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CLS3003-B: Chemistry (Semester 2) > L3 & L4 - Rates of Reaction > Flashcards

L3 & L4 - Rates of Reaction Flashcards

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1
Q

Define ‘reaction rate’.

A

The change in concentration of a species per unit time.

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2
Q

What is the equation for rate for the reaction A → B?

A 
Rate = Δ[B] / Δt
Rate = - Δ[A] / Δt

[A] - concentration of substance A
[B] - concentration of substance B
Δt - change in time

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3
Q

How do you find initial rate?

A

Gradient of a line beginning at t=0s.

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4
Q

How do you find instantaneous rate?

A

Gradient of a line / tangent that falls at the specific point / instant.

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5
Q

How do you find average rate?

A

Gradient of a tangent to the curve or gradient of a straight line.

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6
Q

What are the 3 principles of the collision theory?

A

1) Reactions occur when particles collide, but not all collisions lead to a reaction.
2) Particles must possess at least a minimum amount of energy.
3) Particles must approach each other in a certain relative orientation.

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7
Q

What do the factors that alter rates of reaction affect?

A
  • The overall number of collisions occurring

- The number of particles with enough energy to react

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8
Q

Define ‘activation energy’.

A

The minimum energy required to initiate a chemical reaction.

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9
Q

What is the transition state?

A

The highest energy point in the reaction.

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10
Q

What factors affect the rate of reaction?

A
  • Temperature
  • Catalyst
  • Surface Area (Solid Reagents / Heterogeneous Catalysts)
  • Pressure (Gases)
  • Concentration (Liquids)
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11
Q

Explain how temperature affects the rate of reaction.

A
  • Increasing temperature increases the rate of reaction
  • Particle speeds increase, so collisions are more frequent
  • Particles have more energy, so they can overcome the energy barrier
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12
Q

Explain how catalysts affect the rate of reaction.

A
  • Adding a catalyst increases the rate of reaction
  • Works by providing an alternative reaction pathway, with a lower activation energy
  • More particles now have the energy to react
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13
Q

Explain how surface area affects the rate of reaction.

A
  • Increasing surface area increases the rate of reaction
  • Increases chances of a collision, as more particles are exposed
  • Powdered solids react quicker than large lumps
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14
Q

Explain how pressure affects the rate of reaction.

A
  • Increasing pressure increases the rate of reaction

- Forces gas particles close together; increasing the collision frequency, so rate of reaction increases

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15
Q

Explain how concentration affects the rate of reaction.

A
  • Increasing concentration increases the rate of reaction

- Larger number of particles = more collisions

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16
Q

What does the rate of any reaction depend on?

A

The concentration of reactants.

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17
Q

For any chemical reaction:
wA + xB → yC + zD

What is the general form of the rate law?

A

Rate = k[A]^m [B]^n

k - rate constant
m, n - reaction order with reference to that reagent
(m + n) - overall order of reaction

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18
Q

How can the order of reaction be found?

A

ONLY by experiment

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19
Q

What does it mean if a reaction is zero-order?

A

The rate is independent of the concentration of the reactant.

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20
Q

What is the equation for a zero-order reaction?

A

A0 - At = kt

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21
Q

For zero-order reactions, the rate does not change. What does this mean that the reaction depends on?

A

A catalytic bottleneck

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22
Q

What does it mean if a reaction is first-order?

A

Rate = - Δ[A] / Δt = k[A]

23
Q

What does the integrated rate law describe the concentration of a reactant as?

A

A function of time

24
Q

What does it mean if a reaction is second-order?

A

The reaction has a rate law with a sum of the exponents equal to 2.

25
Q

What is the equation for a second-order reaction?

A 
Rate = k [A] [B]
Rate = k [A]^2
26
Q

Study of reactions that depend on variation of 2 reagents is complicated. So what should be done?

A

Pseudo first-order conditions should be used.

27
Q

Define ‘half life’.

A

The time taken for the concentration of a reactant to drop to half its original value.

28
Q

For a first-order reaction, half-life is ________.

A

constant

29
Q

What do first-order reactions depend on?

