R2.2: How fast?

• The rate of a reaction is the concentration change of either products (volume collection) or reactants (mass loss) over time.
• In a reaction, particles can only collide when they have energy greater than the activation energy of the reaction and have the correct geometry with other colliding particles, according to collision theory.
• Factors affecting ROR?

  1 Concentration of reactants increases frequency of collisions as there are more particles to react

  2 Temperature of reaction as particles gain kinetic energy and more particles are, hence, able to overcome the activation energy barrier

  3 Surface area or particle size increase ROR as they create more points of contact, creating more chances of collisions

  4 Pressure or Volume forces particles to be in tighter spaces in which they have less space to move and collide with one another more.
• Catalysts provide particles in a reaction with a lower-energy, alternate route to overcoming the collision energy barrier, allowing more particles to collide successfully
• Exothermic reactions tend to have lower activation energies as their reactions have more spontaneous tendencies

  
• Reaction rates can be found by observing the tangent of a singular point of the reaction or drawing a secant from the first point of reaction to the end and presenting its gradient as the rate. Below is a graph of concentration change vs time for a reaction.

  
• Order of reaction is the power to which a particular reactant's concentration is raised to in the rate equation
• The rate equation goes as follows:

  R = k[A]^m[B]^n

  From mA+nB-->pC+qD where R is the rate law and the rate constant

  overall order=m+n
• Zero order: mol dm-3 s-1

  First order: s-1

  Second order: mol-1 dm3 s-1

  Third order: mol-2 dm6 s-1

  IN UNITS OF K
• How to experimentally find rate constant, K?

  1. Isolate change in only 1 variable and solve algebraically for its experimental coefficient
  2. Repeat for other reactant
  3. Plug in values with the values of any of the recorded concentrations and rates to solve for K (remember that K's units are determined by the overall order)

  
• Graphs for the rate constant and overall order

  
• Ways to measure ROR?

  Volume change as gas escapes or volume collection in a gas syringe or data logger in a gas pressure sensor

  Color changes measured by eye but also through a colorimeter or spectrophotometer as color gets darker with a more concentrated solution as more color is absorbed and less is transmitted, speeding rate of reaction

  Mass loss as gas is released and concentration of reactants are used up

  Conductivity with different ion concentrations in exo and endothermic reactions
• Examples of rates?

  1 Rate of precipitate formation -> ex. sodium thiosulfate and hydrochloric acid precipitating in a way that clouds the solution

  2 Rate of clock reactions -> involving rapid color change, usually measured in steps after reaction under study is complete.
• Reaction mechanisms are a sequence of elementary steps in a reaction
• Elementary steps are theoretically proposed with kinetic and stoichiometric data to identify mechanisms that best support experimental observations in steps to arrive at original equation (similar to Hess' Law). The same element is canceled if they are found on the reactant's side in one step and the product for another or vise versa- if they are present on the same side, they are added.

  
• The slowest elementary step is the rate determining step and determines the rate of reaction
• Transition step in a reaction is an unstable, quick state the reaction reaches before reaching the next step of the reaction or reverting back to reactants

  The Intermediate stage is more stable and is a state at which the reaction can pause briefly.

  
• Energy profiles are diagrams used to show mechanisms of a reaction and the energy used to overcome specific elementary steps

  
• The overall activation energy (Ea) is the energy needed to take the lowest concentration of the reactants in the reaction to the highest transition state
• Molecularity stands for the number of reactants involved in each elementary step of a reaction. A step can be unimolecular (with 1 reactant), bimolecular (with 2 reactants), or, rarely, terimolecular (with 3 reactants- which is hard because all the reactants need to collide with sufficient energy and the correct geometry)
• Maxwell Boltzmann curves most commonly used to represent the distribution of particles at different energies. As temperature increase, more particles have higher kinetic energy, and as a catalyst is added, the threshold for particles to react is lowered.

  
• To measure the effect of Temperature and the presence of a catalyst Svante Arrhenius' equation is used.

  K=Ae^-Ea/RT

  With A standing for Arrhenius' factor calculated through

  A=K/e^-Ea/RT

  It is also graphed as follow;

  
• In the graph of Arrhenius' equation lnK is y axis, 1/T is x axis, the y-intercept is lnA, (-Ea/R) is the gradient of the line
• The order of reaction can be defined as the power dependence of rate on the concentration of all reactants.
• K = (ln2/t1/2) where K is the rate constant and t1/2 is the time it takes to reach half-life
• A(f)=A(o) x e^-Kt where A(f) is the amount after certain periods of half-life, A(o) is the original amount, K is the rate constant found and t is the time that has passed
• anything (equations) on half-life are only valid for first order reactions
• A graph of concentration over time is of a first order reaction if the change in time between each half life is around the same
• For the rate law, take the power/expression of the rate determining/slowest step ONLY
• Half life is constant (difference-wise) for 1st order reaction