Topic 9 Kinetics I Flashcards
Why does an increase in concentration often increase the rate of a reaction in solution?
The frequency of collisions between reacting solute particles increases as there are more in a given volume & they are closer together. Hence, the frequency of successful collisions increases.
How does a product/time graph show a faster reaction rate?
The curve will have a steeper gradient & levels off faster.
Why does changing the pressure not affect reactions in the solid or liquid phase?
The volumes of liquids/solids don’t change very much under pressure; the particles don’t move much closer together.
Why does an increase in pressure increase rate of reaction in the gas phase?
If the pressure of a gaseous mixture is increased, there will be more reactant molecules in a given volume, so more frequent, successful collisions.
Why does increasing the surface area increase the rate of reaction?
For heterogeneous reactions involving a solid, a large surface area of the solid increases rate of reaction as collisions only on the surface of the solid result in reaction. Powdering a solid catalyst also increases rate of reaction, where a solid catalyst is involved.
Maxwell-Boltzmann distribution curve: y-axis & x-axis
y-axis: Fraction of molecules with energy, E
x-axis: Energy, E
Describe the Maxwell-Boltzmann distributions where T2 is at a higher temperature than T1.
- Neither graph is symmetrical.
- Both graphs start at the origin and finish by approaching the x-axis asymptotically.
- The area under each curve is the same, since the number of molecules has not changed.
- The peak of T2 is displaced t the right and lower than T1.
- T2 finishes above T1.
How does temperature increase rate of reaction?
Increased temperature increases the fraction of molecules that possess the required activation energy. The rate of reaction increases because the number of successful collisions increases.
Catalyst
A substance that increases the rate of the reaction, but is chemically unchanged at the end of the reaction.
Maxwell-Boltzmann distribution for the effect of a catalyst
One curve, two activation energy lines. A catalyst provides an alternative route for the reaction with a lower activation energy, so a greater fraction of the particles have at least the energy required to react, so there are more frequent, successful collisions.
How else can the effect of catalysts be shown?
An enthalpy profile diagram with enthalpy on the y-axis & progress of reaction on the x-axis. Reactants & products on different levels. Show an arrow for delta H & another arrow for Ea (from reactants to the top of the hump). Catalysed reactions have a lower hump than uncatalyzed reactions.
2 economic advantages of catalysts
- Reactions can occur at lower temperatures, so lower energy costs.
- They can increase the rate of a chemical reaction, so more of the desired product is made in a given time period.
