8. Rates of Reaction

make sure your activation energy isn’t too low or you can’t precipitate!

rate of reaction

the change in concentration of a reactant/product per unit time

measuring average rate

total volume/total time

instantaneous rate of reaction

the rate of reaction at any one particular time during the reaction

measuring instantaneous rate

draw tangent at given time, find slope

effective collision

one that results in the formation of products

activation energy

the minimum energy that colliding particles must have for effective collisions to occur

exothermic

a chemical reaction that gives out heat

endothermic

a chemical reaction that takes in heat

factors affecting rate

nature of reactants, particle size, concentration, temperature, catalysts

nature of reactants explanation

ionic compounds in solution - ions are free to move, can react almost instantaneously with ions of opposite charge. covalent compounds - must break bonds before forming new ones

particle size explanation

the smaller the particle size, the greater the surface area, increase in number of collisions, increase in effective collisions, increased rate of reaction

dust explosion conditions

ICODE - ignition, combustible, oxygen, dry, enclosed space

concentration explanation

increased concentration of reactants, increased number of collisions, increase in effective collisions, increased rate of reaction

temperature explanation

increase in temperature, increase in number of collisions, increase in energy, increase in molecules reaching activation energy, increase in effective collisions, increased rate of reaction

catalyst

a substance that alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction

enzyme

a substance that is produced by a living cell and acts as a biological catalyst

enzyme examples

catalase -> H2O2, amylase -> starch

homogeneous catalysis

catalysis in which both the reactants and the catalyst are in the same phase

homogenous catalysis example

elephant toothpaste/iodine snake - 2H2O2 —[I-]—> 2H2O + O2

heterogeneous catalysis

catalysis in which the reactants and the catalyst are in different phases

heterogenous catalysis example

oxidation of methanol using a hot platinum catalyst - 2CH3OH + O2 —[Pt]—> 2HCHO + H2O

oxidation of methanol using a hot platinum catalyst observations

platinum spiral will glow red hot, popping sounds, flask gets hot, cools, repeat

autocatalysis

catalysis in which one of the products of the reaction is used as a catalyst for the reaction

catalytic converter

a device that converts harmful gases into less harmful gases

catalytic converter catalysts

platinum (Pt), palladium (Pd), rhodium (Rh)

catalytic converter reaction

2CO + 2NO -> 2CO2 + N2

catalytic converter type

heterogenous

catalyst poison

a substance that makes a catalyst inactive

catalyst poison example

lead (Pb)

how does a catalyst poison work?

the poison bonds strongly to the surface and blocks it

mechanisms of catalysis

intermediate formation theory, surface adsorption theory

intermediate formation theory

catalyst combines with a reactant to form an intermediate compound

intermediate formation theory example

2H2O2 --[I-]--> 2H2O + O2

intermediate formation theory evidence

oxidation of potassium sodium tartrate using hydrogen peroxide

intermediate formation theory colour change

pink -> green -> pink

surface adsorption theory stages

adsorption, reaction on surface, desorption

adsorption

reactants adsorp to the surface of the catalyst

reaction on surface

reactants now closer together and reaction takes place on surface, bonds break and new bonds form

desorption

product will desorp from surface