6.1 Kinetics Flashcards
Rate of Reaction
The change in concentration of a product per unit of time.
Also viewed as the negative of the change in concentration of a reactant per unit of time.
Rate of Reaction Formula
(avg) = Δ [reactant or product]/Δ time (concentration in mol/L of reactant/product)
Light Absorption
If a reaction produces a precipitate (an insoluble compound) we can time how long it takes the formation of product to cloud the view of a mark made on a piece of paper placed under the reaction vessel. A spectrophotometer (measures wavelength of the color forming in solution)or a colorimeter (filters out a specific color) could also be used.
Collecting an Evolved Gas
The gas produced is collected in a syringe or in a graduated vessel over water (gas can’t be water soluble).
Other Techniques
Electrical conductivity
- Presence of ions allows a solution to conduct. The reaction rate may be found from the changes in conductivity.
Titration
- Taking small samples throughout experiment and testing for amounts of product made at that time.
pH
- (not great method) because dealing with small changes in concentration over the pH scale.
Kinetic Molecular Theory States:
- Large numbers of molecules in continuous motion
- Attractive and repulsive forces are negligible
- Energy is transferred between molecules during collisions, but average kinetic energy is unchanged (as long as temp is constant)
- Kinetic energy of molecules is proportional to the absolute temperature. (in K)
The Collision Theory States:
When a chemical reaction takes place, the reactant particles must collide with a certain minimum amount of energy, called activation energy. This energy is required to break chemical bonds in the reactants.
The energy of each particle is not important, it is the energy of the collision.
Particles must collide with the proper geometry or orientation for atoms to come in direct contact and form the chemical bonds of the products. (steric factor)
Factors which affect Rate of Collision/Reaction
Concentration (and pressure with gases)
Temperature
Amount of surface area
Catalysts
Concentration effect on Rate of Collision/Reaction
A higher concentration of reactants leads to more effective collisions per unit time thus an increased rate of reaction (where in limiting reagent amount is same but speed is faster).
Pressure effect on Rate of Collision/Reaction (Gasses)
Increased pressure means less room for particles to spread out/move and thus more concentration increases collisions.
Temperature effect on Rate of Collision/Reaction
Higher average kinetic energy of molecules and therefore the energy of collision, thus more collisions per unit time.
Catalysts (Situational)
A substance that speeds up a reaction without being used up itself.
Catalysts effect on Rate of Collision/Reaction
Provides surface for reaction to take place (alternative mechanism), where the catalyst breaks bond(s) or weakens bond(s), so that collision may require less energy.
Maxwell-Boltzmann Curves
A curve/graph which shows how the energy is spread out over different molecules in a reaction.
Maxwell-Boltzmann Curve Characteristics
- Area under the curve = the total molecules in the sample (it doesn’t change)
- All molecules have energy (does not ever go back down to 0)
- Few molecules have high energies, but there is no maximum energy for a molecule.
- Must begin at (0,0) as no molecules = no energy
