Topic 6: Chemical Kinetics

rate of change in a concentration (change in concentration per time) • increase in concentration of products per unit time (or decrease in concentration of reactants per unit time)

mol dm^-3 s^-1

• at the start of the reaction and decreases as the reaction proceeds • not constant during reaction

• change in volume of gas produced • change in mass • change in transmission of light colorimetry/spectrophotometry • change in concentration measured using titraion • chnage in concentration measured using conductivity • non-continious methods of detecting change during a reaction; ‘clock reactions’

• collecting gas and measuring chang ein volume as regular time intervals • use of a gas syringe • gas can be collected by displacement of water from inverted burette (if gas law low solubility in water)

• if reaction gives off gas= decrease in mass can be measured by standing reaction mixture directly on balance • doesnt work well with hydrogen as its too light

• good if one of the reactants/products is coloured (gives characteristic absorbtions in visible region) • use of indictator to genarate compounds • colorimeter/spectrophotometer pass light through selected wavelength though solution and measures intesnity of light transmitted by reaction components • allows for continious reading (graph of absorbance) `

• use of standard solutions • samples drawn from reaction mix at regular time intervels and analyse by titration • quenching used (substance is introduced which stops reaction at th emoment its withdraw; freez frame)

total eletrical conductivity; depends on conc of ions + their charges - this changes during reacion; conductivity can be measured using conductivity meter (immersing electrodes in solution)

• use the ‘ed point’ to stop the clock • time taken to reach this point for the same reaction under different conditions shows the different rates of reactions • limitation; only gives average rate over time interval

• values of kinetic energy greater than activation energy 2. appropiate collision geometry

in kelvin (K) is proportional to the average kinetic energy of the particles in a substance

in kelvin (K) is proportional to the TOTAL kinetic energy of the particles in a substance

• distribution grapho of reaction • measures number of particles with kinetic energy E on Y axis • kinetic energy, E on x-axis • area under curbe represents total number of particles in sample

minimum value of kinetic energy which particles must have before they are able to react

temperature concentration particle size pressure catalyst

increase in temp= increase in ROR • increase in kinetic energy= increase in collision frequency • increase in frequency of collisions with articles with required activation energy to overcome barrier which increases the frequency of succesful collisions resulting in reaction

• increasing the conc. of reactants increases the rate • increasing conc.= increasing frequency of collisiosn between reactant particles= increasing frequency of succesful reactions (particles closer together)

• in solids; decreasing particle size increases rate • making solids into smaller parts; increases surface area; allows more contact and probability of collisions between reactants

• in gases; increasing presssure increases arate of reaciton - more pressure on gas; compreses gas; increases its concentration; increases frequency of collisions

• increses ROR without itself undergoing permanent chemical change • provide alternate route for reaction with lower activation energy • means that large fraction of particles will now have sufficent kinetic energy greater than activation energy= able to undergo collisions

substance that icnreases the ROR without itself undergoing permanent chang

enzymes; used in domestic and industrial processes (biotech and industry)