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Chemistry Alevel OCR > 3.2 Physical Chemistry > Flashcards

3.2 Physical Chemistry Flashcards

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1
Q

What are the standard conditions ?

A

Temperature =298K (25 °C)
Pressure= 100kPa (100 000Pa) (0.986 atm)

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2
Q

What form will the substances be in their standard state?

A

The physical state of a substance under standard conditions

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3
Q

What is the symbol for standard conditions?

A

⦵ (plimsoll line symbol)

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4
Q

Explain enthalpy change of reaction

A

Enthalpy change of reaction is the energy change associated with a given reaction.

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5
Q

Define enthalpy change of formation

A

The enthalpy change that takes place when 1 mole of a compound is formed from its elements

-in the elements standards state under standard conditions

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6
Q

Define enthalpy change of combustion

A

The enthalpy change that takes place when 1 mole of a substance is completely combusted

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7
Q

Define enthalpy change of neutralisation

A

The enthalpy change that takes place when 1 mole of water is formed from a neutralisation reaction

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8
Q

What is the equation for heat exchanged with surroundings ?

A

q=mc∆T

• q=heat exchanged with surroundings (J)
• m= mass of the substance heated or cooled (g)
.c = specific heat capacity of the substance that is heated (Jg^-1K^-1)
•ΔT- change in temperature (K).

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9
Q

What is the specific heat capacity of water?

A

4.18 J g-1 K-1

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10
Q

What is 1cm³ of water in grams?

A

1 g

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11
Q

Explain what is meant by average bond enthalpy

A

-the energy required to break one mole of gaseous bonds

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12
Q

Define a catalyst

A

-a substance that increases the rate of a reaction without being used up during the process.

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13
Q

How does a catalyst work?

A

It allows a reaction to proceed via a different route with a lower activation energy

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14
Q

What are 2 types of catalysts?

A

Homogenous catalysts and heterogenous catalysts

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15
Q

What is a homogenous catalyst?

A

-If a catalyst for a reaction is in the same phase as the reactants
-for example a liquid catalyst that is mixed with liquid reactants. It could also include a gaseous catalyst with gaseous reactants.
-Enzymes within bodily fluids such as blood or saliva are examples of homogeneous catalysts.

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16
Q

What is a heterogenous catalyst?

A

-if a catalyst for a reaction is in a different phase from the reactants
E.g. a solid catalyst used in liquid reactants
-however phases do not just include the states of solid ,liquid and gas. If all the chemicals are immiscible liquids (they do not mix) the catalyst would still be heterogenous, as it could be in a different liquid layer to the catalyst
-a catalytic converter in a car is an example of a heterogenous catalyst

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17
Q

Explain the economic importance of catalysts

A

Catalysts have great economic importance because they lower the energy demands of processes and lowering the temperatures needed. By doing so, they reduce costs and also help the environment. Less fossil fuel needs to be combusted to generate the required energy, and this also means lower carbon dioxide emissions.

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18
Q

What has to be weighed against the benefits to the environment and improved sustainability of catalysts?

A

Catalysts often contain toxic substances or substances that are hard to obtain. Chemists have to consider this against the potential benefits they have, such as environmental sustainability and cost.

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19
Q

What is a dynamic equilibrium?

A

-a reversible reaction that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products do not change

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20
Q

What is the position of equilibrium?

A

The extent of a reaction

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21
Q

When is an equilibrium established?

A

-once the concentrations of the reactants and products have become stable and the forward and reverse rates are the same

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22
Q

What 3 factors affect the position of equilibrium?

A

-concentration of the reactants or products
-pressure in reactions involving gases
-temperature

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23
Q

What happens if you increase the concentration of a reactant in a reaction at equilibrium?

A

-the position of the equilibrium will move to the right hand side to decrease the concentration of the reactant
-therefore ,forming more products

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24
Q

What happens if you increase the concentration of a product in a reaction that is at equilibrium?

A

-the position of equilibrium will move to the left-hand side to decrease the concentration of the product
-therefore , more reactants will be formed

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25
Q

What happens when you increase the pressure in a reaction involving gases at equilibrium?

A

Increasing the total pressure of the system causes the position of equilibrium to move to the side with fewer gas molecules, as this will decrease the pressure

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26
Q

What happens when you decrease the total pressure in a reaction involving gases at equilibrium?

A

Decreasing the total pressure of the system causes the position of equilibrium to move to the side with the greater number of gas molecules, as this will increase the pressure.

27
Q

What is the effect of increasing the temperature of a reaction at equilibrium?

A

-it will cause the position of the equilibrium to move in the direction that decreases the temperature
-the position of the equilibrium will move in the endothermic (ΔH is positive) direction

28
Q

What is the effect of decreasing the temperature of a reaction that is at equilibrium ?

A

-causes the position of the equilibrium to move in a way to increase the temperature
-the position of the equilibrium will move in the exothermic (ΔH is negative direction)

29
Q

What is the effect of a catalyst on equilibrium?

