3 + 4 Rates of Reaction Flashcards
Define ‘rate of reaction.’
the change in concentration of a species per unit of time
State the two ways rate of reaction can be expressed for the single step reaction A → B.
Rate = -Δ[A] / Δt
OR
Rate = Δ[B] / Δt
State the units for any rate of reaction.
mol L-1 s-1 (or mol/l/s)
How can the rate of a reaction be calculated directly from a concentration-time graph?
by drawing a tangent and then calculating the gradient of it
When do reactions occur and which two conditions must be met in order for a reaction to occur?
reactions occur when particles collide
particles must possess at least a minimum amount of energy
particles must approach each other in a certain, relative orientation
Which two factors influence the reaction rate?
- the overall number of collisions occurring
2. the number of particles with enough energy to react
Define ‘activation energy, Ea.’
the minimum amount of energy required to initiate a chemical reaction
Define ‘transition state.’
highest energy point in the reaction
the configuration of the atoms at the time of the collision
On the following graph, label (1-4) reactants, products, ΔG and the transition state of the reaction.
1 - reactants
2 - products
3 - transition state
4 - ΔG
Draw out a normal Boltzmann distribution for a reaction.
see document
Explain how increasing temperature increases the rate of reaction.
particle speed increases so collisions are more frequent
particles have more energy, so can overcome the energy barrier
Draw out a normal Boltzmann distribution for a reaction and on the same axes draw another showing the effect of temperature on a rate of reaction.
see document
Explain how adding a catalyst increases the rate of reaction.
works by providing an alternative reaction pathway with a lower Ea
more particles now have the energy to react
Draw out a normal Boltzmann distribution for a reaction and on the same axes draw another showing the effect of a catalyst on a rate of reaction.
see document
Explain how increasing surface area of solid reagents/heterogeneous catalysts increases the rate of a reaction.
increases chances of a collision - more particles are exposed
powdered solids react quicker than large lumps
Explain how increasing the pressure of a gas increases the rate of a reaction.
forces gas particles closer together increasing the frequency of collision so the reaction rate increases
Explain how increasing the concentration of liquids increases the rate of a reaction.
the larger number of particles, therefore, more collisions
What does the rate of any reaction mainly depend on?
concentration of reactants
For the reaction equation, wA + xB → yC + zD, what is the general form of a rate equation?
Rate = k [A]^m [B]^n
where k = rate constant
m,n = reaction orders w.r.t to those reagents
m+n = overall reaction order
What is the only way a reaction order can be found?
by experiment
What is meant by a ‘zero-order’ of reaction?
where the rate is independent of the concentration of reactant
Rate = k
Overall order = 0
The rate does not change
What does a ‘zero-order’ reaction depend on?
depends on a catalytic bottleneck (catalyst)
What is meant by a ‘first-order’ of reaction?
where the rate is proportional to the concentration of a single reactant raised to the first (n^1) power
Rate = [A] k
overall reaction order = 1
How does integrated rate law describe the concentration of a reactant as a function of time?
ln[A]t = -kt + ln[A]0
y = mx+ b
Define ‘half-life.’
the time taken for the concentration of a reactant to drop to half of its initial value
What is half-life for a ‘first-order’ reaction?
half-life is constant; depends on the rate constant (k) and the concentration of reactant
Where is constant half-life usually seen?
In the half-life of radioactive isotopes i.e. medicine, nuclear energy etc…
Draw a graph to show how half-life varies during a ‘first-order’ reaction.
see document
State the formula that can be used to calculate k (rate constant) for a first-order of reaction using half-life (t1/2).
k = ln(2) / t1/2
What is meant by a ‘second-order’ of reaction?
has a rate law with the sum of exponents equal to 2
overall reaction order is 2
Rate = k[A][B] OR Rate = k[A]^2
m + n = 2 OR m = 2
What is meant by a ‘pseudo’ first-order reaction?
A reaction that is not a naturally ‘first-order’ reaction but instead depends on the variation of two reagents
pseudo = fake
Describe the shape of each of the following order rate-concentration graphs in terms of half-life:
a) Second-order reaction
b) First-order reaction
c) Zero-order reaction
a) the curve declines steeply at first then levels out. Half-life increases as the reaction progresses
b) A slightly-sloping curve which drops with a constant half-life
c) a straight line showing a constant decline in concentration. Half-life decreases as the reaction progresses
Describe the relationship between each of the following order rates and concentrations:
a) Second-order reaction
b) First-order reaction
c) Zero-order reaction
a) the rate is proportional to the square of the concentration ([X]^2 x k) so you get a curved line
b) the rate is proportional to the concentration ([X] x k)
the gradient of the line equals the rate constant for the reaction
c) the rate does not depend on the concentration
the line is parallel to the x-axis ([X]^0 x k)
On the following rate-concentration graphs (see document), identify the order of reaction for lines (1-3).
