Equilibrium Flashcards

1
Q

What is collision theory?

A

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.

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2
Q

What is activation energy?

A

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.

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3
Q

What are enthalpy profile diagrams?

A

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.

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4
Q

What is Boltzmann distribution?

A

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.

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5
Q

What is the effect of temperature on reaction rate?

A

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.

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6
Q

What is the effect of concentration on reaction rate?

A

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.

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7
Q

What is the effect of pressure on reaction rate?

A

For gases, increased pressure increases rate of reaction.
The particles are closer together, so collide more often, and so have more chances to react.

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8
Q

What is a catalyst?

A

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.

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9
Q

How do catalysts work?

A

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.

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10
Q

How do catalysts work? (model answer)

A

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.

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11
Q

How does the catalyst affect Boltzmann distribution?

A

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.

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12
Q

What is a heterogenous catalyst?

A

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.

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13
Q

What are homogenous catalysts?

A

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.

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14
Q

What is the use of catalysts in industry?

A

They lower production costs and help make better products.

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15
Q

What is the effect of catalysts on the environment?

A

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.

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16
Q

What is an example of a catalyst for the environment?

A

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

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17
Q

What are the limitations of catalysts?

A

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.

18
Q

What are examples of catalysed reactions?

A

Esterification using H2SO4 as a catalyst:
Ozone depletion, using Cl* as catalyst:
2O3 <—–> 3O2

19
Q

What is reaction rate?

A

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

20
Q

How to measure the progress of reaction by mass?

A

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.

21
Q

How to measure the progress of reaction by gas volume?

A

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.

22
Q

What is dynamic equilibrium?

A

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.

23
Q

What is Le Chateliers principle?

A

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.

24
Q

How does changing concentration affect equilibrium?

A

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.

25
Q

How does changing pressure affect equilibrium?

A

Increasing pressure shifts the equilibrium to the side with fewer gas molecules, which reduces the pressure.
Decreasing the pressure shifts the equilibrium to the side with more gas molecules, so pressure increases again.

26
Q

How does changing temperature affect equilibrium?

A

Increasing adds heat energy, so the equililbrium shifts in the endothermic (+ve enthalpy change) direction to absorb this energy.
Decreasing removes heat energy, so equilibrium shifts in the exothermic direction to try and replace it.

27
Q

Why is temperature in Le Chatelier’s principle important for ethanol production?

A

The industrial conditions are pressure of 60, temperature of 300c, and a phosphoric acid catalyst.
It is exothermic, so lower temperatures favour the forward reaction, so there is a higher yield.
But lower temperature means a slower rate of reaction, so 300c is a compromise between maximum yield and fast rate.

28
Q

Why is pressure in Le Chatelier’s principle important for ethanol production?

A

Higher pressure favours the forward reaction, and increases the rate.
But high pressures are expensive and dangerous to produce.
Stronger pipes are needed to withstand the pressure, and side reactions can also occur.

29
Q

What is the equilibrium constant?

A

For a homogenous reaction that’s reached dynamic equilibrium, Kc is worked out to give an idea of how far to the left or right equilibrium is.
Kc = [D]^d [E]^e / [A]^a [B]^b
Lower case letters are the number of moles.
The square brackets is the concentration in mol dm^-3.
Products are on top line, reactants on bottom.

30
Q

How to estimate the position of equilibrium using Kc?

A

Kc increases when the quantities of the products increase and the reactants decrease.
So Kc increase means equilibrium has shifted to the right.
The greater the increase or decrease the more equilibrium has moved.

31
Q

How to find equilibrium concentrations before calculating Kc?

A

Find out how many moles of each reactant and product there are at equilibrium.
Calculate the molar concentrations of each reactant and product by dividing each number of moles by the volume of the reaction in dm3.

32
Q

How is Kc calculated for heterogenous equilibria?

A

If the mixture is homogenous, all the reactants and products are put into the expression.
If the mixture is heterogenous, only gases and aqueous substances go into the expression.

33
Q

What is partial pressure?

A

Each gas exerts its own pressure within a mixture.
The partial pressure of X depends on the number of moles present.

34
Q

What is a mole fraction?

A

The proportion of a gas mixture that is made up of a particular gas.
Mole fraction = number of moles of gas / total number of moles of gas in mixture.
or
Partial pressure of gas in mixture = mole fraction of gas x total pressure of mixture

35
Q

What is the gas equilibrium constant?

A

Kp is the constant for a reversible reaction where some or all of the reactants and products are gases.
Kp = p(D)^d etc (partial pressure used instead of concentration)

36
Q

How do you work out Kp in an exam question?

A

Work out how many gas moles there are at equilibrium.
Add up the number of gas moles.
Divide each gas mole by the total number of moles then multiply it by the total pressure.
Put into the Kp equation.

37
Q

What is changing the position of equilibrium?

A

If the position moves to the left, you’ll get more reactants.
If the position moves to the right, you’ll get more products.
The larger the value of Kc the further to the right equilibrium lies.

38
Q

How does changing the temperature affect Kc?

A

If the change of equilibrium (by changing temperature) means more product is formed, Kc rises.

39
Q

How does changing the concentration affect Kc?

A

The value of Kc is fixed at a given temperature, so if the concentration of one thing changes then the concentration of the others must change to keep Kc the same.

40
Q

How does changing the pressure affect Kc?

A

Increasing pressure shifts equilibrium to the side with fewer gas molecules, which reduces pressure.
Kp and Kc will always stay the same though.

41
Q

What is the effect of a catalyst on Kc?

A

Catalysts increase the rate of the forward and backward reaction by the same amount so the equilibrium position will be the same, but reached faster.
So Kc stays the same.

42
Q

What do Kc values say about equilibrium?

A

A Kc value of 1 shows equilibrium is in the middle.
A Kc value of more than 1 shows equilibrium is towards the products.
A Kc value of less than 1 shows equilibrium is towards the reactants.