Topic 9: Kinetics I Flashcards
How does temperature affect reaction rate?
Heating particles increases KE which increases the collision frequency. Particles are more likely to have AE.
How does pressure affect reaction rate?
More particles per cm3 which increases the frequency of collisions. AE is not affected.
How does conc affect reaction rate?
More reacting particles in the same volume which increases collision frequency. AE is not affected.
How does surface area affect reaction rate?
More surface for collisions to occur on, which increases the collision frequency. More particles are exposed and ready to react.
How does a catalyst affect reaction rate?
Lowers activation energy by providing a surface for the reaction to take place on. Therefore more collisions are successful.
What must happen for a reaction to take place?
Reactant particles must collide with enough activation energy, at the correct orientation. If not, they will bounce off each other.
Draw a Maxwell Boltzmann distribution curve demonstrating temperature on reaction rate.
A higher temperature moves the peak of the curve and the average kinetic energy to the right.
More particles have energy greater than the activation energy.
The area under the curve is the number of particles, which must remain constant.
x axis= energy. Y axis= fraction of molecules w energy, E
Describe an experiment to see the effect of concentration on rate.
Add sodium thiosulfate to different concentrations of HCl. Record time taken for a cross underneath the conical flask to disappear.
Explain steric hindrance
When shapes of molecules influence reactions, there is a steric factor involved in the reaction.
Sometimes atoms in a molecule hinder the course of a reaction and block an attacking species. Therefore the reaction suffers steric hindrance.
What is the assumption made in temp increasing reaction rate?
The temp argument assumes that the fraction of COLLISIONS with correct activation energy is the same as the fraction of MOLECULES with correct activation energy, which is not strictly true.
But the diff is v small when the fraction is v small, so it is unreasonable to draw them in a molecular energy distribution curve instead of the collision distribution curve.
Explain collisions and reactions in solution.
In gases, molecules move fast so frequently collide. In solution, molecules are more closely packed, w/o much space between them.
In lower concs, most of it is solvent, so solute molecules are surrounded by solvent molecules in a solvent cage.
BUT many solute molecules can get trapped in the same cage, increasing collision Hz.
So for a reaction in solution, molecules must diffuse into the same solvent cage, and they must react.
Give the main economic advantages of using catalysts in industry.
Increased rr =more desired product made in a given time.
Reactions can take place at lower temps, lowering energy costs for the manufacturer.
What kind of catalysts are used in industry? How do they work?
Most catalysts are heterogeneous, meaning it’s in a diff phase to the reactants. Solids are heterogeneous catalysts for reactions with gases.
A solid catalyst provides a surface on which gas molecules can adsorb and then react. The product molecules then desorb from the surface and more reactant molecules take their place.
State the definition for activation energy
The energy required to break bonds in a chemical reaction, and therefore the minimum energy required for a collision to result in a successful reaction
The main reason that reaction rates increase with temperature is that…
a larger proportion of molecules have high energies.
