Ib Hl Chemistry Assessment Statements Topic 6 And 16

Andrew Voyles IB Chemistry Assessment Statements: Topics 6 and 16 6.1.1: The rate of reaction is the decrease in the concentration of one of the reactants per unit time, or the increase in the concentration of one of the products per unit time. 6.1.2: A graph may be plotted of concentration against time, with time on the x-axis and some measure of how far the reaction has gone (ie concentration, volume, mass loss etc) on the y-axis. This will produce a curve and the rate at any given point is the gradient of the tangent to this curve. 6.2.1: The kinetic theory states that all matter consists of particles (atoms or molecules) in motion, as the temperature increases, the average speed of the movement increases, there is a temperature to which we can extrapolate, absolute zero, at which, theoretically, the motion of the atoms and molecules would stop, the pressure of a gas is due to the motion of the atoms or molecules of gas striking the object bearing that pressure, and that there is a very large distance between the particles of a gas compared to the size of the particles such that the size of the particle can be considered negligible. 6.2.2: Activation energy is the energy that must be overcome in order for a chemical reaction to occur. 6.2.3: Collision theory states that reactions take place as a result of particles (atoms or molecules) colliding and then undergoing a reaction. Not all collisions cause reaction, however, even in a system where the reaction is spontaneous. The particles must have sufficient kinetic energy (such that for a given particle of energy E, E ≥ Ea), and the correct orientation with respect to each other for the two to react. 6.2.4: Higher temperature -> greater average kE -> faster reaction. Higher concentration -> more collisions -> faster reaction. Higher pressure -> greater concentration -> faster reaction Smaller particle size -> greater surface area -> faster reaction

6.2.5: For the higher temperature, T2, there are a greater number of particles which have at least the minimum activation energy for this reaction. As a result, the reaction at the higher temperature will proceed at a faster rate. 6.2.6: The presence of a catalyst lowers the activation energy of the reaction by providing an alternate pathway which is more favored. In addition, it also helps to align the molecules in the proper orientation needed for the collisions involved in the reaction. These two factors combined to create a reaction that proceeds at a faster rate than normal, without the catalyst.

6.2.7: The presence of a catalyst lowers the activation energy of the reaction by providing an alternate pathway which is more favored. This creates a greater number of particles which have at least the minimum activation energy for this reaction. As a result, the reaction at the higher temperature will proceed at a faster rate. 16.1.1: The rate constant is a constant which quantify as the speed of a particular chemical reaction, whose units depend on the specific reaction as well. The overall order of reaction is the sum of the orders of the individual reactants. The order of reaction, with respect to a particular reactant, is a measure of the relation between the concentration of a reactants and its effect upon the reaction rate.

16.1.2: The rate expression will be of the form Rate = k[A]m[B]n... (with a term for each reactant). The exponents of the reactants correspond to their coefficients in the slow step of the reaction mechanism. 16.1.4:

Zero Order

First Order

Second

Order 16.2.1: Many reactions are not actually single reactions, but the result of several reactions combined together a process refer to as a reaction mechanism. The products of some reactions become the reactants of other reactions, and all of the steps in a mechanism can be added together to achieve the overall reaction equation. The step which precedes at the slowest rate (the rate determining step) determines the overall rate of the reaction mechanism. 16.2.2: The rate determining step is the slowest step in the reaction mechanism. The order of reaction, with respect to a particular reactant, is a measure of the relation between the concentration of a reactants and its effect upon the reaction rate. The concentration of a reactant on a zero order reaction has no effect on the reaction rate. The change in concentration of a reactant in a first order reaction is proportional to the change in reaction rate. The change in concentration of a reactant in a second order reaction is proportional to the square of the change in reaction rate. 16.3.1: k=Ae(-E /RT) a

16.3.2: When ln(k) is plotted against 1/T, the resulting line has a slope of –Ea/R, where R is the universal gas constant. By multiplying the slope by –R, we could obtain a value of the activation energy (Ea)