Chemical Kinetics II Flashcards
Complex Reactions
aA + bB > products
Mechanism involved a series of steps (elementary reactions)
Overall Rate Equation:Cannot be deduced from stoichiometric equation
k = overall rate constant
m & n = orders of the reaction with respect to A and B (can be 0,1,2 . . . Can even be fractions). The overall order of the reaction is (m + n)
Complex reactions - exceptions:
The stoichiometric coefficients do not appear in the rate equation
Sometimes m or n (or both) might be the same as the values in the chemical equation, but this is coincidence only
A substance not in the chemical equation can be in the rate equation e.g. catalyst, product of reaction
Does the concentration of reactant affect the overall rate of the reaction?
No, unaffected
m = 0, [A]0 = 1 and so _______ is not in the rate equation
concentration
The _____ _____ of the reactant in a zero order reaction is proportional to the initial concentration and inversely proportional to the rate constant. It is not constant.
half life
What does each letter mean in The Arrhenius Equation?
Constant A = Arrhenius Factor (or pre-exponential factor). It has the same units as k.
The value of k increases exponentially as temperature increases
Experimental observations tell us that the reaction rate approximately doubles every 10 ⁰C
A plot of ln k vs 1/T is a straight line
y = mx + c
ln A = y intercept
-Ea / R = m (the slope)
The rate constant k , and therefore the rate of reaction, increase with increasing temperature and decrease with increasing Ea
Measuring k at different temperatures allows for the determination of Ea for the reaction
What is the importance of Ea (activation energy)
Value of Ea determines how sensitive the reaction is to changes in T
Higher Ea = more sensitive to T
If Ea = 0, rate constant (k) is independent of T
If k1 is known at a particular T1 and Ea is known, then k2 at any other T2 can be calculated
Ea can also be determined by measuring k at two different T
first order reaction units of k
s^-1
2nd order reactions units of k
dm^3 mol^1 sec^1
third order reaction units of k
dm^6 mol^-2 sec^-1
zero order reaction units of k
mol dm^-3 sec^-1
Reaction Theory meaning
Based on a model of how a reaction occurs at a molecular level - Collision Theory and Transition State Theory
What is Collision Theory?
Reactant molecules behave like hard spheres. Collisions between the spheres may result in a reaction
Transition State Theory
Assumes the reactants form a transition state at the maximum on the energy profile – more sophisticated theory
how does collision theory work?
1) two molecules must collide in the correct orientation with particular min amount to break existing bonds
2) energy profiles used in collision theory represent the potential energy of twi molecules as they approach then collide and react to from products
Determinations of collision theory?
Only molecules with kinetic energy greater than the Ea will react
There are more of these molecules at T2 than T1
As T increases, Ea/RT decreases and approaches zero, so e-Ea/RT approaches 1
Transition state theory
Transition State: Highest point on energy profile
Ea: Activation Energy, difference between reactants and transition state
∆rU: Internal energy change
Ea (reverse) > Ea (forward) because the reverse reaction is endothermic
Exothermic Reaction: products are at a lower potential energy than the reactants
what is transition state theory?
A pre-equilibrium is established between reactants and the transition state, ABCŦ, which then goes on to form the products
Gibbs energy of activation
Change in Gibbs energy for the formation of the transition state
Transition state theory shows that the rate constant (k) depends on the Gibbs energy of activation ∆GŦ.
Transition state theory shows that the rate constant (k) depends on;
Gibbs energy of activation ∆GŦ.
Explain what a catalyst is;
substance that changes the rate of a reaction without being consumed in the reaction
Catalysis process»_space;>
Does not appear in the overall chemical equation for the reaction
Can appear in rate equations
Does not affect the amount of product formed
Not changed chemically (can be physically altered) so can be recovered at the end of the reaction
Negative catalysts / inhibitors slow down reaction rates
What is the advantage of catalysis?
The catalyst provides an alternative pathway for the reaction with a lower Gibbs activation energy for the rate-determining step
The process of catalysis lowers the activation energy. TRUE or FLASE?
FALSE
It does not lower the activation energy
At a given ___, the rate constant is greater for the catalysed reaction, so the reaction is faster
T = temperature
____ ______ reaction has a single transition state, the catalysed reaction involves the formation of an intermediate
Non-catalysed
What is Heterogeneous Catalysis?
Different physical state than the reactants
Products and any unreacted starting materials are easily separated from the catalyst
What is Homogeneous Catalysis?
Same physical state as the reactants
More difficult to recover catalyst potentially
Enzyme catalyses reactions in cells take place in aqueous solution and involve homogeneous catalysis
Enzymes: what are they?
Protein molecules that catalyse a specific reaction and remain unchanged by the reactions they catalyse
They catalyse a wide range of biochemical reactions (37 ⁰C – Human Body). Processes would happen much to slowly to sustain life without enzymes.
Lock and key model
Enzyme-Catalysed Reactions
Just like with other catalysts, enzymes provide an alternative pathway with a lower activation energy for the rate-determining step
