reaction kinetics Flashcards
rate of reaction definition
the rate of reaction is defined as the change in concentration of a reactant or a product per unit time
what is the rate equation
rate = k [A]^m [B]^n
what does zero order mean
rate of reaction is independent of the concentration of A
changes in [A] does not change the rate of reaction
what does first order mean
rate of reaction is directly proportional to [A]
what does second order mean
rate of reaction is directly proportional to [A]^2
what is the Arrhenius equation
k=Ae^(−Ea/RT)
A = pre exponential factor
what is the rate constant k affected by
temperature (increase in temperature = increase in rate constant)
activation energy (decrease in activation energy = increase in rate constant)
unaffected by changes in concentration
how to deduce the units of k
units of k = (mol dm^-3 s^-2) / (mol dm^-3)^overall order of reaction
definition of t 1/2 (half life)
the half life of a reaction is the time taken for the concentration of a reactant to decrease to half its initial value
what does concentration-time graph show
straight line = rate is constant as concentration decreases = 0 order
curve with constant half life = first order with respect to [R]
curve with half lives not constant = order of reaction with respect to [R] is not first order. (plot a rate-concentration graph to find out)
half life equation
half life = ln2 / k
what does rate-concentration graph show
horizontal line = rate is independent of [R], zero order
straight line passing through the origin = rate is directly proportional to [R], first order
parabola = order is not 1, plot a straight line graph to determine (lg graphs, gradient is order of reaction)
what does rate-time graph show
horizontal line = 0 order
curve = first order
curve (with decrease much faster compared to first order reaction) = second order
what is the collision theory
the collision theory states that to react, the reactant particles must collide with sufficient energy and with the correct orientation
activation energy definition
the activation energy of a reaction is the energy barrier the reactants must overcome in order to become the products
draw the maxwell-boltzmann energy distribution curve
y axis = number of particles
x axis = kinetic energy
index (shaded area) = proportion of molecules with KE >/= activation energy
deduction of r.d.s with rate equation
fast equation before slow equation: reactants must be included in the rate equation
slow equation: r.d.s
fast equation after slow equation: ignore
deduction of rate equation with r.d.s
write rate equation for r.d.s (rate = k2[reactant])
substitute intermediate reactants by writing a kc equation
write:
overall rate equation is rate= k [reactant] where k = k2kc
factors affecting rates of reaction
collision frequency
proportion of the particles that have sufficient energy to overcome the activation energy
effect of concentration or pressure on rate
at a higher concentration, there are more reactant particles per unit volume. the reactant particles are close together and collide more frequently. frequency of effective collisions increases. hence rate of reaction increases
effect of temperature on rate
when temperature is increased, reactant particles have a greater average kinetic energy. they move more quickly and particles collide more frequently. the proportion of particles that have kinetic energies greater than or equal to the activation energy increases. Frequency of effective collisions and hence rate of reaction increases
catalyst definition
a catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway of lower activation energy, while remaining chemically unchanged at the end of the reaction
effect of catalyst on rate
a catalyst provides an alternative reaction pathway of lower activation energy. the proportion of particles that have kinetic energies greater than or equal to the lower activation energy increases. Frequency of effective collisions increases and hence rate of reaction increases.
what are the 2 different types of catalysis
homogenous catalysis
heterogeneous caatalysis
how does the homogeneous catalysis work
homogeneous catalysis involves the formation of an intermediate between the catalyst and one of the reactants. the catalyst is regenerated in a later step of the reaction. the catalysed reaction may involve two or more steps, each of which has lower activation energy than the uncatalysed reaction.
catalytic role of Fe2+ in the I-/S2O8 2- reaction
the uncatalysed reaction is slow as the repulsion between the two negative ions (I- and S2O8 2-) results in a high activation energy. the catalyst Fe2+ and intermediate Fe3+, being positively charged, easily attract the negative ions. Thus, the catalysed pathway has a lower activation energy than the uncatalysed reaction.
the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
how does the heterogeneous catalysis work
reactants are adsorbed onto the active sites by formation of weak attraction forces. This brings the reactant molecules closer together, weakening the covalent bonds in the reactant molecules and lowering the activation energy. It also orientates the reactant molecules correctly for the reaction to occur. the reaction occurs where bonds are broken and new bonds are formed. The product desorbs from the catalyst surface so that active sites are exposed for further reaction.
details of haber process (technically chemical equilibria)
catalyse: fine iron catalyst (cheap and can be used in a large scale)
pressure = 250 atm (pressure cannot be too high as it is expensive even though high pressures would favour the forward reaction and increase the yield)
temperature = 450°C (moderately high temperatures to increase rate of reaction by providing reacting particles with enough energy even though low temperatures would favour the forward exothermic reaction)
the yield of NH3 is further increased by removing it as it forms to continuously shift the position of equilibrium to the right
what are enzymes
enzymes are homogeneous catalysts, through the mechanism of action that resembles heterogeneous catalysis. Enzymes are highly specific and catalyses only one reaction.