A

Rate constant, k

30
Q

What do first-order reactions not depend on?

A

Concentration of reactant

31
Q

Describe zero-order reactions with reference to a rate vs. concentration graph.

A
  • The rate does not depend on the concentration

- Line is parallel to the x-axis

32
Q

Describe first-order reactions with reference to a rate vs. concentration graph.

A
  • The rate is proportional to the concentration

- Straight line of fixed gradient (Gradient of the line = rate constant for the reaction)

33
Q

Describe second-order reactions with reference to a rate vs. concentration graph.

A
  • The rate is proportional to the square of the reaction

- A curved line, showing rate or reaction increasing with concentration overall

34
Q

Describe zero-order reactions with reference to a rate vs. time graph.

A
  • A straight line showing a constant decline in concentration
  • Half life decreases as reaction progresses
35
Q

Describe first-order reactions with reference to a rate vs. time graph.

A
  • A slightly sloping curve which drops with a constant half-life
36
Q

Describe second-order reactions with reference to a rate vs. time graph.

A
  • A curve which declines steeply at first but then levels out
  • Half life increases as reaction progresses
37
Q

What is the overall rate of a multi-step reaction governed by?

A

Rate Determining Step

38
Q

What does the rate equation include?

A

All of the molecules involved in the Rate Determining Step

39
Q

Define ‘molecularity’.

A

The number of individual molecules of the reacting species taking part in the rate determining step of a reaction.

40
Q

A reaction may give different products depending on the __________.

A

conditions

41
Q

A fast reaction (low activation energy) is referred to as ___________ __________.

A

kinetically favourable

42
Q

Thermodynamically, a reaction that leaves the system in a more ______ state is favourable.

A

stable

43
Q

What does a catalyst do?

A

Reduce activation energy of a reaction.

  • Brings reactants closer together
  • Weakens bonds in reactants
  • Stabilises transition state
44
Q

What is an enzyme? Describe some of the properties of an enzyme.

A

A biological catalyst

  • Mostly globular proteins
  • Specific, so only catalyse one reaction / type of reaction
45
Q

List the different types of enzymes. (x6)`

A
  • Oxidoreductases
  • Transferases
  • Hydrolases
  • Lyases / deXases
  • Isomerases
  • Ligases
46
Q

What do the names of enzymes depend on?

A

The substrate or reaction being catalysed.

47
Q

Describe the lock and key model of enzyme action.

A
  • Both enzyme and substrate have a unique, fixed shape
  • Only one substrate (key) can fit into the enzyme’s active site (lock)
  • Doesn’t explain all experimental observations
48
Q

Describe the induced fit model of enzyme action.

A

1) Substrate enters active site of enzyme
2) Enzyme-substrate complex is complete
3) Enzyme changes shape slightly as the substrate binds
4) Enzyme/products complex is formed and products leave the active site of enzyme

E + S ⇌ ES → EP → E + P

49
Q

Which factors influence enzyme activity?

A

Non-Specific

  • Temperature
  • pH

Kinetics
- Concentration

Specific

  • Cofactors
  • Coenzymes
  • Inhibitors
50
Q

Describe how temperature influences enzyme activity.

A
  • Enzymes have optimum temperature

- Enzyme becomes denatured beyond this temperature

51
Q

Describe how pH influences enzyme activity.

A
  • Affects protonation of enzyme and active site
  • Affects protonation of substrate
  • Affects conformation of enzyme
  • Affects stability of enzyme
  • Each enzyme has different optimum pH
52
Q

Describe how concentration influences enzyme activity.

A
  • Small Km = strong E-S binding
  • Large Km = weak E-S binding, little activity at low concentrations of substrate
  • Km is related to the rate constants for each step in the enzyme reaction
53
Q

What does the lineweaver-burke plot show?

A
  • Inverse initial velocity (1/V0) plotted against the inverse of the substrate concentration (1/[S])
  • Vmax can be accurately determined and thus KM can also be determined with accuracy because a straight line is formed

CLS3003-B: Chemistry (Semester 2) (12 decks)

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