5.1 Rates, Equilibrium and pH

• Half life:

  time taken for concentration of reactant to decrease by half
• Rate determining step:

  slowest step in a reaction mechanism
• Order of reaction:

  the power that the concentration is raised to in the rate equation
• Rate:

  change in concentration per unit time
• Conc-time graph of zero order?


  straight line down
• Conc-time graph of 1st order?


  negative exponential curve
• Initial rate of conc-time graph?


  tangent at t=0
• Conc-time determination of k?

  k=ln2/t1/2
• For first order conc-time graph half life is constant
• Rate-conc graph for 0 order?


  straight line with 0 gradient
• Rate-conc graph of 1st order?


  straight line up
• Rate-conc graph of 2nd order?


  exponential curve upwards
• Gradient of first order rate-conc graph?


  k
• Rate determining reaction?


  -use compounds which appear in rate equation

  -reactants not in the equation should be used in the next reaction step
• Effect of temp on K?


  -rate of reaction is increased, more particles above activation energy

  -k must increase as the rate increases

  -k increases exponentially
• Log arrhenius equation interpret?


  lnk is y axis

  1/T is the x axis

  lnA is +c

  -Ea/R is the gradient
• Mole fraction?


  of a gas is the same as its proportion by volume to the total volume of the gas mixture

  mole fraction=moles in gas A/total moles of gas in mixture
• Partial pressure?


  mole fraction of gas A * total pressure in the system
• ICE method:

  -calculate number of moles of each substance

  -use inital moles, change and eqm moles to find the desired value

  -concentrations of them at eqm

  -equation
• Homogeneous system?


  all reactants and products in the same physical state
• Heterogeneous system?


  when one of the reactants or products are in a different state, any solids and liquids are viewed as constant so are emitted from calculation
• Kp pressure?


  kPa
• Kp equation?


  $Kp=$$partialpressureproduct/partial pressure reactants$
• Kc and Kp do not change when?


  concentration/pressure is changed
• Kc and Kp do change when?


  temperature is changed
• If system is cooled?


  temp decreases and Kc increases
• If system is heated?


  temp increases so Kc must decrease
• Kc tells us about the extent of a reaction so it shows how far the reaction has gone

  large Kc means eqm is right

  small Kc means eqm is left
• the rate constant:

  only changes with temperature

  increase temp increase in k
• Monobasic acid?


  only has one H+ to donate
• Dibasic acid?


  has 2 H+ ions to donate
• Tribasic acid?


  has 3 H+ ions to donate
• Conjugate acid-base pairs?

  a pair of species which are linked by the gain or removal of a proton
• Ka is used to measure?


  strenght of a weak acid

  Ka=[H+][A-]/[HA]
• conversion of ka to pka

  pKa=-log10[Ka]
• stronger acid?


  small pKa and larger Ka
• Weak acid?


  large pKa and small Ka
• pH equations?


  pH=-log10[H+]

  [H+]=10^-pH
• Strong mono basic acid assumption?


  [H+] is same as conc of acid
• Strong dibasic acid assumption?


  [H+]=2[HA]