L3 & L4 - Rates of Reaction Flashcards
Define ‘reaction rate’.
The change in concentration of a species per unit time.
What is the equation for rate for the reaction A → B?
Rate = Δ[B] / Δt Rate = - Δ[A] / Δt
[A] - concentration of substance A
[B] - concentration of substance B
Δt - change in time
How do you find initial rate?
Gradient of a line beginning at t=0s.
How do you find instantaneous rate?
Gradient of a line / tangent that falls at the specific point / instant.
How do you find average rate?
Gradient of a tangent to the curve or gradient of a straight line.
What are the 3 principles of the collision theory?
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.
What do the factors that alter rates of reaction affect?
- The overall number of collisions occurring
- The number of particles with enough energy to react
Define ‘activation energy’.
The minimum energy required to initiate a chemical reaction.
What is the transition state?
The highest energy point in the reaction.
What factors affect the rate of reaction?
- Temperature
- Catalyst
- Surface Area (Solid Reagents / Heterogeneous Catalysts)
- Pressure (Gases)
- Concentration (Liquids)
Explain how temperature affects the rate of reaction.
- 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
Explain how catalysts affect the rate of reaction.
- 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
Explain how surface area affects the rate of reaction.
- 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
Explain how pressure affects the rate of reaction.
- Increasing pressure increases the rate of reaction
- Forces gas particles close together; increasing the collision frequency, so rate of reaction increases
Explain how concentration affects the rate of reaction.
- Increasing concentration increases the rate of reaction
- Larger number of particles = more collisions
What does the rate of any reaction depend on?
The concentration of reactants.
For any chemical reaction:
wA + xB → yC + zD
What is the general form of the rate law?
Rate = k[A]^m [B]^n
k - rate constant
m, n - reaction order with reference to that reagent
(m + n) - overall order of reaction
How can the order of reaction be found?
ONLY by experiment
What does it mean if a reaction is zero-order?
The rate is independent of the concentration of the reactant.
What is the equation for a zero-order reaction?
A0 - At = kt
For zero-order reactions, the rate does not change. What does this mean that the reaction depends on?
A catalytic bottleneck
What does it mean if a reaction is first-order?
Rate = - Δ[A] / Δt = k[A]
What does the integrated rate law describe the concentration of a reactant as?
A function of time
What does it mean if a reaction is second-order?
The reaction has a rate law with a sum of the exponents equal to 2.
What is the equation for a second-order reaction?
Rate = k [A] [B] Rate = k [A]^2
Study of reactions that depend on variation of 2 reagents is complicated. So what should be done?
Pseudo first-order conditions should be used.
Define ‘half life’.
The time taken for the concentration of a reactant to drop to half its original value.
For a first-order reaction, half-life is ________.
constant
What do first-order reactions depend on?
Rate constant, k
What do first-order reactions not depend on?
Concentration of reactant
Describe zero-order reactions with reference to a rate vs. concentration graph.
- The rate does not depend on the concentration
- Line is parallel to the x-axis
Describe first-order reactions with reference to a rate vs. concentration graph.
- The rate is proportional to the concentration
- Straight line of fixed gradient (Gradient of the line = rate constant for the reaction)
Describe second-order reactions with reference to a rate vs. concentration graph.
- The rate is proportional to the square of the reaction
- A curved line, showing rate or reaction increasing with concentration overall
Describe zero-order reactions with reference to a rate vs. time graph.
- A straight line showing a constant decline in concentration
- Half life decreases as reaction progresses
Describe first-order reactions with reference to a rate vs. time graph.
- A slightly sloping curve which drops with a constant half-life
Describe second-order reactions with reference to a rate vs. time graph.
- A curve which declines steeply at first but then levels out
- Half life increases as reaction progresses
What is the overall rate of a multi-step reaction governed by?
Rate Determining Step
What does the rate equation include?
All of the molecules involved in the Rate Determining Step
Define ‘molecularity’.
The number of individual molecules of the reacting species taking part in the rate determining step of a reaction.
A reaction may give different products depending on the __________.
conditions
A fast reaction (low activation energy) is referred to as ___________ __________.
kinetically favourable
Thermodynamically, a reaction that leaves the system in a more ______ state is favourable.
stable
What does a catalyst do?
Reduce activation energy of a reaction.
- Brings reactants closer together
- Weakens bonds in reactants
- Stabilises transition state
What is an enzyme? Describe some of the properties of an enzyme.
A biological catalyst
- Mostly globular proteins
- Specific, so only catalyse one reaction / type of reaction
List the different types of enzymes. (x6)`
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases / deXases
- Isomerases
- Ligases
What do the names of enzymes depend on?
The substrate or reaction being catalysed.
Describe the lock and key model of enzyme action.
- 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
Describe the induced fit model of enzyme action.
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
Which factors influence enzyme activity?
Non-Specific
- Temperature
- pH
Kinetics
- Concentration
Specific
- Cofactors
- Coenzymes
- Inhibitors
Describe how temperature influences enzyme activity.
- Enzymes have optimum temperature
- Enzyme becomes denatured beyond this temperature
Describe how pH influences enzyme activity.
- 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
Describe how concentration influences enzyme activity.
- 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
What does the lineweaver-burke plot show?
- 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
