Chapter 10- Reactions and Rates of Equilibrium. Flashcards
State Le Chantilier’s Priniciple.
When a system is in equilibrium and is subjected to an external change the system readjusts itself to minimise the effect of the change.
Describe how the position of equilibrium changes due to concentration and how this can be demonstrated.
It can be demonstrated by the reaction between chromate ions and dichromate ions.
2CrO4^2- + 2H^+ ⇌ Cr2O7^2- + H2O
When acid is added the increase in H^+ ions causes a shift to the right producing more products and making the reaction orange. If more OH^- ions are added the reaction shifts to the left producing more reactants to counteract the change and so the colour becomes yellow.
Describe how the position of equilibrium changes due to temperature and how this can be demonstrated.
If you increase the temperature the equilibrium position shifts to the endothermic direction to counteract the change and vice versa for the decrease in temperature.
Cobalt can be used to show this by…
[Co(H2O)6]^2+ +4Cl^-(aq) ⇌ CoCl4^2- + 6H2O
Cobalt chloride is dissolved in water and HCl is added then it is subjected to different temperatures; with an increased temperature it goes blue and more cobalt chloride is produced, with decreased temperature it goes pink as more reactants are produced.
What is a homogeneous catalyst?
It has the same physical state as the reactants and acts as an intermediate (so breaks down to give the product and regenerates as a catalyst).
Give an example of a homogeneous catalyst.
- H2SO4 is a catalyst for the formation of esters from carboxylic acids and alcohols.
- Cl. radicals in ozone depletion.
What is a heterogeneous catalyst?
It has a different physical state from the reactants; they’re usually solids in contact with gaseous or solution reactants.
Describe how a heterogeneous catalyst works.
The reactant molecules are absorbed (weakly bonded) on the catalyst surface ( bonding of e^- weakens them making the subsequent reactions easier). They’re then held at the right orientation for the reaction to occur (e^- are rearranged) and finally desorption occurs and they’re released.
Give an example of a heterogenous catalyst.
- Fe in the haber process.
- Ni in hydrogenation.
How do you calculate the exact position of equlibrium?
aA + bB ⇌ cC + dD
Kc = [C]^c [D]^d / [A]^a [B]^b
[ ] = the concentration of
Describe the position of equilibrium if Kc is…
1
> 1
< 1
1 = The position of equilibrium is half-way between reactants and products.
> 1 = indicates it is more towards the products.
< 1 = indicates it is more towards the reactants.
What does Boltzmann distribution represent?
The spread of molecular energies in gases.
Describe the features of a typical Boltzmann distribution graph.
- No molecules have 0 energy despite the graph starting at the origin.
- The area under the curve is the total number of molecules.
- There is no maximum energy for a molecules so the graph should never touch the x-axis further along.
- Always draw the Ea on it with a dashed line, any particles with energy beyond this point can successfully react.
- The y-axis is the number of molecules with a given energy and the x-axis is energy.
Describe Boltzmann distribution at a higher temperature.
- More molecules have an energy greater than or equal to the Ea.
- A greater proportion of collisions will lead to a reaction (more successful collisions), increasing the rate of reaction.
- Collisions will also be more frequent as the molecules are moving faster.
What other factor can impact Boltzmann distribution other than temperature?
A catalyst as it provides an alternative pathway with a lower Ea.
How can the progress of a reaction be determined?
If a reaction produces gas you can determine the rate of reaction using gas collection or monitoring mass loss. Gas can be collected using a gas syringe or a measuring cylinder and water. The volume of gas lost in a certain time is then used to calculate the rate of reaction. With the loss of mass experiment, the initial mass is recorded at regular time intervals and the mass is then recorded again. The reaction is complete when no more gas is produced so the mass no longer decreases.
