining Rate Law Exponents: Ex 2 Rate Laws Determining Rate Law Exponents: Ex 2 Rate = k[SO2]2[O2]1 1st order in [O2] 2nd order in [SO2] 3rd order overall Can use any experiment to find k Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Expt # [NO] M [O2] M Initial Rate M s–1 Group Problem Using the following experimental data, determine the order with respect to NO and O2 . A. 2, 0 B. 3,1 C. 2, 1 D. 1, 1 Expt # [NO] M [O2] M Initial Rate M s–1 1 0.12 0.25 1.5  10–3 2 0.24 6.0  10–3 3 0.50 1.2  10–2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E