Chemistry - Chapter 16: Rates of Reaction

rate of a chemical reaction

the change in concentration per unit time of any one reactant or product

average rate: total volume of oxygen given off (cm³) / time for reaction to complete (secs) x 100

or unit of conc/unit of time

instantaneous rate of reaction

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

Collision Theory

for a chemical reaction to occur, the reacting particles must collide with each other with enough energy to produce a product. Energy required is called the activation energy (insufficiency results in no reaction)

activation energy

the minimum energy required to start a chemical reaction // the minimum energy colliding particles must have in order to start a chemical reaction & for effective collisions

effective collision

one that results in the formation of products

large activation vs small activation energy required

large AE required → small no. of molecules have enough energy to pass over the barrier per second (slower reaction rate)

small AE required → faster rate

factors: nature of reactants

• ionic reactions = fast + covalent reactions = slow

• ionic → don’t involve bond breaking but simply the union of oppositely charged species at room temperature (precipitate forms quickly)

• covalent → reactions that involve the breaking of covalent bonds at room temperature (precipitate forms slowly)

factors: surface area/particle size

• more exposed particles → more frequent and successful collisions

• the smaller the particle size, the faster the reaction

• e.g. powdered CaCo3 vs marble chips

factors: concentration

• the no. of moles of a given substance in a certain volume of solvent

• an increase in concentration results in more particles per unit volume, hence more frequent collisions between particles

• changing the conc. of reactants in a chemical reaction alters the rate of reaction (HCl vs sodium thiosulfate → black cross on paper experiment)

factors: temperature

• temperature is proportional to the kinetic energy of particles. The more energy the particles have, the faster they move around

• increase in temperature → higher kinetic energy, leading to more frequent collisions & successful ones as collisions themselves occur with more energy

• @ rough, approx. 10 degrees C increase doubles the rate of reaction

factors: catalysts

increases/alters the rate of a chemical reaction without being consumed in the process (provides alternative route w/ lower activation energy)

properties of a catalyst

• recovered chemically unchanged at the end of an experiment

• tends to be specific (e.g. enzymes)

• only needs to be present in small amounts

• catalytic poisons → action of catalyst may be destroyed by these

• in equilibrium reactions, catalysts help the reaction achieve equilibrium more quickly…doesn’t affect equilibrium’s position or final composition of mixture in the reaction

homogenous catalysis

both the reactant and catalyst are in the same place

heterogenous catalysis

reactants and catalysts are in different places

autocatalysis

one of the products of the reaction catalyses the reaction (e.g. Mn7+ + Fe3+ → Mn2+ + Fe2+

mechanisms of catalysis: surface adsorption theory

• explains most heterogenous catalysis

• adsorption - the accumulation of substances at the formation of another substance

• e.g. ethene & hydrogen

mechanisms of catalysis: intermediate formation theory

Catalyst and reactant combine to form an unstable intermediate. These intermediates decompose forming products and regenerating the catalyst.​

e.g. oxidation of potassium sodium tartrate by h2o2

phase

a specific state of matter that implies a boundary

heterocatalysis example

Oxidation of methanol by a platinum catalyst:

• The hot spiral cools in transfer to conical flask then glows hot again once inside → platinum glows due to exothermic reaction

• popping sound → hydrogen gas exploding

• smell of methanol → methanol is oxidised

• spiral ceases glowing then heats up again → CO2 is produced due to incomplete combustion, poisons the catalyst, then leaves

catalyst

a substance that alters the rate of a chemical reaction but is not consumed in the reaction

catalytic converters

• a device in vehicles that converts pollutants to less harmful substances in the exhaust pipe of cars

• cars produce NO, CO, NO2, all of which are dangerous to the environment

• catalytic converters contain a metal (Pt, Pd, Rh) which convert using the surface adsorption theory

catalytic poison

a substance that makes a catalyst inactive

exothermic reaction

products that have less energy than the reactants = heat is given out

endothermic reaction

products have more energy than the reactant = heat is taken in