1 - first-order
2 - second-order
3 - zero-oder
Using the initial rates table on the document, determine the order with respect to reactants A and B.
For [A]: Initial conc → 0.5M/0.25M = 2 Rate → 0.0052/0.0013 = 4 So, when [A] x2 Rate x 4 therefore, second-order w.r.t. [A]
For [B]: Initial conc → 0.04M/0.02M = 2 Rate → 0.0026/0.0013 = 2 So, when [B] x2 Rate x 2 therefore, first-order w.r.t. [A]
Complete the flow chart about the order of reaction starting with the following question following both answers yes and no:
Plot [A] Vs t
Is graph linear?
Yes OR No
See document
What do many reactions consist of?
a series of separate stages
each step has its own rate and rate constant
What governs the overall rate of a multi-step reaction?
the (slowest) rate-determining step
What does the rate equation of a reaction only include?
the molecules involved in the rate-determining-step (RDS)
Define ‘molecularity.’
the number of individual molecules of the reacting species taking part in the rate determining step of the reaction
State the general rate determining step and rate law for a unimolecular reaction.
RDS: A →
Rate law: Rate = k [A]
State the general rate determining step and rate law for a bimolecular reaction.
RDS: A + A →
OR A + B →
Rate law: Rate = k [A]^2
OR Rate = k[A][B]
State the general rate determining step and rate law for a trimolecular reaction.
RDS: A + A + A → OR A + A + B → OR A + B + C → Rate law: Rate = k [A]^3 OR Rate = k[A]^2[B] OR Rate = k[A][B][C]
Why may a reaction give different products?
depending on conditions
What is a fast reaction referred to as?
low Ea
and kinetically favourable
Thermodynamically, when is a reaction favourable?
when it leaves a system in a more stable state
Fill in the labels 1-8 on the graph on kinetic and thermodynamic control (see document).
1 – kinetic control 2 – thermodynamic control 3 – transition state 4 – intermediate 5 – ΔG 6 – starting materials 7 – kinetic products 8 – thermodynamic products
Which 4 things does a catalyst do for a reaction?
reduces the Ea of a reaction
brings reactants closer together
weaken bonds in reactants
stabilise the transition state
Draw 2 enthalpy profile diagrams for a reaction with and without a catalyst. For each diagram label Ea and ΔH.
See document
Define ‘enzyme.’
a biological catalyst
What type of proteins are enzymes and what do they catalyse?
mostly globular proteins
catalyse one reaction/type of reaction
What do ‘oxidoreductases’ catalyse?
reduction (add hydrogen atoms or electrons)
What do ‘transferases’ catalyse?
transfer groups from one molecule to another
What do ‘hydrolases’ catalyse?
break bonds by adding water
What do ‘lyases’ catalyse?
(deXases); remove a functional group from a molecule i.e. decarboxylases/dehydrogenases
What do ‘isomerases’ catalyse?
switch molecules between isomers
What do the names of enzymes tend to depend on?
the substrate/reaction catalysed
Name an example of a hydrolase enzyme and the reaction it catalyses drawing out the molecules too.
lactose → galactose
lactase
see document for diagrams
Briefly explain 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)
What does the ‘lock-and-key’ model of enzyme action not explain?
all experimental observations
Briefly explain the ‘induced-fit’ model of enzyme action.
substrate enters active site of the enzyme
forms an E-S complex and enzyme changes shape slightly as the substrate binds
an E-P complex is formed
products leave the active site of the enzyme
State the 4 non-specific factors influencing enzyme activity.
temperature
pH
kinetics
concentration
State the 3 specific factors influencing enzyme activity.
cofactors
coenzymes
inhibitors
Fill in the processes 1-2 on the diagram of an enzyme.
1 - denaturation
2 - renaturation
What temperature do all enzymes have and what happens to the enzyme above it? Provide an example for each.
an ‘optimum’ temperature i.e. 37°C in humans
enzyme becomes denatured above it i.e. fever, pyrexia does this
Give an example of when the effect of temperature on enzymes is exploited.
In the polymerase chain reaction (PCR)
i.e. Taq polymerase, primer etc…
Describe the effect of pH on enzymes.
each enzyme has a different optimum pH
affects protonation of enzyme and active site
conformation and stability of enzyme
Which features do concentrations of enzyme/substrate affect?
the Km and Vmax of enzymes
What is the ‘michaelis-menten’ equation and what is it used for?
V0 = Vmax[S]/{Km + [S]}
V0 - velocity or reaction rate
Vmax[S] - maximum velocity or maximal reaction rate
Km - Michaelis constant (where concentration is working at half of Vmax)
[S] - Substrate concentration
reflection of the affinity of the enzyme for its substrate
Explain the significance of Km in enzyme activity.
