12.7 Catalysis
What is true about a homogeneous catalyst?
- It is present in the same phase as the reactants. - It is regenerated during the overall reaction. - It speeds up the rate of the reaction.
Heterogeneous catalysis involves
- adsorption of the reactant onto the surface of the catalyst - activation of the adsorbed reactant - reaction of the adsorbed reactant - diffusion of the product from the surface into the gas or liquid phase (desorption)
A heterogeneous catalyst _______.
- is not consumed in the course of the reaction - provides a surface on which reactions can occur
Reaction mechanisms will always have a minimum of how many steps?
1 It is possible for a reaction mechanism to be only one step, though it will often consist of multiple steps.
2N2O5(g)→4NO(g)+3O2(g) is 0.425Lmol s at 630. K and 5.02Lmol s at 680. K. Using the Arrhenius equation, what is the activation energy for this reaction?
1.76 x10^5 J/mol The Arrhenius equation is k=Ae−EaRT, where R is the ideal gas constant (in this case, we use 8.314Jmol K), T is the temperature in kelvin, Ea is the activation energy in joules per mole, and A is the frequency factor. Given two temperatures and rate constants for the same reaction, with the same activation energy, we can substitute these into the Arrhenius equation and divide the resultant equations by each other (such that A cancels out) to obtain: Ea=−R[lnk2−lnk1(1T2)−(1T1)] In this case,k1=0.425Lmol s,T1=630K,k2=5.02Lmol s, andT2=680.K. Thus, the activation energy is: Ea=(−8.314Jmol K)[ln5.02−ln0.425(1680. K)−(1630. K)]=1.76×105 J/mol
How many elementary steps are involved in the following reaction mechanism for the decomposition of ozone? Overall=2O3→3O2Step 1=O3→O2+OStep 2=O+O3→2O2
2 The overall reaction does not count as an elementary reaction, as it is the sum of the elementary reactions. Looking at the mechanism itself, in the first step, ozone yields O2 and O, and in the second step, O and O3 collide to yield 2O2.
For a particular catalyzed reaction, the change in enthalpy is 26kJmole and the activation energy is 67kJmole. Which can be the change in enthalpy and the activation energy for the *uncatalyzed* reaction?
26kJmole,84kJmole The change in enthalpy must be the same, and the activation energy must be greater for the uncatalyzed reaction.
For a particular reaction, the change in enthalpy is 51kJmole and the activation energy is 109kJmole. Which of the following could be the change in enthalpy (ΔH) and the activation energy (Ea), respectively, for the catalyzed reaction?
34kJmole,83kJmole 34kJmole,120kJmole 51kJmole,120kJmole For a particular reaction, the change in enthalpy is 51kJmole and the a34kJmole,83kJm The change in enthalpy must be the same in each case, and the activation energy must be lower for the catalyzed reaction.
The rate constant for the reaction C12H22O11(aq)+H2O(l)→2C6H12O6(aq) is 4.52×10−4 s−1 at 355 K and 6.71×10−3 s−1 at 405 K. Using the Arrhenius equation, what is the activation energy for this reaction?
6.45 x 10^4
Which is an example of homogeneous catalysis?
Acid catalysis, organometallic catalysis, and enzymatic catalysis
heterogeneous catalysts are An example of a heterogenous catalyst can be found in automobile catalytic converters. They often function by furnishing an active surface upon which reactions can occur. They are often present in excess of the reactants.
An example of a heterogenous catalyst can be found in automobile catalytic converters. They often function by furnishing an active surface upon which reactions can occur. They are often present in excess of the reactants.
Identify the option below that is FALSE about homogeneous catalysts. Select the correct answer below: They are present in the same phase as the reactants. They are always regenerated by the end of the reaction. An example of a homogeneous catalyst is a layer of carbon (graphite) providing a reactive surface for an all-gas reaction. They typically interact with reactants to form intermediates, which eventually react away to regenerate the catalyst.
An example of a homogeneous catalyst is a layer of carbon (graphite) providing a reactive surface for an all-gas reaction. Homogeneous catalysts are present in the same phase as the reactants. Like all catalysts, they interact with reactants to form intermediates, which then react in another step to regenerate the original catalyst. In this case, graphite is not present in the same phase as the reactants, and therefore cannot be classified as a homogeneous catalyst.
Which of the following are FALSE about catalysts?
Catalysts affect the energy of products. Catalysts affect the energy of reactants. Catalysts lower the activation energy of a reaction (in both directions), often by providing an alternate reaction mechanism.
Identify the option below that is FALSE regarding heterogeneous catalysts. An example of a heterogenous catalyst can be found in automobile catalytic converters. They often function by furnishing an active surface upon which reactions can occur. They are often present in excess of the reactants.
None of the above A heterogeneous catalyst is always present in a different phase from that of the reactants. Heterogeneous catalysts often function by furnishing an active surface upon which reactions can occur. Two examples of heterogeneous catalysts are nickel in the hydrogenation of polyunsaturated fats and oils and automobile catalytic converters.
