Flashcards in Lecture 19: Kinetics II Deck (45)
How does temperature affect the rate constant?
Each ten degree rise in temperature causes a 2-3x increase in reaction rate.
As you increase the heat, the molecules in the reaction acquire more kinetic energy, collide more often, resulting in a higher reaction rate (collision theory)
What is the collision theory?
A reaction consists of bond breaking and bond forming. For a molecule to react, it must be in the right orientation and be collided with the right amount of energy.
What is the activation energy?
The minimum energy required for a reaction to proceed
How can we visualise energy changes ( on what kind of graph?)
Reaction coordinate diagram:
Transition state = highest point on the curve
Activated complex = species at the transition state
Activation energy = the energy between the reactants and the activated complex
What happens as the reactant gets up to the transition state?
It will transit and become the product
What is the arrhenius equation?
k = A e (-Ea/RT)
k = reaction rate
A = arrhenius factor or frequency constant
Ea = activation energy
R = universal gas constant. Either 1.987 kCal/°mol or 8.314JK-1mol-1
T = absolute temperature in kelvin
What does the arrhenius equation state?
The relationship between reaction rate and temperature.
What does the frequency factor represent?
The likelihood that reactions will occur when reactants align in the proper orientation
What can we obtain from an arrhenius plot?
Activation energy : from the slope of lnk vs. 1/temperature
The frequency factor: the y intercept of lnk vs. 1/temperature
What unit should the frequency factor be?
How are the points of an arrhenius plot obtained?
They are the determination of the rate constant at different temperatures. Then the axis are natural logged to obtain a straight line
What happens if we dont want to have to conduct 11 experiments to do this?
Just doing 2 experiments and obtaining two different rate constants at two different temperatures
Then we can use a calculation method by using 2 arrhenius equations to work out the frequency constant and activation energy
What is the equation to obtain frequency constant and activation energy of a reaction using just two reaction rates at two different temperatures?
ln(k2/k1) = Ea/R (T2-T1/T2xT1)
You work out the activation energy first, then use that in the original arrhenius equation to work out the frequency factor.
How does the concentration of reactants affect the reaction rate?
The chemical reaction is proportional to the product of the molar concentration of the reactants
The number of collisions per second depends on the number of particles per litre (kinetic molar theory)
This is true for all types of reactions which are NOT zero order
How do solvents affect the rate of reaction?
For non electrolytes: Polarity - predominate factor
How does polarity affect reaction rate in non electrolytes?
In reality the reactant and product will not always have a similar solubility.
Polar solvents accelerate reactions that form products having higher internal pressures than reactants, i.e. To form products more polar than reactants
If however, products are less polar than reactants, they are accelerated by solvents of low polarity or internal pressures are retarded by polar solvents.
What is the integrated rate law that describes the solubility parameters of solvent and solute?
lnk = lnko + V/RT (ΔδA + ΔδB - ΔδP)
Where V = molar volume of solvent and solute
Δδ is the difference in solubility parameters between solute and solvent. If there is a big difference, it will be a big number.
ko is the rate constant in an infinitely dilute solution
What causes the reaction rate to be high? (Polarity in non electrolytes)
ΔδA and ΔδB have big differences and ΔδP has a small difference
If internal pressure (polarity) of a product is similar to solvent AND polarity of reactants are different to solvent
If vice versa, the reaction rate will be small
How does ionic strength increase the rate of reaction (in electrolytes)
If both reactants are similarly charged, e.g. Positive and positive, and negative and negative, then the rate of reaction will be high,
However if reactants do not have similar charge, like positive and negative, the rate of reaction will be low
What is the integrated rate law that describes the ionic strength of an ion in solution?
lnk = lnko + 1.02 x ZA x ZB x √μ
Where ko = the rate constant in an infinitely dilute solution (when 1.02ZAZB√μ = 0)
ZA or ZB = ionic charges of reactants
μ = ionic strength, = 1/2C1Z1^2
Why are interactions between ion and dipolar molecules and those between neutral molecules less sensitive to ionic strength?
They are less sensitive due to neutralising effects
What happens if the reaction products are oppositely charged ions?
There is a considerable increase in the reaction rate obtained with increasing ionic strength of the reaction mixture
What is the dielectric constant?
One of the earliest, simplest parameter used to describe the ability of a solvent to store charge
What does the ability of a co solvent to increase the solubility of non polar drugs depends on?
The co solvents lack of polarity in water by reducing the intermolecular interactions and in particular, hydrogen bonding between solute and solvent
What does a large dielectric constant indicate.
A very polar solvent
What is the integrated rate law which describes the effect of dielectric constant on the rate constant of an ionic reaction?
lnkE = infinity - NZAZBe^2I / RTr E
Where E is the dielectric constant of the solution, this is approximately = to the solvent in dilute solutions
N = avogadros number
Zx is the charge of the ionic species
e = unit of ionic charge
r = distance between ions
How does the dielectric constant affect the rate of reaction?
Reactions involving oppositely charged ions OR neutral molecules are accelerated by solvents non polar solvents (low DC)
Reactions involving similarly charged ions are accelerated by polar solvents (high DC)
How fast is the reaction if it includes oppositely charged reactants in a high dielectric constant solvent?
A low reaction rate. Like dissolves like!
What is a catalyst?
Substance that influences the speed of a reaction without being changed chemically. E.g. Enzymes in the human body
What can catalysts do to a reaction?
Positive speed it up,
How else can catalysts be classified?
Homogenous and heterogenous catalysts
What is a homogenous catalyst?
Where the catalyst and substrate are in the same phase e.g. Acid base catalyst
What is a heterogenous catalyst?
Where catalyst and reactant are in different phases e.g, finely divided solid in liquid or liquid in contact with solid
How do catalysts work?
They combine with the reactant and change the mechnism by which the process occurs
How do catalysts increase the rate of reaction?
By decreasing the activation energy of the reaction
What are the three types of catalysts important to pharmaceutical rections?
Specific acid/base catalyst
General acid/base catalyst
What is a specific acid base catalysis?
E.g. Reaction catalysed by H+ or OH- ions from a specific acid or base
E.g. HCl NaOH, CH3COOH
How can we limit the undesired acid base reaction?
We can put in a buffer
However this doesn't always solve all our problems because buffers can affect our products.
What happens in the example of sodium hydrogen phosphate, which is a basic species, and the addition of a buffer?
In sodium hydrogen phosphate, there are 2 proton donating species which act as a catalyst by proton anions.
What is general acid/base catalysis?
When a reaction is catalysed by a proton gaining or proton donating ion to form an acidic or basic component in the buffer system
Or a reaction is catalysed by a molecule that is not specifically an acid or base
E.g. Sodium hydrogen phosphate
What happens when a drug is combined with this phosphate buffer (sodium hydrogen phosphate)!
The 2 proton donating functional groups which are weakly basic exerts catalysis by the phosphate anions so we need to pre assess the buffer effect on the stability of drugs especially if they are pH sensitive and prone to acid/base hydrolysis
What is the primary salt effect?
When the reaction products are oppositely charged ions, a considerable increase in the reaction rate is obtained with increasing ionic strength of the reaction mixture.
What is the secondary salt effect?
The rate of reaction can be changed by the acidity of the system
What are catalytic ions?
Ions that increase the reaction rate by changing the pka of the solvent