chemistry c18 and c22 test - rates and energy

rate of reaction

change in concentration of a product or reactant per unit time

in a zero order reaction…

concentration does not influence rate as anything raised to the power of 0 is 1

in a first order reaction…

if the concentration is doubled, the rate increases by a factor of 2

in a second order reaction…

if the concentration is doubled, the rate increases by a factor of 2²

2 methods of continuous monitoring of rate

gas collection and mass loss

concentration-time graph of a zero order reaction

straight line with neg gradient, k is equal to gradient, rate does not change through course of reaction

concentration-time graph of a first order reaction

downwards curve with decreasing gradient over time and so decreasing rate, half life of concentration is constant

concentration-time graph of a second order reaction

downward curve steeper at start and tailing off slower

half life (only from concentration-time graph)

time taken for the concentration of a reactant to decrease to half its original value

initial rate

instantaneous rate at start of a reaction when t=0

clock reaction

way of obtaining initial rate by taking single measurement, time from start of experiment measured until a visual change occurs, initial rate is proportional to 1/t, clock reaction repeated several times with diff concs

rate determining step

slowest part of reaction sequence

importance of rate determining step

rate equation only includes species in rate determining step, orders in rate equation match number of species in rate determining step

arrhenius equation

k = e^(-Ea/RT)

logarithmic form of arrhenius equation

ln k = -Ea/R 1/T + ln A

lattice enthalpy

measure of strength of forces between the ions in an ionic solid, involves ionic bond formation from separate gaseous ions, exothermic so enthalpy change always negative

born haber cycle

r1: form gaseous neutral atoms, form ions from gaseous ions, combine ions to form ionic solid, r2: convert elements in standard state directly to ionic lattice

electron affinity

enthalpy change when one mole of gaseous atoms acquires one mole of electrons to form one mole of gaseous negative ions

why are second electron affinities endothermic

second electron is being gained by a neg ion so like charges repel, energy must be put in to overcome this force

what happens when an ionic solid dissolves

ions are hydrated (attracted by polar bonds in water molecule) or solvated (attraction to polar solvent molecules)

standard enthalpy change of solution

enthalpy change when 1 mol of ionic substance dissolves in water to give a solution of infinite dilution

infinite dilution

water is in excess so adding more doesn’t result in anymore heat being absorbed or evolved

what does enthalpy change of solution depend on

lattice enthalpy of solid/dissociation, hydration enthalpy of ions

enthalpy of hydration

enthalpy change when one mol of gaseous ions dissolves in water to form one mol of aqueous ions to form a solution of infinite dilution

entropy

measure of the disorder of a system, a system becomes energetically more stable when it becomesmore disordered, spontaneous reactions go in the direction of increasing entropy

entropy in exothermic reactions

more energy in surroundings, more ways of arranging energy, entropy increases

entropy in endothermic reactions

less energy in surroundings, fewer ways of arranging energy, entropy decreases

delta s of surroundings

-delta H/T

gibbs free energy equation

delta G = delta H -TdeltaS system

how to know if reaction is feasible

if G less than or equal to 0

limitations of predictions made for feasability

gibbs onlt provides info about energetic feasability, activation barrier may be too high for reaction