Extent and Rate of Reaction 1 Flashcards
What does molecular motion give insight into?
- The transfer of energy and variation of pressure
- Collision frequency
What do transport properties allow us to understand?
How molecules move through phases:
- Ionic movement
- Effect of viscosity
- Diffusion
What is the movement of molecules in solid form treated as?
A vibration about mean lattice position
State the ideal gas equation.
PV = nRT
What are the four principles of explaining behaviour of an ideal gas?
- A gas is made of a very large number of molecules so small compared to the distances between them
- The molecules are in constant, rapid, straight line motion, colliding frequently
- Collisions are perfectly elastic - no kinetic energy is lost or gained during collision - do not exert any attractive or repulsive forces on each other
- The average kinetic energy is proportional to temperature (K)
What is the speed of molecules proportional to?
- The temperature and inversely proportional to the molecular mass
What is collision frequency proportional to?
- Temperature at constant volume and to pressure at constant volume
What is viscosity?
For a molecule to move it requires energy to escape its neighbour
When does viscosity decrease?
With an increase in temp. as molecules have more kinetic energy
Describe ionic movement.
- Ions move through a solvent by the application of a potential difference
- Resistance is used to ionic motion and inverse of conductance
- Motion of an ion remains random but the application of an electric field biases its motion and subsequent migration through solution
When does conductance decrease and increase?
- Conductance decreases with length and increases with cross sectional area of a medium
- Conductance increases with an increase in the number of ions present, Molar conductivity,
Lm = k/c
What are ion channels?
Highly selective proteins that move ions down a potential gradient driven by the hydrolysis of ATP
Give the thermodynamic definition of diffusion.
A force that represents the spontaneous tendency of molecules to disperse to a situation of greater disorder
What does Fick’s law state?
- Diffusion is greater when the concentration varies steeply with position
Define phenomenological diffusion.
- Diffusion is the movement of a substance from a region of high concentration to one of low concentration without bulk motion
Define atomistic diffusion.
- Random walk of molecules, not so random, molecules are propelled by collisions in high
concentration areas
Define kinetic energy.
Energy that an object possesses because of its motion
Describe how energy in a molecule is distributed.
- Rotation is the movement of a molecule about an axis
- Translation is the movement from one place to another
- Vibration is the motion of one atom in relation to another within the molecule
Which motion is only allowed for molecules in the solid state?
Vibration
Why do we need to know about the movement of molecules?
- To assist in understanding reaction mechanisms
and optimise synthetic reactions - Improve yield
- Speed up processes
- Minimise by-products
- Predict or extend shelf-life
- e.g. aspirin hydrolysis is pH dependant
Describe the mechanism of breathing.
- Lungs expand, increase in volume in alveoli causes a partial vacuum and a pressure gradient
- Get a bulk flow of air down the pressure gradient
- Air arriving in the lungs has a high molecular O2 concentration
than the blood in the capillaries surrounding the alveoli - See diffusion of O2 molecules down this concentration
gradient into the blood
Describe rate of reaction.
- the greater the number of collisions, the faster the reaction
- the smaller the activation energy, the greater the number of collisions with sufficient energy to react
- Molecules need to be in the correct orientation
How to calculate rate of reaction?
rate of reaction = (number of collisions per unit time) x (fraction with sufficient energy) x (fraction with appropriate orientation)
State and describe the factors that affect rate.
- Physical state – reactants have to be in the same phase i.e. dissolved.
- Concentration – increasing concentration increases frequency of collision
- Temperature – increasing the kinetic energy of the atoms and the frequency of collisions provides an increase in the number of collisions with sufficient energy to exceed the activation barrier.
- Catalysts – No effect on overall energies, catalyst effects the activation energy
Give the rate equation?
Rate = k[A]^n[B]^m
- n is the order of the reaction with respect to A
- m is the order of reaction with respect to B
- (n+m)
State the effect of temp. on reaction rates.
- The rate constants increase with temp.
State the Arrhenius equation.
k = Ae^ -Ea/RT
- k is the rate constant (sec-1)
- A the frequency factor, representing the number of collisions occurring with the correct orientation to react
- e^-Ea/RT represents the fraction of collisions with the minimum energy to react
- where Ea is the activation energy (kjmol-1)
- R is the universal gas constant (8.314x10-3 kjmol-1K-1)
- T is temperature (K)
Describe the effect of catalysts upon reaction rates.
- A catalyst participates in the reaction but is not consumed by it
- The stability of reactants and products does not change from the un-catalysed reaction
- A catalyst increases the rate of a reaction as it modifies the reaction energy pathway, such
that ΔG‡ is smaller
What are the three ways catalysts increase the rate of reaction?
- Activate the reactant TM chemistry
- Stabilise (and reduce energy of) transition state
- Different reaction mechanism
How can we measure the rate of reaction?
- Follow the concentration of a component of the reaction, a reactant, as a function of time
- Decrease in reactant concentration
- Increase in product concentration
A + B → C
Rate =−ΔA/Δt =−ΔB/Δt =−ΔC/Δt
