Chapter 10 - Reaction Rates + Equilibrium

rate of reaction

how fast a reactant is used up

factors that affect reaction rates

-temperature

-concentration

-pressure

-surface area

-catalyst

increasing TEMPERATURE

-the particles gain more kinetic energy so more frequent collisions

-the particles move faster so more frequent collisions

so increases rate

increasing CONCENTRATION

more particles in a given volume so more frequent collisions so increases rate

increasing PRESSURE

less space between particles so molecules are closer together so more frequent collisions so increases rate

catalysts

-lower the activation energy needed by providing an alternative reaction pathway

-so more particles have energy greater than the activation energy so more frequent collisions so increases rate

effects of catalyst on BOND ENTHALPY (draw)

.

types of catalyst -homogenous

has the same physical state as the reactants - forms an intermediate

examples:

-esters with sulfuric acid catalyst - all liquids

-ozone depletion with chlorine radical catalyst - all gases

types of catalyst -heterogenous

has a different physical state from reactants

-reactant molecules are adsorbed onto surface of catalyst where reaction takes place, product molecules leave surface of catalyst by desorption

examples:

-the Haber process - making ammonia

-hydrogenation of alkenes

how are catalysts sustainable?

increase sustainability

-lower the temperature needed so less energy is used so has cheaper costs

-less combustion of fossil fuels means less carbon dioxide produced which is a greenhouse gas

Boltzmann Distribution

-bell-shaped curve

-the area under the curve = the total number of particles

-curve starts at the origin = no molecules have no energy

-curve does not touch x-axis = no maximum energy

-only the molecules with energy greater than Ea can react

-the area under the curve does not change

changes to Boltzmann curve if TEMP increases

-shifts to the right and peaks lower

-more molecules have greater energy than activation energy so plateaus above and peaks lower

changes to Boltzmann curve if CATALYST added

-curve does not change but activation energy is lower

-catalysts lower the activation energy needed so more molecules have energy greater than activation energy

example of reversible reactions

Haber Process: nitrogen + hydrogen → ammonia

dynamic equilibrium

the rate of forward reaction = the rate of reverse reaction

-the concentration of both reactants and products remains constant - but may still continue to react

-only reached within a closed system

Le Chatelier’s Principle

the position of a dynamic equilibrium shifts to minimise the effect of any change

Le Chatelier’s Principle -temperature

-if temp increases, equilibrium position shifts to endothermic reaction to absorb more heat to reduce the temp

-if temp decreases, equilibrium position shifts to exothermic reaction to release more heat

Le Chatelier’s Principle -pressure

-if pressure increases, equilibrium position shifts to side with fewer molecules of gas to reduce pressure

-if pressure decreases, equilibrium shifts to side with more moles of gas to increase pressure

Le Chatelier’s Principle -concentration

-if conc of reactants increases, equilibrium shifts to the right to use up reactants + form more products

-if conc of products increases, equilibrium shifts to the left to use up products + form more reactants

-decrease = opposite effect

Le Chatelier’s Principle -catalysts

have no effect on the position of equilibrium as they increase the rate of both the forward and reverse reaction by the same amount

equilibrium constant Kc -expression

[products]^{no. of moles} / [reactants]^{no. of moles}

square brackets = concentration

Kc value

-if Kc < 1 means high conc of reactants + equilibrium is towards the left

-if Kc > 1 means high conc of products + equilibrium is towards the right

what affects Kc?

-only temperature

-in endothermic = if temp increases, Kc increases + if temp decreases, Kc decreases

-in exothermic = if temp increases, Kc decreases + if temp decreases, Kc increases