How Fast/Far

• If the initial rate doesn't change when the concentration of a reactant is doubled then it is said that it is zero order in respect to that reactant.
• If the concentration of a reagent is doubled and the initial rate also doubles, then this is known as first order in respect to that reagent.
• If the concentration of a reagent is doubled and the rate quadruples, then this is said to be second order in respect to that reagent.
• Rate = K x [A]^a x [B]^b
• Rate has the units moldm^3s^-1
• 'K' is known as the rate constant and reflects the ease that a reaction takes place. The value for K changes with each reaction and different conditions.
• Large values of K result in a greater rate of reaction.
• An increase in temperature causes a greater value for K which thus increases the initial rate of reaction.
• rate doubles roughly every 10°c.
• Orders multiply the units of concentration: [A]^2 = moldm^-3 x moldm^-3
• The overall order is the sum of individual orders.
• If initial rate changes by the same factor as initial concentration, ( both double, triple, etc... ) then this is said to be first order in respect of that reagent.
• Initial rate changes by the square of initial concentration factor (concentration doubles but rate quadruples) then this is said to be second order in respect to that reagent.
• In a concentration time graph, the gradient of the line determines the rate and the Shape of the graph determines the order in respect to that reactant.
• In a concentration time graph, a straight, descending line indicates that the reaction is zero order in respect to that reactant as gradient, and therefore rate, remains constant with changing concentration.
• To find the initial rate of a reaction, draw a tangent to the graph when time is equal to zero and calculate its gradient.
• the half life, (t½) is the time taken for the concentration of a variable to decrease by half.
• In a concentration time graph of a reaction which is first order in respect to that reactant, half life should be roughly constant as the reaction progresses.
• In a rate concentration graph, if the rate is proportional to the concentration, then it is first order in respect to that reactant.
• In a rate concentration graph, if the rate is not effected by changing concentration and remains constant, then this is zero order in respect to that reactant.
• In a rate concentration graph, if rate is proportional to the square of the concentration, then this is second order in respect to that reactant.
• Rate = K[A]^a  Where :

  • K is the rate constant
  • [A] is the concentration of A
  • ^a is the order of A
• Therefore, K = Rate / [A]^a
• Reaction mechanisms often occur across multiple steps.
• The rate determining step is the slowest step of the reaction.
• The rate equation tells us the molecularity of the rate determining step e.g. the chemicals and their quantities involved in the reactants of the RDS.
• For a reaction mechanism to be valid:

  • The reactants in the rate determining step are consistent with the rate equation.
  • The sum of all the steps must give the overall reaction equation.
• Clock reactions can be used to obtain the initial rate of reaction of a reaction by measuring the time taken from the start of a reaction until a visible change.
• You can repeat a clock reaction at the same temperature whilst changing the concentration of one reactant to determine what order the reaction is in respect to that reactant.
• From a clock reaction, initial rate can be represented by 1/Time
• During the period of a clock reaction where we are measuring time, we assume that rate is constant and so average rate is the same as the initial rate.
• The longer the measured time of a clock reaction, the less accurate the initial rate.
• Arrenhius' Equation:

  K=Ae^-(Ea)/(RT)
• In Arrhenius equation, A is the pre-exponential (frequency) factor. It takes into account the frequency of collisions with the correct orientation.
• In Arrhenius equation, the exponential factor is a measure of the proportion of molecules that have enough energy to react.
• In arrhenius equation, activation energy is measured in Jmol-1, The gas constant in measured in Jk-1mol-1, and temperature is measured in kelvin.
• Homogenous equilibria is when all species are in the same state and/or phase.
• In heterogenous equilibria, some species can be in different states and/or phases.
• When a reaction is in heterogenous equilibria, liquids and solids are not included when calculating a value for KC, as their concentrations are constant.
• When given equilibrium concentrations, substitute them straight into the expression to calculate Kc