Small Km = strong E-S binding
Large Km = weak E-S binding, little activity at low concentrations of substrate
What is Km is related to?
the rate constants for each step in the enzyme reaction
Fill in labels 1-3 on the substrate concentration vs reaction rate graph on the document.
1 - Km
2 - Vmax
3 - 1/2Vmax
Fill in labels 1-5 on the Lineweaver-Burke plot on the document.
1 – Km/Vmax 2 – 1/Vi 3 – -1/Km 4 – 1/Vmax 5 – 1/[S]
What do cofactors and coenzymes do?
Complete the structure of conjugated enzymes
Briefly describe how enzymes are activated.
Apoenzyme becomes active by binding of coenzyme or cofactor to enzyme
Holoenzyme is formed when associated cofactor/coenzyme binds to the enzyme’s active site
State 6 examples of cofactors/coenzymes.
Zn^2+ Mg^2+ Vitamin B12 NAD(P)H FAD+ Coenzyme A
Identify the 4 cofactors/coenzyme structures on the document.
1 – Vitamin B12
2 – NAD(P)H
3 – Coenzyme A
4 – FAD+
State the 5 ways non-specific enzyme inhibition/deactivation work.
acids and bases temperature alcohol reducing agents heavy metals
Define ‘competitive’ reversible inhibition.
bind to free enzyme only - usually in an active site
E + I ⇌ EI
Define ‘irreversible’ enzyme inhibition.
bind so tightly to the enzyme they cannot be displaced
How does irreversible enzyme inhibition work.
often form covalent bonds, modifying the structure and removing the activity of the enzyme active site
Give 3 examples of irreversible enzyme inhibition.
aspirin
organophosphates
suicide inhibitors
Identify each of the following irreversible enzyme inhibitors on the document.
1 – Aspirin
2 – catalytically inactive
3 – parathion
4 – Novichock agents
Define ‘allosteric’ enzyme inhibition.
bind in a regulatory site, not an active site
Give an example of allosteric enzyme inhibition.
strychrine
Define ‘non-competitive’ enzyme inhibition.
bind free enzyme and enzyme-substrate complex equally well; don’t bind in exact site occupied by substrate
E + I → EI
and/or ES + I → ESI
Define ‘uncompetitive’ enzyme inhibition.
bind the enzyme-substrate complex
ES + I → ESI
Identify the types of enzyme inhibition labelled 1-3 in each diagram.
1 – competitive reversible
2 – allosteric
3 – uncompetitive/non competitive
Beta-lactamases deactivate penicillin-type antibiotics by the mechanism shown below (see document). Clavulanic acid inhibits the deactivation of penicillin; its mechanism of action is shown below. What type of inhibitor is clavulanic acid?
A competitive inhibitor
State the integrated rate law equation for a ‘zero order’ equation.
A0 – At = kt
Rearrange “-d[A]/[A] = kdt” for the integrated rate law and state what form it holds.
-d[At]/[A0] = kdt (divide by -d)
[A]t/[A]0 = -kt (apply logs)
ln{[A]t/[A]0} = –kt (apply log law)
ln[A]t – ln[A]0 = –kt (+ln[A]0)
ln[A]t = –kt + ln[A]0
This corresponds to:
y = mx + bState the integrated rate law for:
a) a zero-order reaction
b) a first-order reaction
c) a second-order reaction
a) [A]t = -kt + [A]0
b) ln[A]t = -kt + ln[A]0
c) 1/[A]t = -kt + 1/[A]0
For a first-order reaction, rearrange the integrated rate law for the formula to find k.
ln[A]t = -kt + ln[A]0 ln½[A]0 – ln[A]0 = –kt½ ln(½) = –kt½ ln2 = kt½ k = ln2/t½
In what instance is a reaction a pseudo-first-order reaction?
when one reagent is present in very large excess
Why is there two Ea values for an enzyme catalysed enthalpy diagram?
because there is a different intermediate product to get to before the product
Describe the four main stages of PCR and how it exploits enzymes.
1) denature DNA and separate the two strands by heating to a high temperature (approx. 95°C)
2) anneal primer to join the two strands together
3) free nucleotides are left free to join as base-pairs again (elongation)
4) this results in the DNA splitting which is repeated until the DNA is amplified
What is the role of Taq polymerase in PCR?
Taq polymerase (adapted for high temperatures) attaches nucleotides to a DNA template copying the DNA
For each of the following types of enzyme inhibition, state, in very basic terms, how it works:
a) Competitive
b) Allosteric
c) Non-competitive
d) Uncompetitive
a) binds to same active site as enzyme
b) changes the shape of the enzyme’s active site by binding to a regulatory site
c) Has a different active site and results in a conformational change of substrate active site
d) binds to E-S complex to stop it from working
