Flashcards in Rates Deck (18):
Definition and units of rate of reaction
Change in conc. of reactants or products/time
mol dm ^-3 s^-1
How is rate measured?
Plot graph of conc. against time
At any time, the rate is equal to the slope/gradient of a tangent at that point on the graph
What is initial rate of reaction?
The change in concentration of a reactant or product per unit time at the start of a reaction when t = 0
How can we obtain data for a conc. ?
a) Reactions involving acids or bases
b) Reactions that produce gases
c) Reactions that produce visual changes
a) Measure pH changes using titration or a pH meter
b) Change in volume or pressure, or change in mass of reactants
c) Time taken to form precipitate or colour change
What does zero order mean?
The rate is unaffected by changing the conc. of this reactant
What does first order mean?
If this reactant is increased by s.f A, rate also increases A times
What does second order mean?
Rate increased by square of factor conc. this reactant is increased by
What is the rate constant?
Rate = k x concentrations of reactants to the power of their order
How do concentration and half life vary for zero order reactions and what is the shape of the concentration - time graph?
Concentration decreases at a constant rate
Half life decreases with time
Straight line with neg. gradient
Shape of rate-conc. graph of zero order reaction
How do concentration and half life vary for first order reactions and what is the shape of the concentration - time graph?
Concentration halves in equal time intervals
Half life is constant
Exponential decay curve
Shape of rate-conc graph of first order reaction
How do concentration and half life vary for second order reactions and what is the shape of the concentration - time graph?
Conc. decreases rapidly but then rate of decrease slows down
Half life increases with time
Steeper exponential decay
Shape of rate-conc graph of second order reaction
What effect does temp. have on rate constant?
Higher temp = larger value of k
What is a reaction mechanism?
A series of steps that, together make up the overall reaction
What is the rate determining step?
The slowest step in the reaction mechanism of a multi-step reaction