Rate

what 4 principles can be used to explain the behaviour of an ideal gas?

1. a gas is made of a very large number of molecules that are so small compared to the distance between them that their size can be ignored (dimensionless particles)

2. molecules are in constant, rapid, straight line motion, colliding frequently with each other and the walls of any container in which they are placed

3. these collisions are perfectly elastic (no kinetic energy is lost or gained during collision) - molecules do not exert any attractive or repulsive force on each other

4. although there is a wide distribution of kinetic energies, the average kinetic energy is proportional to temperature (K)

what is speed of molecules proportional to?

the temperature (+ inversely proportional to the molecular mass)

what is collision frequency proportional to at constant volume?

temperature and pressure

what is viscosity?

for a molecule to move it requires energy to escape its neighbour (measure of resistance to flow)

viscous = thick, high resistance to flow

how viscosity change with temperature?

decreases with an increase in temperature as molecules have more KE

what is the thermodynamic definition of diffusion?

a force that represents the spontaneous tendency of molecules to disperse to a situation of greater disorder (increase in entropy)

what is kinetic energy?

energy that an object possesses because of its motion

distributed amongst a molecule’s rotational, translational and vibrational motion

how do ions move through a solvent?

by the application of a potential difference

what is conductance (ion movement)?

the ease with which charged ions move across a membrane or through a solution

inverse of resistance

how does conductance change with length, the cross sectional area of a medium and the number of ions present?

decreases with length

increases with cross sectional area

increases with an increase in the number of ions present

what is potential energy?

the energy of a molecule due to its position

eg, on a gradient

what is the rotation, translation and vibration motion of a molecule?

rotation - movement of a molecule about an axis

translation - movement from one place to another

vibration - motion of one atom in relation to another atom within the molecule - only motion allowed for molecules in the solid state

what determines the rate of a reaction?

number of collisions per unit time x fraction with sufficient energy x fraction with appropriate orientation

describe how physical state, concentration, temperature and catalysts affect rate

physical state - reactants have to be in the same phase (ie dissolved)

concentration - increasing concentration increases frequency of collision

temperature - increasing the KE of the atoms and the frequency of collisions provides an increase in the number of collisions with sufficient energy to exceed the activation barrier

catalysts - no effect on overall energies, but lowers the activation energy

what is the rate law?

an experimentally determined equation relating the rate to the molar concentrations of reactants

what is rate law expressed as?

rate = k [A]ⁿ [B]^m

n is the order of reaction with respect to A

m is the order of reaction with respect to B

(n+m) is the overall order of the reaction

how do the rate constants of most reactions change with temperature?

increase with temperature

eg, k for a given reaction may be 2x higher at 35^oC than at 25^oC

state the Arrhenius equation

k = Ae^-Ea/RT

k is the rate constant (sec^-1)

A is the frequency factor, representing the number of collisions occurring with the correct orientation to react

e^-Ea/RT represents the fraction of collisions with the minimum energy to react

where E_a is the activation energy (KJ mol^-1)

R is the universal gas constant (8.314 ×10^-3 KJ mol^-1 K^-1)

T is temperature (K)

what is a catalyst?

a substance that participates in a reaction but is not consumed by it

increases the rate of a reaction as it modifies the reaction energy pathway, such that delta G is smaller

in what 3 ways can a catalyst alter the reaction energy pathway?

activate the reactant

stabilise (and reduce energy of) transition state

different reaction mechanism

how do you measure the rate of a reaction (generic)?

follow the concentration of a component of the reaction as a function of time

• decrease in reactant concentration or

• increase in product concentration

requires a change in physical or chemical property associated with the component

utilise a feature of a component being consumed eg, gaseous - increase in pressure