Reaction Rate, Reaction Order & Rate constant Flashcards
What do reaction mechanisms consist of?
A series of elementary reaction steps that make up the overall reaction.
What is the rate of reaction expressed as?
A change of the concentration of a given reactant (or product) as a function of time:
What factors influence the rate of reaction?
- Concentration of reactants
- Temperature and pressure of the
- Solvent (polarity, ionic strength)
- Surface area of a solid reactant
- Action of catalysts
What is the rate law?
Relation between rate reaction and the reactant concentration
What is the rate constant dependent on?
Independent of concentrations but depends on T or P. Units depends on the overall order of the reaction
What is the rate law of a zero-order reaction?
Reaction proceeds at constant arte and is independent of the reaction concentration
What are the units of k for a zero-order reaction?
Concentration.time-1
mol.L-1.s-1
What is half-life?
The half (t1/2) of a reaction is the time taken for the concentration of reactant to fall to half of its original value
What is the shell-life (t0.9 or t10%)?
Is the length of time the product may safety be stored on the dispensary shelf before significant decomposition occur. It is the time taken for decomposition of 10% of the active drug to occur
What is the rate law of a first order reaction?
Rate depends on the concentration of one reactant
What are the unit of a first- order reaction?
Units of k: time-1
s-1 (or similar)
What is the plot of the logarithms of the concentration of the analyse (as ordinate) against time (as abscess) for a zero and first order reaction?
Linear with a gradient of -k
What is the rate constant in a second-order type I reaction?
Rate depend on the product of two concentration terms which refer to same reactant
What are units of k for a second order type I reaction?
Concentration-1.time-1
L.mol-1.s-1 (or similar)
What is the plot of the inverse of the concentration of the analyse (as ordinate) against time (as abscess) for a second order type I reaction?
Linear with a gradient of k
What is the rate law for a second order reaction type II?
Rate depends on the product of two concentration terms
What is Pseudo first order?
Rate depends on the concentration of one reactant
What are the units for a second order reaction?
Units of k: time-1
Ex: s-1
What are the units of Ea?
J.mol-1 (R= 8.31 J.K-1)
What are the units of T?
K
What are the units of A?
Units of the rate constant
What is happens to the half-life for n=0?
It increases with increasing concentration
What happens to the half-life for n=1?
It does not change with change in concentration
What happens to the half-life for n=2?
Decreases with increasing concentration
What must reactant molecules do in order to react?
Must collide with the appropriate orientation and with a minimum energy
What is activation energy?
Minimum energy needed for the reaction to occur. Ea comes from the thermal excitation of molecules
What does a large Ea mean?
Strong T dependence and a slow reaction
What does a small Ea mean?
Weak T dependence and a fast reaction
What is a catalyst?
Speeds up a reaction by providing an alternate reaction mechanism with a lower energy of activation compare to the unanalysed pathway
How is the rate changed?
By lowering Ea
What can enzymes do to a reaction rate?
Increase the A factor
Decrease the energy of activation. Increase the reaction rate without increasing the temperature
What mechanism describes the behaviour of many enzymes?
Michaelis-Mennen mechanism
What is Km?
A constant. It is the concentration of substrate [S] required to reach half-maximal velocity (Vmax/2). Under true conditions it is the estimate of the dissociation constant of the ES complex
What does a small Km mean?
Tight binding between E and S
What does a large Km mean?
Weak binding between E and S
What is Vmax?
A constant. Is the theoretical maximal rate of the reaction but it is never achieved in reality. Vmax is asymptotically approached as substrate is increased.
What would need to be required to reach Vmax?
All enzymes molecules to be tightly bound with substrate
What is kcat?
Turnover number. It is the number of substrate molecules converted to product per unit of time and per enzyme molecule (when E is saturated with S). It a measure of the catalytic activity of the enzyme
What is kcat/Km?
The catalytic efficiency. It is an estimate of “how perfect” the enzyme is. It is used to rank enzymes
What does a big kcat/Km mean?
That an substrate binds tightly to E (small Km), with a fast reaction of ES complex.
What is reversible competitive inhibition?
The substrate and the inhibitor are chemically similar. S and I compete for binding to the same active site: S and I cannot bind to the enzyme at the same time
What can overcome reversible competitive inhibition?
Increasing [S]
What happens to the constants in reversible competitive inhibition?
Maximum velocity (Vmax) unchanged Km is increased
What is non competitive inhibition?
When the substrate and the inhibitor are not similar. I does not bind to the active site of the enzyme. I binds equally well to both free E and the enzyme-substrate complex. Increasing [S] cannot overcome inhibition
What happens to the constants in reversible non competitive inhibition?
Km unchanged
Vmax is decreased
