Chemistry Lab/Kinetics

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Kinetics was a topic for the event Chemistry Lab in 2015 and 2016.

Kinetics (2015)

Kinetics: Reaction Rates

[math]\text{rate}=\dfrac{[\text{limiting reagent}]}{\text{time}}[/math]. To find reaction rates, you need a table or graph showing the concentration of one of the products or reactants over a period of time. If given:

  • a line graph showing concentration of a reactant, you can find reaction rate at a given instant for that reactant. It will be equal to the slope of the line tangent to the point on the graph at that instant.
  • a line graph showing concentration of a product, you can find the reaction rate at at given instant for that product. It will be equal to the opposite of the slope of the line tangent to the point on the graph at that instant.
  • the reaction rate for one reactant or product and the reaction equation, you can find the reaction rates for another reactant or product. Balance the equation, if necessary. Take the rate you are given, multiply by the coefficient of the reactant or product you want the rate for, and divide by the coefficient of the reactant or product whose rate you were given.

Kinetics: Reaction Conditions

  • Increasing temperature increases reaction rate.
  • Increasing concentration increases reaction rate.
  • Increasing particle size decreases reaction rate.
  • Adding catalysts increases reaction rate.

Kinetics: Rate Laws

  • In an equation with rate constant k and reactants A, B, and C, [math]Rate=k([A]^x)([B]^y)([C]^z)[/math]. x, y, and z are whole numbers that indicate the concentrations' effect on reaction rate. They can be determined by using a table of data showing concentrations of reactants and resulting reaction rates. When the concentration of all reactants but one (A) stay the same, the concentration of that one product is multiplied by a factor of p, and the reaction rate is multiplied by a factor of q, the whole number exponent (x) of that product in the rate law is equal to log(q)/log(p). The reaction is considered to be x-order with respect to A and (x+y+z)-order overall.
  • In reactions that are 0th-order overall (see first bullet), the [math]rate=k[/math]. The graph of the concentration of a product or a reactant over time yields a straight line, and the slope of this line equals -k. The half-life of the reaction equals [math][A]_0/2k[/math].
  • In reactions that are 1st-order overall (see first bullet), the [math]rate=k[A][/math]. The graph of the natural log of the concentration of a product or a reactant over time does, and the slope of this line equals -k. The half-life of the reaction equals [math]ln(2)/k[/math].
  • In reactions that are 2nd-order overall (see first bullet), either [math]rate=k[A]^2[/math] or [math]rate=k[A][B][/math]. The graph of the inverse of the concentration of a product or a reactant over time does, and the slope of this line equals k. The half-life of the reaction equals [math]1/k[A]_0[/math].

Sample Questions: Kinetics

Questions in the kinetics section might involve...

  1. Reaction Rates
  2. Reaction Conditions (Temperature, Concentration, Particle Size, Cataylsts)
  3. Rate Laws (at state and national levels)
  4. Rate Constants (at state and national levels)

Links

  • Upper level reaction links [1]