Question

True/False?

Part 1.The rate law for the elementary reaction A + B → products might not be R = k[A][B].

Part 2. If we know the mechanism of a reaction including the values of the elementary rate constants, we can find the rate law (assuming the differential equations can be solved).

Part 3. The Arrhenius equation holds exactly.

Part 4. In homogeneous catalysis, doubling the catalyst concentration will change the rate.

Part 5. A catalyst does not appear in the overall reaction.

Answer 1

Part 1. The rate law for the elementary reaction A + B → products might not be R = k[A][B].

True.....(As an example, the reaction NO₂ + CO → NO + CO₂ is second-order in the reactant NO₂ and zero       order in the reactant CO. R=k[NO₂]² not R=k[NO₂][CO])

Part 2. If we know the mechanism of a reaction including the values of the elementary rate constants, we can find the rate law (assuming the differential equations can be solved).

TRUE....( If we know the mechanism, we can express the rate law in terms of rate constants and the concentrations of the involving species. If any intermediate is involved we can find its concentration applying the steady state principle and substituting it in the rate law we can express the rate law in terms of the products and reactants only. the differential equations can be solved can be solved from the rate law. )

Part 3 : The Arrhenius equation holds exactly.

False....The Arrhenius Law assumes a Boltzman distribution of energy states e(-E/RT). This exponential term falls in the region 0 to 1 and acts as a probability of the molecule having enough energy (E) to undergo the reaction.
The Arrhenius equation fails when (i) we have a system which does not follow standard Boltzman energy distributions i.e. has a non-equilibrium distribution of energies; (ii) when -E/RT is not constant, e.g. in an exothermic process, heat is evolved so (unless steps are employed to keep the temperature constant) the temperature increases. As T changes so does the term e(-E/RT).....so your reaction would speed up (or slow down for an endothermic process).

Part 4 . True. (If the amount of an enzyme is high then it will increase the rate of reaction. This is because if there is a greater surface area of an enzyme then there are more molecules exposed and available for collisions. This will increase the amount of collisions therefore increasing the rate of reaction.)

Part 5. True. (A catalyst is a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction. A catalyst will appear in the steps of a reaction mechanism, but it will not appear in the overall chemical reaction (as it is not a reactant or product).