evil chem test last week of school
average rate =
delta C / delta t
measuring pH change
pH probe or meter can monitor change if the reactants;/products include H3O (acid) or OH (base)
NaOH + HCl -> NaCl + H2O
measuring change in volume of gas produced
(one of the products MUST be gas). measure at regular time intervals to create a volume vs time graph.
Mg + 2Hcl -> MgCl2 + H2
measuring change in mass
if a reaction produces a gas, the reaction mixture can be measured by placing it on a balance (H2 gas too light to give significant changes)
CaCO3 + 2HCl -> CaCl2 + CO2 + H2O
measuring change in the transmission of light
used if one of the reactants or products has colour. sample absorbs light in visible region and the indicator can be added to generate a coloured compound that can be followed in the reaction. use colorimeter or spectrophotometer.
CV + OH -> CVOH
as time goes down,
absorbance also goes down
measuring change in concentration using titration
reaction samples are removed at regular time intervals. agent is added to samples to stop the reaction (quenching). samples are then titrated with a known standard (acid or base)
measuring change in concentration using conductivity
total electrical conductivity of a solution depends on total concentration of its ions and charges. charge must change as the reaction takes place! conductivity meter is used to measure.
BrO3- + 5Br- + 6H+ -> 3Br2 +3H2O
large decrease in ions (charge) corresponds with decrease in electrical conductivity of the solution as the reaction happens.
measuring rate with time
clock reaction: measure time taken to reach observable end point. end point can be measured under different conditions. compare different end points to determine rates of reactions.
ex. Iodine clock reaction (iodine to reacts with something and reacts again to turn into iodide)
kinetic energy and temperature
all particles are in constant motion and move and collide randomly. since motion is random, not all particles in a substance at one time have the same kinetic energy. therefore, we can only take the average of all of them.
collision theory
1. particles must collide
2. particles must collide in the correct orientation
3. particles must collide with sufficient kinetic energy to break bonds of reactants
activation energy
for a collision to lead to a reaction, particles must have a certain minimum value of kinetic energy (known as activation energy Ea) this is the energy needed to overcome the repulsion between molecules and to break some of the bonds in the reactants before they can react.
if a particle has kinetic energy >/= Ea, they will have successful collisions. Ea varies reaction to reaction
energy pathway of a reaction
shows progression of a reaction. measures amount of energy of products and reactants.
Activated Complex
point at which bonds are [artially being broken and formed. very unstable, has highest kinetic energy.
factors that affect the rate of chemical reactions
1. increase in T = increase in rate
2. adding a catalyst = decrease in Ea = increase in rate
3. increase in concentration = increase in rate
4. increase in size of particle = increase in rate
catalysts
provides alternate pathway for the reaction (increases rate of reaction). more particles have necessary Ea for reaction to occur
homogenous: same state as reactants
heterogenous: different state (usually solid)
homogenous catalyst example
ozone breakdown by UV light (g)
O3 -> O2 + O•
O• + O2 -> O3 + heat
(this is a natural process)
CF2Cl2 -> CF2CL + Cl•
(CFCs speed up O3 breakdown)
Cl• + O3 -> ClO• + O2
ClO• + O3 ->Cl• + 2O2
(Cl• catalyses breakdown O3 and forms ClO•)
this has two transition states (when you normally see just one) and Ea is much lower with catalyst.
O3 is good in stratosphere (absorbs UV light) but bad in troposphere (causes respiratory issues)
heterogenous catalyst ex
catalytic car converter that converts pollutants to less harmful substances.
2NO -> N2 + O2
2NO2 -> N2 + 2O2
2CO + O2 -> 2CO2
CH3CH2CH2 + 5O2 -> 3CO2 + 4H2O
NO, NO2, and CO are toxins (get broken down by catalyst)
Catalyst speeds up breakdown of fuel
•
• means free radical (unpaired electron(s))
increasing temperature raises rate because
particles collide with each other more often because kinetic energy increases.
concentration raises rate because
there are more particles available to react meaning more collisions.
surface area raises rate because
there are more particles exposed which means they collide more
pressure and its affect on rate
P increases, V decreases. When V decreases, C increases. Therefore, the more P, the more C! the higher C, the more collisions, the higher rate.
amount of product based on amount of C reactant
increasing concentration of reactant in EXCESS will increase the rate but you will produce the same amount of product. however, increasing the concentration of the LR will increase the rate and also increase the amount of product made!