How is reaction rate usually expressed ?
A change in reactant or product concentration per unit change in time
What are the units of concentration ?
Describe the convention with regards to expressing reaction rates
Rates of reactions are expressed as positive numbers, as a result, a negative sign is required for reactants.
What are the units of reaction rate ?
What is the rate expression ?
Rate = K [A]m [B]n
Where K = rate constant
m = order of reaction with respect to [A]
n = order of reaction with respect to [B]
Describe the rate constant K ?
Reaction specific, and directly proportional to the rate of the reaction.
What is the impact of increasing the temperature on the rate constant, K ?
It increases since the proportion of molecules with energy greater than the activation energy Ea of the reaction increases with temperature.
How are the orders of reactions determined ?
They need to be determined experimentally.
What is the value of any number to the power zero ?
Describe a zero order reaction ?
The rate of the reaction is independent of the concentration of reactant [A] and therefore has a constant reaction rate.
The rate of the reaction is therefore equal to the rate constant, K.
How would you calculate the overall order of a reaction ?
[A]order + [B]order
Describe a first order reaction ?
The rate of the reaction is directly proportional to the concentration of the reactant.
Rate = k[A]1 or R=k[B]1
Describe a second order reaction ?
The rate is proportional to the square root of the reactant concentration.
Rate = K[A]2
Sketch and explain the graph of reactant concentration over time for a Zero order reaction
As the concentration of reactant A decreases over time, the slope (rate!) is constant.
The rate is not decreased by a decrease in reactant concentration.
Sketch and explain the concentration over time graph of a first order reaction
As the concentration of reactant [A] decreases, the rate decreases proportionally.
(CF exponential decay)
Sketch and explain the graph of a reactant concentration over time for a second order reaction
The rate of the reaction decreases proportionally to the square of the reactant [A] concentration.
CF exponential decay.
What is the half-life of a reaction ?
The time taken for the concentration of the reactant to decrease by half of its original value.
NB. This is different for each rate order.
What do you need to do to determine the exponents of the rate law for a reaction ?
The only way to do this certainly is through experimentation via the "initial rates" or "isolation method".
If there are two or more reactants involved in the reaction, the concentrations are usually varied independently of each other.
What are the units of a zero order reaction ?
Rate = K [A]0
What are the units of a first order reaction ?
Rate = K [A]1
What are the units of a second order reaction ?
Rate = K [A]2
Name the three steps involved in free radical formation ?
Describe the formation of HCl from H2 + Cl2
What is the rate determining step of a reaction ?
The slowest step of a reaction
How would you predict the rate law from a chemical equation ?
The chemical equation of an elementary step reflects the exact molecular process that transforms its reactants into its products.
In an elementary process, the orders with respect to the reactants are equal to the corresponding stochiometric coefficients
What is the Arrhenius constant (A) ?
Also known as the frequency factor.
What is the collision frequency (z) ?
The number of collisions that molecules acquire per unit time.
What is the orientation factor (p) ?
The proper orientation reactant molecules require for product formation.
What is Ea ?
Describe how it works
For reactants to transform into products, the reactants must go through a high energy "transition state" which is the minimum energy required for reactants to transform into products.
What factors increase K (and therefore, rate?)
- Increase in temperature
- Decrease in activation energy (e.g. addition of a catalyst)
What is an activated complex ?
This is also known as a transition state.
It is the species formed during an efficient collision, before the reactants transform into the final product.
Sketch a graph and decribe the potential energy over reaction progress for an EXOthermic reaction
Energy products > Energy of reactants
Sketch a graph and describe the potential energy over time for an ENDOthermic reaction
What happens to the reaction rate if the difference between the total energy of reactants, and Ea increases ?
The reaction rate slows.
Describe the difference between kinetically controlled and thermodynamically controlled reactions
If the reaction has two possible products, one with a low Gibbs free energy (most thermodynamically stable) product, and one with a smaller activation energy.
If the product formed is the one with the lowest gibbs free E, it is said to be thermodynamically controlled, and vice versa.
Describe a Catalyst ?
A catalyst increases the rate of a reaction, without itself being used up in the reaction.
They provide an alternative mechanism for the reaction, with a lower Ea.
Describe a typical biological catalyst ?
They are very specialised biological catalysts.
They are protein molecules with a large Mr, containing one or more active sites.
How do catalysts work ?
They stabilise the transition state of a reaction by lowering the energy barrier between reactants and the transition state.
They do not change the energy difference between the reactants and the products.
What is the rate of an enzyme-catalysed reaction ?
Rate = K [ES]
Where [ES] is the Enzyme-substrate complex
Describe the principle of saturation kinetics ?
When the concentration of the substrate is large enough for the substrate to occupy all the available active sites of the enzyme, any further increase would have no effect on the rate of the reaction.
Describe equilibrium in most chemical reactions (Dynamic equilibrium)
Once a product is formed, it reacts in such a way to yield back the initial reactants. Eventually, the system reaches a state where there are as many molecules of products being formed as there are molecules being generated through the reverse reaction.
These will not necessarily be equal at equilibrium, but they will be constant.
How would you calculate the equilibrium constant, K ?
Keq = Concn Products/Concn Reactants
How does temperature change affect Keq ?
If temperature change, the equilibrium constant also changes.
What is the law of mass action ?
At a given temperature, changing the concentration of a product or reactant causes the system to resolve to maintain Keq.
When can the equilibrium constant K be directly calculated ?
When the equilibrium concentrations of products or reactants are known or obtained.
What is the significance of a Keq > 1 ?
The forward reaction is favoured, and the reaction favours product formation.
N.B. If K is very large, the equilibrium mixture will contain very little reactant compared to product.
What is the significane of Keq = 1 ?
Neither the forward nor the reverse reactions are favoured
What is the significance of a Keq < 1 ?
The reverse reaction is favoured, therefore the reaction does not proceed very far towards product formation and thus very little product is formed.
Describe the equilibrium constant in relation to pure solids and liquids ?
Concentrations remain constant throughout in heterogenous equilibria, therefore Keq = 1
What is Le chateliers principle ?
When any system at equilibrium is subjected to a change in concentration, temperature, volume or pressure, then the system readjusts itself to counteract the effect of the applied change, and equilibrium is re-established.
Describe the effect of an increase in pressure on reaction direction ?
This will favour the direction which decreases the total number of compressible moles
What happens to the direction of a reaction if temperature at equilibrium is increased ?
The equilibrium evolves in the direction of the endothermic reaction.