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Rates of Reaction

Rates of Reaction. Chapter 6. Chemical Kinetics. Info Given : (deals only with very beginning and very end) Balanced equation tells us nature, state, and relative amounts of reactants and products.

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Rates of Reaction

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  1. Rates of Reaction Chapter 6

  2. Chemical Kinetics Info Given: (deals only with very beginning and very end) • Balanced equation tells us nature, state, and relative amounts of reactants and products. • +92kJ heat term, tells us reaction is EXOTHERMIC, and provides 92000J of energy per 2 moles of NH3 (g) • ΔG°f is negative (-ve), thus the reaction is SPONTANEOUS at 25°C, and 100kPa. (Nitrogen gas will combine with hydrogen gas to form ammonia without any external energy being applied) • ΔS° = -ve value, does not favour spontaneity; less of factor than ΔH

  3. Chemical Kinetics cont… Info NOT Given: (the stuff between the start and end states) • How quickly or slowly does the reaction proceed? Does it take seconds, minutes, hours, years…to ”get” as far as it will go? • Are there any intermediate steps? Do nitrogen and hydrogen combine directly to form ammonia, or are there sub-reactions that must take place before the final product is formed.

  4. Questions to consider… • What makes some reactions occur within a few seconds while others occur over days, or even years? • What can you do to speed up or slow down a reaction? • Why do factors such as temperature, surface area, and concentration affect reaction rates?

  5. Refresher from yesterday… • What is Kinetics? • The rate at which chemical reactions occur • The change in concentration of reactants over time • The change in concentration of products over time • Reaction rate = -[A]/Δt or [B]/Δt

  6. How do we Measure Rates? • Collect Gas • if the reaction produces a gas then it is easy to measure the rate of reaction based on the amount of gas collected. • Change in conductivity • if the reaction produces ions, the conductivity of the solution will change • Eg. (CH3)CCl(aq) + H2O(l)  (CH3)COH(aq) + H+(aq) + Cl-(aq) • Change in colour • using spectrophotometer you can measure the change in colour intensity to determine the rate of reaction. • Eg. ClO-(aq) + I-(aq)  IO-(aq) + Cl-(aq) • Change in Mass • Eg. CO2 escapes during a reaction • Change in pH • Change in volume • Change in pressure • Change in concentration • Change in light absorption

  7. Example 1: Reaction Rates C4H9Cl(aq)+ H2O(l) C4H9OH(aq) + HCl(aq) In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times, t. Rate = Δ[C4H9Cl] Δt • Note: by convention, rates are positive. So, if you are working with reactants disappearing you must multiply by -1!

  8. Calculating Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) The average rate of the reaction over each interval is the change in concentration divided by the change in time:

  9. Reaction Rate Determination C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between the reacting molecules.

  10. Reaction Rates • Slope of a line tangent to the curve at any point is the instantaneous rate at that time. • Secants yield the average reaction rate. C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

  11. Sample Problem Measurements taken during the following reaction showed a concentration of carbon monoxide (CO) of 0.019 mol/L at 27 min and of 0.013 mol/L at 45 min. Calculate the average rate of the loss of carbon monoxide (CO) AND the gain of carbon dioxide (CO2). CO (g) + NO2 (g) CO2 (g) + NO (g)

  12. Examples of Reaction Rate using Quantities Example 1: 0.25mol of propane burns in 50s – what is the rate at which propane is burning? ANSWER: 5.0x10-3mol s-1

  13. Examples of Reaction Rate using Quantities Example 2: 4.8 g of Mg(s) reacts with HCl in 20 s – calculate the rate of this rxn. ANSWER: 9.87 x 10-3mol s-1 Example 3: 15mL of 0.1 M KMnO4 is decolourized by sodium oxalate in 75 s – calculate the rate of this rxn. ANSWER: 2.0 x 10-5mol s-1

  14. Reaction Rate and Stoichiometry • Now apply it to an equation. Which substance is measured?? Consider the reaction: C3H8 + 5O2 3CO2 + 4H2O IF 0.25 mol of propane burns in 50s – the rate at which propane is burning is 5.0 x 10-3mol s-1 (previous slide) Therefore the rate at which the OXYGEN is being consumed would be… 5 x 5.0 x 10-3mol s-1 or 25 x 10-3mol s-1 And the rate at which the CARBON DIOXIDE is being produced would be… 3 x 5.0 x 10-3mol s-1 or 15 x 10-3mol s-1

  15. Reaction rates and Stoichiometry

  16. Reaction Rates and Stoichiometry • Suppose that the mole ratio is not 1:1? Example:

  17. Try It!! Write the Rate Expression: 2. Look at the ratio in the equation: for every mole of NO2 made, ½ N2O5 is decomposed

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