PHYSICAL CHEM - KINETICS (A2)
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What is the rate equation for the general equation A + B –> C + D?
rate = k[A]^m[B]^n (m and n are the reaction orders in respect to A and B)
What is k in the rate equation?
• the rate constant • always the same for a specific reaction at particular temp
What is an order of reaction?
• the power to which the conc. of its reactant is raised in the rate equation • tells you how the conc. of a reactant affects the rate
How would a tripling the conc. of reactant with a reaction order of 1 affect the rate?
the rate would also triple
How would a doubling the conc. of reactant with a reaction order of 2 affect the rate?
the rate would x4
How would a doubling the conc. of reactant with a reaction order of 0 affect the rate?
the rate would stay the same
How does changing temp affect k and why?
• if temp is increased, k increases • when temp is increased rate of reaction increases but conc. of reactants and reaction orders stay the same so k must increase for the rate equation to balance
How does the shape of rate-conc. graphs tell you the reaction order?
• straight horizontal line - 0 order as changing conc. doesn’t change rate • straight line through origin - 1st order as rate is proportional to conc. • curved line - 2nd order (or higher)
What is the rate determining step?
the step with the slowest rate (determines the overall rate of the reaction)
How can you figure out which reactants are involved in the rate determining step?
• if a reactant appears in the rate equation it must affect the rate so it (or something derived from it) must be involved in the rate determining equation • if a reactant doesn’t appear in the rate equation then it won’t be involved in the rate determining step • however the rate determining step can involve an intermediate that isn’t in the rate equation
What information does the order of reaction with respect to a reactant give about the rate determining step?
shows the number of molecules of that reactant that are involved in the rate determining step
What is the arrhenius equation?
k = Ae^-Ea/RT - k is the rate constant - A is the arrhenius constant - Ea is the activation energy - R is the gas constant - T is temp
How is Ea calculated using the arrhenius equation?
Ea = (ln A - ln k) RT
What is the equation of the line for a graph of ln k against 1/T?
ln k (y) = -Ea/R (m) x 1/T (x) + lnA (c) (y = mx + c)
How can Ea be calculated from a graph of ln k against 1/T?
Ea is the gradient x R