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Flashcards in Rates Deck (18):
1

Definition and units of rate of reaction

Change in conc. of reactants or products/time
mol dm ^-3 s^-1

2

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

3

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

4

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

5

What does zero order mean?

The rate is unaffected by changing the conc. of this reactant

6

What does first order mean?

If this reactant is increased by s.f A, rate also increases A times

7

What does second order mean?

Rate increased by square of factor conc. this reactant is increased by

8

What is the rate constant?

Rate = k x concentrations of reactants to the power of their order

9

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

10

Shape of rate-conc. graph of zero order reaction

horizontal line

11

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

12

Shape of rate-conc graph of first order reaction

Straight line

13

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

14

Shape of rate-conc graph of second order reaction

Exponential curve

15

What effect does temp. have on rate constant?

Higher temp = larger value of k

16

What is a reaction mechanism?

A series of steps that, together make up the overall reaction

17

What is the rate determining step?

The slowest step in the reaction mechanism of a multi-step reaction

18

What three things should a proposed mechanism be consistent with if it is to be regarded as plausible?

- Slow step should match rate equation e.g
A + B + C -> D doesn't match rate= k[A][B]
- No multibody collisions, max. two molecules
- Equations in mechanism must add up to overall equation