Arrhenius Equation, Kinetics
Write the expression of Kp for: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Don't forget the P
How do you work out reaction rate from a graph?
Gradient of a graph of amount of product/reactant against time. Gradient is the reaction rate. If the graph is a curve, then work out the tangent.
How can you determine A's value?
It can only be determined experimentally
What do the order of reactions tell you?
It tells you how the reactant concentrations affect the rate. It also gives you information on the rate determining step.
Gas constant units?
J K⁻1 mol⁻1
Units for activation energy?
J mol⁻¹
Is the rate of an uncatalysed reaction MORE or LESS affected by temperature compared to the rate of a catalysed reaction.
Uncatalysed reaction rate CHANGES MORE as temperature changes compared to the rate of a catalysed reaction
units for A?
Units of A are identical to the rate constant. if reaction is first order, units are s⁻¹. If 2nd order, then A = mol⁻¹ dm³ s⁻¹
How do you work out Kp?
Use partial pressures in equation
So you have collected data for concentration of reactants at different times. How do you determine the order of the reaction with respect to that reactant?
You plot a graph of concentration vs time. You can find the rate at different times by finding the gradient at different times. Then you plot the rate against concentration. After that, you can find the order of the reaction.
How do you find the activation energy using the arrhenius equation? How do you find ln A?
You plot a graph of ln (rate constant) against 1/temperature. Gradient will be -Ea/R. Multiply the gradient by -R to find Ea. ln A is where the graph crosses the y axis.
Tell me about continuous monitoring
You record the amount of reactant or product at regular time intervals. You use this to work out how the rate changes over time.
Roughly what activation energy does the rough guide of"increase temp by 10 degrees, rate of reaction doubles" apply to?
about 50 000J activation energy
What do you know about catalysts and the rate determining step?
catalysts can appear in rate equations, so they can appear in the rate determining steps too
What is the colour of I⁻(aq) in solution?
colourless bruh
What is the largest value k can be in an equation for a specific reaction?
if Ea is 0, then k=A
What does k of a rate equation do?
k relates the reactant concentrations to the rate AT A PARTICULAR TEMPERATURE. If the temperature rises, then k rises.
Is A constant for all values of T?
no
What are the properties of A? Why is this?
A is a constant. It exists to quantify the configuration of molecules, it factors in the orientation of the molecule, and factors in the changes in rate due to frequency of collisions in the correct orientation. Each chemical reaction will have a different value of A.
Why can you not extrapolate to find the value of A?
A is only linear over a narrow range of temperatures.
What does A mean?
A is the pre-exponential factor, or frequency factor. (In exam say Arrhenius constant)
What does a rate equation do?
A rate equation links the concentrations of reactants to the rate constant. It tells you how the rate of reaction is affected by the concentration of reactants. A rate equation also allows you to work out the mechanism of a chemical reaction.
Will A stay the same if you add a catalyst?
No. A will change. A catalyst is likely to be involved in the slow step of a reaction, and a new rate equation will have to include a term relating to the catalyst.
How do you find the orders of reactions?
Only by experiment.
A reactant doesn't appear in the rate equation. What does this tell you?
That reactant won't be involved in the rate determining step, and neither will anything derived from it.
what does e^-(Ea/RT) mean?
The fraction of the molecules present in a gas which have (kinetic?) energies equal to or in excess of the activation energy at a particular temperature.