3.2 physical chemistry

what does system mean in a chemical reaction?

the atoms and bonds involved in the chemical reaction

explain the law of conservation

the amount of energy in an isolated system remains the same

energy cannot be created or destroyed, only transferred from one form to another

what energy change occurs in breaking bonds?

endothermic (energy is taken in)

what energy change occurs in making bonds?

exothermic (energy is released)

what is an endothermic reaction?

a reaction with an overall positive enthalpy change (+ΔH)

enthalpy of product is more than enthalpy of reactants)

what is an exothermic reaction?

a reaction with an overall negative enthalpy change (-ΔH)

enthalpy of products is less than enthalpy of reactants

show an enthalpy change diagram for an exothermic and endothermic reaction

what does activation energy mean?

minimum energy needed for a reaction to take place

which way does the arrow for activation energy point on an enthalpy profile diagram?

upwards ↑

what are the standard conditions?

• 100kPA

• 298 K

what does in standard state mean?

the state an element/compound exists in at standard conditions (110kPA, 298 K)

define enthalpy change of formation

energy change that takes place when 1 mole of a compound is formed from its elements in their standard state under standard conditions

H₂(g) + ½ O₂(g) → H₂O(l)

define enthalpy change of combustion

energy change that takes place when 1 mole of a substance is completely combusted

C(s) + O₂(g) → CO₂(g)

define enthalpy change of neutralisation

enthalpy change when 1 mole of water is formed from a neutralisation reaction

what does enthalpy change of reaction mean?

energy change associated with a given reaction

how can you calculate enthalpy change from experimental data?

Q=mcΔT

enthalpy change = mass of substance being heated x specific heat capacity x change in temperature

draw a simple calorimeter

what are the advantages of using a bomb calorimeter?

• minimise heat loss

• pure oxygen used to ensure complete combustion

why might experimental methods for enthalpy determination not be accurate?

• heat is lost to surroundings

• not in standard conditions

• reaction may not go to completion

what does average bond enthalpy mean?

the mean energy required to break 1 moles of bonds in gaseous molecules

why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule therefore they are more accurate

how to calculate enthalpy change of reaction using average bond enthalpies?

Δ/H = bond enthalpies of reaction - bond enthalpies of products)

what equation is used to calculate rate?

rate = change in concentration / time

rate = (volume/mass of product/reactant formed) / time

what is the unit?

• mol dm ⁻³s⁻¹

• cm³s⁻¹

• g s⁻¹

• mol s⁻¹

what must particles do in order to react?

collide with sufficient energy (activation energy) and the correct orientation

do most collisions result in a reaction?

no

what factors affect rate of reaction?

• temperature

• pressure

• concentration

• surface area

• catalyst

why does increasing temperature increase the rate of reaction?

• increases rate of reaction

• higher proportion of particles have energy greater than the activation energy

• more successful collisions per second

• increased rate

why does increasing concentration/pressure increase the rate of reaction?

• increases rate of reaction

• more particles in a given volume

• more frequent successful collisions

• increased rate

what are variables in an experiment that can be monitored to calculate the rate of reaction?

• concentration of reactants or product

• gas volume of products

• mass of substances formed

how to calculate rate from a concentration time graph?

• draw a tangent

• work out gradient of tangent

what is a catalyst?

a substance that increases rate of reaction without being used up

how do catalyst increases rate of reaction

they provide an alternate reaction pathway with a lower activation energy

due to lower activation energy, more particles have energy higher than the activation energy so more frequent successful collisions so increased rate

what does homogeneous catalyst mean?

a catalyst that is in the same phase as the reactants

(e.g. liquid catalyst with liquid reactants)

what does heterogeneous catalyst mean?

catalyst used in reaction is in different phase to the reactants

(e.g. gaseous reactants passed over a solid catalyst)

what are catalytic convertors?

they are present in vehicles to reduce toxic emission and prevent photochemical smog

define activation energy

minimum energy particles must collide with for a reaction to occur

name important features of Boltzmann distribution

• area under curve = total number of molecules

• area under curve does not change when conditions alter

• curve does not start at origin

• curve does not touch or cross the energy axis

• only the molecules with energy greater than activation energy can react

what are the axis in a Boltzmann distribution?

X-axis = energy

Y-axis = number of molecules with a given energy

Boltzmann curve:

Boltzmann curve with catalyst:

what does dynamic equilibrium mean?

it occurs when the rate of forward reaction equals the rate of reverse reaction and the concentration of reactants and products remain constant in a closed system

what factors can alter the position of equilibrium?

• concentration of reactants or products

• pressure

• temperature

explain Le Chatelier's principle

if a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance

if the forwards reaction is endothermic, what effect would increasing the temperature have?

equilibrium shifts to the right (endothermic direction) to decrease the temperature

if there are more molecules of products what effect would increasing the pressure have?

reduces yield of products becasue equilibrium shifts to the side with less gaseous molecules

why are the optimal conditions not always used?

because you need to reach a balance between optimum conditions and a fast rate of reaction

for example, low pressure and temperature will have a very low rate of reaction

high pressure and temperature may be dangerous and expensive to maintain

what effect does a catalyst have on the position of equilibrium?

no effect

(because catalyst affects rate of forward and reverse reactions equally)

what condition affects the value of K꜀

temperature

for the following reaction, deduce the expression for K꜀ and the units

2[A] + 3[B] + [C] ⇌[D] + 4[E]

K꜀ = [D] [E]⁴ / [A]² [B]³ [C]

K꜀ = 5(mol dm⁻³) / 6(mol dm⁻³)

= mol⁻¹ dm⁻³

what type of system is K꜀ relevant for?

homogeneous systems in equilibrium

what does K꜀ being greater or less than 1 suggest about the position of equilibrium?

greater than 1 = over to the right

less that 1 = over to the left

what effect does decreasing the temperature of an endothermic reaction have on K꜀?

K꜀ decreases

the equilibrium shifts in the exothermic direction (left) to increase the temperature

what effect does increasing the temperature of an endothermic reaction have on K꜀?

K꜀ increases

becasue equilibrium shifts in the edothermic direction (right) to decrease temperature

what effect does decreasing the temperature of an exothermic reaction have on K꜀?

K꜀ increases

equilibrium shifts in the exothermic direction (to the right) to increase temperature

what effect does increasing the temperature of an exothermic reaction have on K꜀?

K꜀ decreases

equilibrium shifts in the endothermic direction (left) to decrease temperature