Unit 2 - Chapter 6 Flashcards
Rate of reaction
speed at which a chemical change occurs
obtained by measuring the rate at which a product is formed, or the rate at which a reactant is consumed over a series of time intervals
generally expressed in changes in concentration
unit is usually mol/(L)(s)
always expressed as an absolute value
relative to molar coefficient in balanced equations, so it is important to indicate what was measured when reporting reaction rate
Types of rates of reaction
average rate of reaction = the speed at which a reaction proceeds over a period of time
instantaneous rate of reaction - the speed at which a reaction is proceeding at a particular point in time, decreases over time as it is less likely for reactions to occur as the reaction proceeds
Measuring reaction rates
ideally, the experimenter can take direct measurements of a reactant or product without disturbing the progress of the reaction
reactions that produce a gas = collect and measure the volume/pressure of the gas over time
reactions that involve ions = measure conductivity of solution over time
reactions that change colour = measure the intensity of a coloured reactant or product using a spectrophotometer
Collision theory
if particle A is to react with particle B, then particles A and B must collide, but not all collisions result in chemical reactions
successful collisions are ones that yield products, and two conditions must be met for a collision to be successful = activation energy and collision geometry
reaction rate therefore depends on rate of successful collisions
Activation energy
colliding particles must possess some minimum kinetic energy that will allow particles to overcome any repulsion forces, and weaken/break bonds in reactant particles
this minimum energy requirement is the activation energy
Collision geometry
particles must collide in a defined 3D geometric alignment
allows the correct alignment of atoms between reactants so that old bonds can break and new bonds can form
correct collision geometry allows for the formation of a highly unstable high energy transitional species called the activated complex
only activated complex with activation energy will produce products, otherwise it will revert to reactants
Reaction mechanisms
most reactions occur in a sequence of steps each involving 1 to at most 2 reactant particles
this sequence of steps = reaction mechanism, which converts reactant to products
each step is called an elementary process, which produce unstable products called reaction intermediates which act as reactants in subsequent steps but do not appear as final products
elementary processes combine to yield the overall reaction
Chain reaction mechanism
chain initiating step = begins the chain reaction
chain propagating steps = these reactions keep the chain going by producing more free radicals
chain terminating steps = these reactions remove free radicals from the system, shutting down the reaction
Reaction mechanisms and collision theory
each elementary process in the reaction mechanism will have its own activation energy, collision geometry, and forms a unique activated complex
the elementary process with the highest energy activated complex will be the slowest step, and therefore the rate determining step (RDS)
the activation energy for the overall reaction depends on RDS
in order to change the rate of a reaction, we must change the rate of the RDS
Nature of reactants
number and type of bond = reactions that must break more or stronger bonds will be slower than those reactions in which bonds are fewer or weaker
dissolving ionic compounds therefore leads to a faster reaction rate
surface area and state of reactant = solid has a small SA to V ratio and therefore a low reaction rate, liquid (and solutions) has a higher SA and a faster reaction rate, gas has maximum SA and is a very fast reaction
Concentration of reactants
generally, higher concentration of reaction = faster rate, as there is a higher frequency and probability of successful collisions
this is because at a higher concentration, the fraction of particles that are greater than the activation energy is larger in number
Temperature
generally, higher temperature leads to a faster rate of reaction
this is because the average kinetic energy is shifted so that more particles have a kinetic energy greater than the activation energy
more frequent and more forceful collisions lead to more successful collisions
Catalysts
catalyst = substance that will increase the rate of a reaction without being consumed by the reaction
catalysts provide a new reaction mechanism with a lower activation energy that does not affect the change in enthalpy
without a catalyst, only a small fraction of particles have kinetic energy greater than the activation energy, and therefore the fraction of successful collisions is small
Types of catalysts
homogenous catalyst = catalyst is in the same phase as the reactant (eg. both are aqueous solutions)
heterogeneous catalyst = catalyst and reactants are in different phases (eg. solid catalyst and gaseous reactants)
provide a surface upon which reactants are temporarily absorbed (form weak attraction), reaction occurs on this surface and products are released
The rate law
the rate of a chemical reaction is directly related to the concentration of the reactants
the rate law quantifies the effect of the concentration of each reactant on the reaction rate
this proportionality can be converted to an equality by introducing a constant called the rate constant (k), which is constant at constant temperature
