Rates of Reaction Chapter 6

Chemical Kinetics Info Given: (deals only with very beginning and very end) 1. Balanced equation tells us nature, state, and relative amounts of reactants and products kJ heat term, tells us reaction is EXOTHERMIC, and provides 92000J of energy per 2 moles of NH 3 (g) 3. 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) 4. ΔS° = -ve value, does not favour spontaneity; less of factor than ΔH

Chemical Kinetics cont… Info NOT Given: (the stuff between the start and end states) 1. How quickly or slowly does the reaction proceed? Does it take seconds, minutes, hours, years…to ”get” as far as it will go? 2. 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.

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?

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

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. (CH 3 )CCl (aq) + H 2 O(l)  (CH 3 )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. CO 2 escapes during a reaction  Change in pH  Change in volume  Change in pressure  Change in concentration  Change in light absorption

Example 1: Reaction Rates C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times, t. Rate = Δ[C 4 H 9 Cl] Δt  Note: by convention, rates are positive. So, if you are working with reactants disappearing you must multiply by -1!

Calculating Reaction Rates C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) The average rate of the reaction over each interval is the change in concentration divided by the change in time:

Reaction Rate Determination C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (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.

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. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Sample Problem Measurements taken during the following reaction showed a concentration of carbon monoxide (CO) of mol/L at 27 min and of mol/L at 45 min. Calculate the average rate of the loss of carbon monoxide (CO) AND the gain of carbon dioxide (CO 2 ). CO (g) + NO 2 (g)  CO 2 (g) + NO (g)

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 -3 mol s -1

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 mol s -1 Example 3: 15mL of 0.1 M KMnO 4 is decolourized by sodium oxalate in 75 s – calculate the rate of this rxn. ANSWER: 2.0 x mol s -1

Reaction Rate and Stoichiometry  Now apply it to an equation. Which substance is measured?? Consider the reaction: C 3 H 8 + 5O 2  3CO 2 + 4H 2 O IF 0.25 mol of propane burns in 50s – the rate at which propane is burning is 5.0 x mol s -1 (previous slide) Therefore the rate at which the OXYGEN is being consumed would be… 5 x 5.0 x mol s -1 or 25 x mol s -1 And the rate at which the CARBON DIOXIDE is being produced would be… 3 x 5.0 x mol s -1 or 15 x mol s -1

Reaction rates and Stoichiometry

Reaction Rates and Stoichiometry  Suppose that the mole ratio is not 1:1? Example:

Try It!! 1.Write the Rate Expression: 2. Look at the ratio in the equation: for every mole of NO 2 made, ½ N 2 O 5 is decomposed