topic 16: kinetics II

this works if there is a change in the number of moles of gas in the reaction. using a gas syringe is a common way of following this

this works if there is a gas produced which is allowed to escape. works better with heavy gases such as CO2 - the mass loss provides measure for reactants - gas must be sufficiently dense or change in mass is too small to measure

small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent HCOOCH3(aq) + NaOH(aq) —> HCOONa(aq) + CH3OH(aq) - the NaOH could be titrated with an acid BrO3–(aq) + 5Br–(aq) + 6H+(aq) —> 3Br2(aq) + 3H2O(l) - the H+ could be titrated with an alkali CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + H+(aq) + I–(aq) - the I- can be titrated with sodium thiosulfate

the change in concentration of a substance in unit time Its usual unit is mol dm-3s-1

the rate equation relates mathematically the rate of reaction to the concentration of the reactants r = k[A]m[B]n

orders are usually integers 0,1,2 0 means the reaction is zero order with respect to that reactant 1 means first order 2 means second order

the total order for a reaction is worked out by adding all the individual orders together (m+n)

rate=k[A][B]

the slowest step will control the overall rate of reaction

a straight diagonal line

the rate of reaction is directly proportional to the concentration of A r = k[A]1

straight line parallel to x axis

the concentration of A has no effect on the rate of reaction r = k[A]0 = k

exponential graph

the rate of reaction is proportional to the concentration of A squared r = k[A]2

from taking gradients from concentration /time graphs

independent of the concentration and is constant

the rate stays constant as the reactant is used up. this means the concentration has no effect on rate

rapidly increase

s-1

mol-1dm3s-1

mol-2dm6s-1

increases the value of the rate constant k

k = Ae-EA/RT A = Arrhenius constant Ea = activation energy R = gas constant T = temp