Chemical Kinetics - General Chemistry - Notes | CHEM 1202, Study notes of Chemistry

Download Chemical Kinetics - General Chemistry - Notes | CHEM 1202 and more Study notes Chemistry in PDF only on Docsity! Chapter 14  Chemical Kinetics o Factors that affect reaction rates  Physical state of the reactants  When reactants are in different phases, the reaction is limited to their area of contact – therefore if surface area of a solid increases, the reaction rate increases  Medicine in a powder reacts faster than a tablet  The more readily molecules collide with each other, the faster the reaction  Concentration of the reactants  As concentration increases, reaction time increases  Increases frequency of collisions  Temperature at which the reaction occurs  As temperature increases, reaction time increases  This is why we refrigerate food  Temperature increases energy of molecules which increases speed which increases the chance of a collision happening  Presence of a catalyst  Catalyst- agent that increases reaction rates  Enzymes are types of catalysts  Overall – anything that increases the frequency of collisions between molecules will increase the reaction rate o Reaction Rates  Speed of an event = change that occurs over a time interval  Reaction rate – speed of a chemical reaction in concentration of products per unit of time (M/s)  Negative – products are consumed  Positive – products are formed o Change of Rate with time  Instantaneous rate of change in a reaction is evaluated by the slope of that point  If aA + bB  cC + dD  Then Rate = -(1/a)(Δ[A]/ Δt) = -(1/b)( Δ[B]/ Δt) = (1/c)( ΔC/ Δt) = (1/d)( ΔD/ Δt) o The Rate Law: The Effect of Concentration on Rate  Rate = k[A][B] when [A] and [B] increase/decrease proportionally (Linearly) to the change in concentration  Generally speaking Rate = k[A]m[B]n  K = rate constant, the magnitude changes with temperature  M and N are small whole numbers  How to Determine M and N  M and N in a rate law are called reaction orders  Overall reaction order is the sum of all the orders with respect to each reactant in the rate law  If a reaction is first order, doubling a product will double the rate  Second order reaction – doubling a product with quadruple (22) the rate  Having a 0 order reactant means that changing its concentration will not change the rate of reaction as long as some reactant is present  Unit of rate constant (k) = 1/(M*s)  In conclusion, rate of a reaction depends on concentration but the rate constant does not o Change of Concentration with Time  First-Order Reaction  Rate = -( Δ[A]/ Δt) = k[A]  -kt = ln[A]t – ln[A]0  Or –kt = ln([A]t/[A]0)  Second-Order Reaction  Rate = -( Δ[A]/ Δt) = k[A]2  Or (1/[A]t) = kt + (1/[A]0)  If ln[A]t is linear, the reaction is first order  If 1/[A]t is linear the reaction is second order o Half-Life  Half-life is the time required for the concentration of a reaction to reach one half its initial value  T1/2 = 0.693/k  Doesn’t depend on the starting concentration (in first order reaction)  T1/2 = 1/(k[A]0)  for 2 nd order reaction o Temperature and Rate  Usually rate of reaction increases with the temperature  Collision model – based on kinetic-molecular theory, molecules must collide to react  As molecules move faster, they collide more forcefully (more energy) and more frequently increasing reaction rates  Orientation Factor – atoms must collide in the right orientation to react also  Activation energy – molecules must possess a certain minimum energy to react  Energy is needed to stretch, bend, and break bonds, Ea o Arrhenius Equation- lnk = -(Ea/RT) + lnA o Reaction Mechanisms – the process by which a reaction occurs  Elementary reactions – when a reaction happens in one step  Molecularity – the number of molecules that participate as reactants in an elementary reactions  Unimolecular – one molecule