CH0002 - Lecture 3
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Another Gibbs free energy equation using the Equilibrium constant
ΔGo = -RT lnK
Thermodynamics tell us whether or not a reaction is ... and ... but it cannot tell us ...
- ...how far the reaction will go
- ...how fast it will be
What does Kinetics tell us?
- How quickly a reaction will take place.
- i.e. the rate of reaction.
What is the rate?
Rate = Change in concentration / Time
What can you use to measure the concentration?
- Volumetric methods - Change in volume of gas or increase in pressure
- Gravimetric methods - Change in mass (if a gas is consumed or evolved by the reaction)
- Instrumental methods - Change in physical property which correlates with concentration. e.g colour, electrical conductivity, pH
For a reaction
aA + bB --> cC + eE
What is the rate?
- rate =
- -1/a x d[A]/dt = -1/b x d[B]/dt =
- +1/c x d[C]/dt =
- +1/e x d[E]/dt
- Where t = time
- Where d = change in
The rate quation for a chemical reaction shows...
...how the rate depends on the concentrations of the various reactants and products
What is the rate equation?
rate = d[A]/dt = -k[A]n
Where k is the rate constant and n is the reaction order.
The minus sign indicates that the concentration of A falls with time. i.e. it is a reactant not a product
In the rate equation, what do the powers to which the concentrations are raised indicate?
The individual orders of reaction.
Rate = (with powers)
Rate = k[A]x[B]y[C]z
Total order of reaction is given by...
sum of the individual orders.
Total order = x+y+z
First order Units
- Rate = k[A]1
- Mol dm-3s-1 = k(mol dm-3)1
- s-1 = k
Second order units
- Rate = k[A]2
- Mol dm-3s-1 = k(mol dm-3)2
- dm3mol-1s-1 = k
What would you like to do?
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