Equilibrium Flashcards
What is collision theory?
A reaction only takes place between two particles if they collide in the right direction and with at least a certain minimum amount of kinetic energy.
What is activation energy?
The minimum amount of kinetic energy particles need to react.
This energy is needed to break the bonds to start the reaction.
Heating particles provides extra energy.
What are enthalpy profile diagrams?
Used to work out the enthalpy change of a reaction, and then if it is endothermic or exothermic.
Endothermic reactions have a positive enthalpy change.
Enthalpy change is the difference in enthalpy of products - enthalpy of reactants.
What is Boltzmann distribution?
The curve starts at 0,0 because no molecules have zero energy.
A few molecules are moving slowly.
Most molecules are moving at moderate speed so their energies are in this range.
Some molecules have more than the activation energy, only these react.
What is the effect of temperature on reaction rate?
As you increase the temperature, the molecules will have more kinetic energy, move faster, and so a greater proportion of molecules will have the activation energy to react.
The Boltzmann curve moves right.
The molecules also collide more often so reaction rate increases.
What is the effect of concentration on reaction rate?
As the concentration of reactants in solution increases, the particles will be closer together, so collide more often, and so have more chances to react.
What is the effect of pressure on reaction rate?
For gases, increased pressure increases rate of reaction.
The particles are closer together, so collide more often, and so have more chances to react.
What is a catalyst?
Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
The catalyst is chemically unchanged at the end.
Only a small amount is needed to have large effect, and save lots of money industrially.
But they only work for a particular reaction.
How do catalysts work?
The reactant molecules bind to the catalyst, which makes it easier to break the bonds, so activation energy decreases.
The broken reactant molecules then form product molecules, and break away from the catalyst.
How do catalysts work? (model answer)
The catalyst lowers the activation energy so there are more particles with enough energy to react when they collide.
It does this by providing a different route for the reaction.
So in a certain amount of time more particles react.
How does the catalyst affect Boltzmann distribution?
The molecules still have the same amount of energy (curve unchanged) but the activation energy is lowered so more molecules have energy above the threshold and react.
What is a heterogenous catalyst?
A heterogenous catalyst is in a different phase from the reactants.
E.g. in the Haber process gases are passed over a solid iron catalyst.
The reaction happens on the surface of the heterogenous catalyst, so increasing its surface area increases the number of molecules that can react at the same time, increasing the rate of reaction.
What are homogenous catalysts?
Homogenous catalysts are in the same physical state as the reactants.
Usually it is an aqueous catalyst for a reaction between two aqueous solutions.
It forms an intermediate species, in which one or more reactants combine with the catalyst to form the intermediate.
The intermediate then reacts to form the products and reform the catalyst.
What is the use of catalysts in industry?
They lower production costs and help make better products.
What is the effect of catalysts on the environment?
Lower temperatures and pressures can be used.
Energy is saved, so less carbon dioxide is released, and fossil fuel reserves are preserved.
They can also reduce waste by allowing a different reaction to be used with a better atom economy.
What is an example of a catalyst for the environment?
Catalytic converters on cars are made from alloys of platinum, palladoim and rhodium.
They reduce the pollution released into the atmosphere by speeding up this reaction:
2CO + 2NO -> 2CO2 +N2
What are the limitations of catalysts?
Catalysts don’t last forever and eventually need to be disposed of.
Many contain toxic compounds which can leach into the soil if not directly sent to landfill.
But if they contain valuable metals (platinum),it can be recycled, depending on the economic and environmental factors.
What are examples of catalysed reactions?
Esterification using H2SO4 as a catalyst:
Ozone depletion, using Cl* as catalyst:
2O3 <—–> 3O2
What is reaction rate?
The change in the amount of reactants or products per unit time. gs^-1 or cm^3 s^-1
Rate of reaction = amount of reactant used or product formed / time
How to measure the progress of reaction by mass?
When the product is a gas, its formation can be measured using a mass balance.
Measure the mass at regular time intervals.
When the mass stops decreasing the reaction is finished.
It is accurate but can release toxic/flammable gas into the room so use a fume cupboard.
How to measure the progress of reaction by gas volume?
Collect the gas given off in a gas syringe and measure at regular time intervals.
This is accurate but vigorous reactions can blow the plunger out of the syringe.
It is good for toxic or flammable gases.
What is dynamic equilibrium?
As the reactants get used up, the forward reaction slows down, and as more product is formed, the reverse reaction speeds up.
After a while, the forward and backward reaction will go at the same rate so the concentration of products and reactants won’t change anymore.
This happens in a closed system.
What is Le Chateliers principle?
If there’s a change in concentration, pressure or temperature, the equilibrium moves to help counteract the change.
If position moves to the left, the backwards reaction is faster, so there’s more reactants.
For right, fowards is faster so more products.
Catalysts have no effect on the position.
How does changing concentration affect equilibrium?
Increasing concentration of the reactant means the equilibrium tries to get rid of the extra reactant, by making more product, so the position shifts to the right.
Increasing concentration of the product means the equilibrium tries to remove the extra product and so the reverse reaction is faster and it shifts to the left.