Which of the following is a homogeneous catalyst for the overall reaction described by the reaction mechanism shown below? Step 1:2NO2(g)→NO3(g)+NO(g)Step 2:CO(g)+NO3(g)→CO2(g)+NO2(g)
There is no homogeneous catalyst. A homogeneous catalyst is consumed somewhere in the reaction mechanism and regenerated in the end, and is present in the same phase as the reactants. In this case, there is no homogeneous catalyst. not NO2
Homogenous catalysts are
They are present in the same phase as the reactants. They are always regenerated by the end of the reaction. They typically interact with reactants to form intermediates, which eventually react away to regenerate the catalyst. NO2 + O3 Homogeneous catalysts are present in the same phase as the reactants, and so can be included as reactants in the chemical equation. They interact with reactants to form intermediates, which then react in another step to regenerate the original catalyst. One example of a homogeneous catalyst is nitric oxide with ozone.
Identify the options below that are functions of catalysts. (select all that apply)
They decrease the activation energy for both the forward and the reverse directions of a reaction. They increase the rate of reactions by lowering activation energy. Catalysts lead to alternate reaction mechanisms, often with additional steps. They speed up the rate of reactions by lowering the activation energy for both forward and reverse reactions, allowing systems to reach equilibrium more quickly. Although catalysts can affect the speed with which equilibrium is achieved, they do not affect the equilibrium position or the energy of reactants and products.
Heterogeneous are
They function by furnishing an active surface upon which reactions can occur. An example of a homogenous catalyst is elemental nickel used in the hydrogenation of polyunsaturated oils.
A certain reaction involves the conversion of cyclohexene, a liquid, into cyclohexane (also a liquid). The rate of this reaction is dramatically increased by heterogeneous catalysis. Which of the following could be the catalyst used in this process?
a mix of solid palladium and carbon Heterogeneous catalysis requires the catalyst to be present in a different state from that of the reactants; thus, the palladium-carbon solid surface is the catalyst in this case.
The second step in heterogeneous catalysis is:
activation of adsorbed reactant Once the reactant is adsorbed, it must be activated by the catalyst to prepare for reaction.
The first step in heterogeneous catalysis is:
adsorption of the reactant onto catalytic surface
Which is an example of homogeneous catalysis?
aqueous acid catalysis enzyme catalysis catalytic depletion of ozone with chlorofluorocarbons These are all examples of catalysis in homogeneous solution.
When compared with the normal reaction pathway, a catalyzed reaction pathway will always have:
depends on the reaction A catalyzed reaction pathway is a completely different pathway from the normal pathway, so it could have the same number of steps, or it could potentially have fewer or additional steps. CONTINUE
The impact of homogeneous catalysis on a reaction can be seen:
energy diagram
A catalyst will:
have no effect on the change in enthalpy of a reaction A catalyst only affects the energy of the transition state, it will have no impact on the change in enthalpy for the overall reaction.
Which is an example of heterogeneous catalysis?
hydrogenation of fatty acids with nickel catalyst and a catalytic converter The decomposition of ozone is homogeneous in the gas phase, and aqueous acid catalysis is homogeneous in the liquid phase, but hydrogenation of fatty acids with nickel catalyst involves liquid phase substrate approaching a solid catalyst.
True about catalysts in forward and reverse reactions?
lower the activation energy of both the forward and reverse reactions A catalyst will lower the activation energy of both the forward and reverse reactions, as it lowers the energy of the transition state between them.
The presence of homogeneous catalyst will not affect the:
molecularity of the overall chemical equation The catalyst will not impact the overall chemical equation, in terms of stoichiometry or molecularity, but it will be present in the rate-determining step so it has an impact on the molecularity for it.
A catalyst will:
overall not be used up as it can regenerate
Reaction mechanisms tell us the:
sequence of collisions that occurs A reaction mechanism tells us which molecules collide and in what sequence in order to generate the product of the overall reaction.
Homogeneous catalysis typically does NOT occur in the:
solid phase In a solid there is no relative movement of the particles, which makes catalysis or any chemical reaction in general very difficult.
Most modern catalytic converters in automobiles have a surface with a platinum-rhodium catalyst. For which of the following reactions is this catalyst used?
the conversion of nitric oxide into elemental nitrogen and oxygen gas the conversion of carbon monoxide into carbon dioxide the conversion of octane into carbon dioxide and water vapor The platinum-rhodium catalyst catalyzes the conversion of nitric oxide into dinitrogen and oxygen, the conversion of carbon monoxide into carbon dioxide, and the conversion of octane into carbon dioxide and water vapor.
For a particular reaction, the change in enthalpy is −42kJmole and the activation energy is 28kJmole. Which of the following could be the change in enthalpy (ΔH) and the activation energy (Ea), respectively, for the catalyzed reaction?
−42kJmole,13kJmole The change in enthalpy must be the same in each case, and the activation energy must be lower for the catalyzed reaction.
