Lab 2 (N) Flashcards
The rate at which a chemical reaction occurs is dependent upon several factors:
the chemical nature of the reactant(s), the reactant concentration(s), and (sometimes) the presence of a catalyst. temp too
Collision Theory
An important part of the kinetic analysis of a chemical reaction is the determination of the
activation energy, Ea. The energy needed to bring reactants close enough together to allow for the formation of the transition state (activated complex).
In Collision Theory, one assumes that reactant molecules must collide with a total energy equal to or greater than the activation energy, Ea.
The larger the activation energy, the slower the reaction.
EX: collision of two negatively charged ions is expected to have a larger activation energy than a
comparable collision of two oppositely charged ions.
hypothetical elementary reaction, A + B → Y + Z,
The reactant entities A and B colliding with a total energy equal to or greater than the activation energy, Ea, can bind to form an activated complex [AB]≠ at the transition state.
The highly unstable activated complex [AB]≠ can either break down to give the reactant entities A and B or internally rearrange the bonds to form the product entities Y and Z.
The activation energy threshold for the reaction is reached by conversion of the kinetic energy of the reactant entities A and B to potential energy at moment of collision, when the entities A and B bind to form the highly energetic activated complex [AB]≠.
Activation energy (Ea)
Is the minimum collisional energy needed to overcome the mutual repulsion of the electron clouds of the colliding molecules and bring them close enough together to permit the formation of the bonds in the product molecules while breaking the appropriate bonds in the reactant molecules.
It constitutes an energy barrier for the reactant entities A and B; the higher the barrier is, the fewer the number of successful collisions of A and B that can overcome this barrier.
If we compare two reactions with the same overall order occurring at the same temperature, the reaction with the larger Ea has the smaller rate constant, k.
The activation energy of a reaction is independent of both temperature and initial reactant concentrations. The activation energy is a characteristic property of a reaction. Always remember that the rate of a reaction and the rate constant k for a reaction are temperature-dependent, whereas the activation energy Ea is not.
Arrhenius Theory on the Temperature Dependence of Reaction Rate Constants
when we repeat a reaction several times at the same fixed temp, each time varying the initial concentrations of reactants arbitrarily, we observed that
the value of the rate constant k remains constant.
However, if we repeat a reaction several times starting with the same initial concentrations of reactants, but each time carrying out the reaction at a different fixed temp, we will observe that k increases as temperature increases.
This is because as temperature increases, the reactant molecules have higher kinetic energy on average (because they move faster), and as a result, a greater number of collisions will have enough energy to overcome the activation energy of the reaction and will form activated complexes that can transition into the product molecules.
The relation between the rate constant k and the absolute temperature T (in kelvin) is given by the Arrhenius equation:
k = A e-Ea/RT
A is called the pre-exponential factor or Arrhenius A-factor, R is the gas constant (R = 8.3145 J K–1 mol–1) and e is the Euler’s number (e = 2.718…), which is the base of natural logarithms.
If we take the natural logarithm (ln) of each side of equation (3), we obtain the linearized Arrhenius
equation:
in k = - Ea/R X 1/T + in A
This equation represents the equation of a straight line in the form y = mx + b, in which “ln k” is the y-variable, “1/T” is the x-variable, “−Ea / R” is the slope m, and “ln A” is the y-intercept b.
And The calculated slope is equal to −Ea / R.
