Topic 16.1: Rate expression and reaction mechanism Flashcards Preview

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Flashcards in Topic 16.1: Rate expression and reaction mechanism Deck (26)
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1
Q

How can the rate law between two reactants (A / B) be determined?

A

Experimentally

Not from stoichiometric equations

2
Q

Mathematical relationship of reaction rate with reactant concentration

A

Rate = k [A]^x [B]^y

3
Q

Rate constant (k)

A

Particular for a reaction at a specific temperature

4
Q

Overall order (a+b)

A

Sum of the magnitude orders

5
Q

Method to determine the rate law

A

The initial rate is worked out using a fixed amount of B and changing the concentration of A.

6
Q

Zero order reaction

a) Description
b) Rate expression
c) Units of rate constant

A

a) The rate does not depend on concentration
b) Rate = k
c) mol dm-3 s-

7
Q

First order reaction

a) Description
b) Rate expression
c) Units of rate constant

A

a) The rate is directly proportional to concentration.
b) Rate = k[A]
c) s-1

8
Q

Second order reaction

a) Description
b) Rate expression
c) Units of rate constant

A

a) The rate is proportional to the square of the concentration.
b) Rate = k[A]^2
c) mol-1 dm3 s-

9
Q

Third order reaction

a) Description
b) Rate expression
c) Units of rate constant

A

a) The rate is proportional to the cube of the concentration.
b) Rate = k[A]^3
c) mol-2 dm6 s-

10
Q

Graphs (Rate vs Concentration)

a) Zero order
b) First order
c) Second order

A

a) Constant rate
b) Linear rate
c) Square rate

11
Q

Graphs (Concentration vs Time)

a) Zero order
b) First order
c) Second order

A

a) Negative linear
b) Negative curve with constant half-life
c) Negative curve with no constant half-life

12
Q

Half-life definition

A

Time required for the concentration of a reactant to reach half its initial value.

13
Q

Half life for a first order rate law

A

It does not depend on the initial concentration of any reactant, so it remains constant throughout the reaction.

14
Q

Reaction mechanism definition

A

Sequence of steps by which a chemical reaction occurs

15
Q

Elementary reactions definition

A

Reactions that occur in a single step

16
Q

Special feature of the rate law of an elementary reaction

A

Its rate law is based directly on its molecularity

17
Q

Molecularity definition

A

Number of molecules that participate as reactants in an elementary reaction

18
Q

Why do most chemical reactions occur by mechanisms that involve two or more elementary reactions?

A

Since the chances of a successful collision between three or more particles are extremely small compared to the one between two.

19
Q

What is the overall result of the chemical equations for the elementary reactions in a multistep mechanism?

A

Chemical equation of the overall process

20
Q

Intermediates definition

A

Substances that are consumed in one elementary reaction and consumed in the next

21
Q

The rate law of a multistep mechanism based on the ones of the elementary reactions

A

The overall rate of reaction depends upon the rate of the slowest step, also known as rate determining step.

22
Q

Rate law for mechanisms with a slow initial step

A

a) The rate of the overall reaction depends on the one of step 1.
b) The rate law of the overall reaction equals the one of step 1

23
Q

Rate law for mechanisms with a fast initial step.

A

We can solve for the concentration of an intermediate by assuming that an equilibrium is established in the fast step.

24
Q

Rate Determining Step (RDS)

A

The step with the highest Ea, which acts as a limit on the rate of reaction (slow)

25
Q

How do catalysts speed up a chemical reaction?

A

By changing a reaction mechanism that has a lower Ea than its original one.

26
Q

Rules for writing mechanisms (4)

A

a) The mechanism must agree with the overall stoichiometric equation.
b) A maximum of two particles can react in any step.
c) All species in the rate equation must appear in the mechanism in or before the RDS.
d) The power of a particular reactant’s concentration in the rate equation indicates the number of times it appears in the mechanism up to and including RDS.