chemical kinetics: reaction profiles and measuring reaction rates

factors that affect reaction rate

• concentration

• temperature

• surface area

• pressure

• rate of reaction does not scale linearly with these factors

reaction pathway

• sequence of steps reactants undergo on the way to form products

• progress along a reaction pathway is followed through a reaction coordinate

• activation energy must be overcome by reactants to form products - larger E_a = more difficult for reaction to proceed, reactants usually proceed by pathway with lowest activation barrier

• as molecules cross activation energy they proceed via a transition state - occurs at energy maxima

• reactions may proceed by a number of steps via a number of intermediates which can undergo further reaction to form products - intermediates occur at energy minima

measuring reaction rates

• can measure rate of change of product/reactant over time

measuring reaction rates - stopwatch method

• mix reactants in a vessel and start a stopwatch

• measure concentration of reactant/product at some instant in time

measuring reaction rates - continuous flow 

• used for faster reactions

• 2 reagents mixed and passed along a flow-tube

• observe product formation in flow-tube - concentration probed at different points in tube

• rate of reaction depends upon distance from mixing point and flow-rate

• plot conc of reaction/product as function of time

measuring reaction rates - stopped flow

• 2 reagents mixed and passed via an observation cell into a receiving/stopping syringe

• reagents flow through until receiving syringe full and hits end-stop - this starts data acquisition period

• transmission of light through observation cell monitored as a function of time

• plot conc of reactant/product as a function of time

measuring reaction rates - relaxation methods

• disturb equilibrium and monitor changes in concentration spectrophotometrically

• disturbing equilibrium - eg increase temperature by passing electrical current through a sample

flash photolysis

• used for photochemical reactions - reactants absorb light and converted into product

• laser generates flashes of light

• products produced absorb specific wavelength of light so give characteristic absorption band which can be measured

• concentration is proportional to absorbance so the rate of reaction can be determined

• once generated, product decays back into reactant