Ch10 Rates of Reaction and Equilibrium

ROR = change in concn of a reactant or a product in a given time.

Rate = change in concn / time

mol dm-3 s-1

• Particles must collide before a reaction can take place. • Not all collisions lead to a reaction • Reactants must possess at least a minimum amount of energy (= activation energy) • Partciles must approach each other in a certain relative way (= steric effect)

• More FREQUENT collisions - increased particle speed, have more particles present • More SUCCESSFUL collions - give particles have more energy, lower activation energy

• Increase the SA of solids • Increase temp • Shine light (radical reactions) • Add a catalyst • Increase pressure of any gas • Increase the conc of reactants

• Increasing SA increases chance of collision because more particles are exposed • Powdered solids react quicker than larger lumps • Catalysts are finely divided for this reason

• Increasing conc = more frequent collisions = increased ROR Increasing the conc of some reactants have greater effects than others.

• Increasing pressure forces gas particles closer together in a given volume • This increases the frequency of collisions, so ROR increases. • Increasing the pressure could have an affect on the position of equilibrium and therefore the yield.

• shining a suitable light source onto reactants increases ROR • light (often UV) provides energy to break bonds and initiate reactions • the greater the intensity of light, the greater the effect • e.g. radical reactions • Initiation, propogation, termination

• Increasing temp increases ROR • As particles get more energy so they can overcome the energy barrier • particles speeds up, also, increases the frequency of collisions. • Only the reactants with energy equal to or greater than Ea (Activation Energy) will react. Look at energy profile diagrams

• Catalysts provide an alternative reaction pathway with lower Ea. • A lower Ea = more particles have sufficient energy to overcome energy barrier + react

A catalyst lowers Ea without being chemically changed/used up at the end of a reaction.

• Catalysts do not affect the position of equilibirum, therefore do not affect yield in an equilibrium system where an increase in temp = lower yield. (e.g. Haber + Contact) • They are safer and cheaper to use, as it means less energy is required to help reactants reach Ea.

• Homogenous Catalyst: same physical state/phase as reactants • Heterogenous Catalyst: different physical state/phase as reactants.

A catalyst gets affected by impurities in a chemical reaction and stops working.

A dynamic equilibrium is an equilibrium that exists in a closed system, when rate of forward reaction is equal to the rate of the reverse reactions, and the concentrations of reactants and products does not change.

• Rate of forward reaction equal to rate of reverse reaction • Concentrations of reactants + products do not change • A dynamic equilibrium occurs in a closed system

When reaction has reached dynamic equilibrium and a change occurs, (change in conc, pressure, temp) the position of equilibrium will move to counteract change and restore equilibrium.

• If equilibrium lies to the right; increase in conc of reactants decreases conc of products. • If equilibrium lies to the left; decrease in conc of reactants increases conc of products. If more products are formed, equilibrium lies to right If more reactants are formed, equilirbium lies to left.