A

-a catalyst does not change the position of equilibrium as it increases the rate of both the forward and reverse reactions in an equilibrium by the same amount
-however a catalyst will increase the rate at which equilibrium is established

30
Q

What do you say if the concentration of the products is much higher than the reactants?

A

-you say the equilibrium lies to the right / favours the products

31
Q

What do you say if the concentration of the reactants is much higher than that of the products?

A

-you say the equilibrium lies to the left / favours the reactants

32
Q

What is the equation for Kc?

A

Kc=[products]
———————
[reactants]

33
Q

What does a Kc value of 1 mean?

A

-would indicate that the position of equilibrium is halfway between reactants and products
-so the ratio of the product concentration to reactant concentration was 1:1

34
Q

What does it mean if the Kc value is greater than 1?

A

-the reaction favours the products (lies to the right)
-the products on the right-hand side predominate at equilibrium

35
Q

What does it mean when the Kc value is smaller than 1?

A

-reaction favours the reactants (lies to the left)
-the reactants on the left-hand side predominate at equilibrium

36
Q

What is the system?

A

-the actual chemical reaction -> that is the atoms and bonds involved

37
Q

What is the surroundings ?

A

Everything except the atoms and bonds involved in the reaction

38
Q

What is the equation for rate of reaction?

A

Rate= change in concentration
——————————————-
Time

Units=moldm^-3s^-1

39
Q

What are some factors that affect the rate of a chemical reaction?

A

-temperature
-pressure
-concentration
-surface area
-adding a catalyst

40
Q

What is collision theory?

A

For two molecules to react, they must first collide. This collision must have sufficient energy to overcome the activation energy of the reaction, and the collision must have the correct orientation

41
Q

What is the effect of concentration on reaction rate?

A

• Increased concentration means more molecules in the same volume.
• The molecules will be closer together and so there is a greater chance of them colliding with sufficient energy to overcome the activation energy.
• Collisions will be more frequent - more collisions will occur in a certain length of time.
Therefore, rate increases with increased concentration.

42
Q

What is the effect of pressure on reaction rate?

A

.When the pressure of a gas is increased,the molecules are pushed closer together
• The same number of molecules occupies a smaller volume
• More collisions are likely to occur with sufficient energy ot overcome the activation energy.
Therefore, rate increases with increased pressure (for gaseous reactions only).

43
Q

How do you calculate the rate of reaction from a graph?

A

Use the gradient

Gradient= change on y-axis
——————————-
Change on x-axis

Which is also gradient=rise
————-
Run

44
Q

What are some techniques to get data for reaction rates?

A

• collecting gas in a gas syringe during a reaction
• monitoring a reaction’s mass during a reaction
•recording the time it takes for a particular change to occur, such as a precipitate forming
• using a colorimeter to monitor changes in concentrations of coloured reactants or products.

45
Q

What is specific heat capacity?

A

-energy required to raise the temperature of 1g of a substance by 1K

46
Q

What is the method for the coffee cup calorimeter experiment?

A

The reaction vessel can be the insulated coffee cup held in a beaker. You could use a polystyrene lid with a hole for the thermometer to reduce heat lost to the surroundings.
• Add a measured mass of the first liquid reactant. Take the temperature
every minute until it is stable.This usually takes around 4 minutes.
.At 5 minutes, add the second reactant. Do not take or record the temperature for the fifth minute.
• Monitor the temperature of the reaction mixture every minute for a further 5 minutes.
• Plot a graph to infer the maximum temperature change generated by the reaction

47
Q

What is the copper calorimeter method?

A

When a fuel is combusted, the heat energy can be used to increase the temperature of a known mass of water.
•Measure the starting mass of the fuel.
• Add a known mass of water to a copper calorimeter.
•Mount the copper calorimeter over the fuel and take the starting temperature of the water.
•Combust the fuel for a few minutes and take the final temperature of the water.
• Take the mass of the unused fuel and calculate the mass of the fuel burnt.

48
Q

How is a bomb calorimeter the most accurate calorimeter?

A

-it is a sophisticated piece of equipment that minimises heat loss at much as possible
-it uses pure oxygen to ensure complete combustion in achieved

49
Q

What is the chemical equation for the haber process?

A

N2(g) +3H2(g) ⇌2NH3(g)

50
Q

What is the enthalpy change for the haber process?

A

-92 kJmol^-1

51
Q

Where are the raw materials for the haber process obtained from?

A

• Nitrogen is obtained from the air by fractional distillation.
• Hydrogen is prepared by reacting together methane (from natural gas) and water.

52
Q

What are the optimum conditions for the haber process?

A

-a low temperature-this would produce a high equilibrium yield.However the rate of reaction would be slow, as few of the reactant molecules would have enough energy to overcome the required activation energy

-a high pressure-would increase the yield and the reaction rate. However large amounts of energy are required this would add to the cost. As well a high pressure could potentially be dangerous if their are any failures in the system

53
Q

What is the compromise between yield and rate in the haber process?

A

• Temperature- this must be high enough to allow the reaction to proceed at a realistic rate, whilst still producing an acceptable equilibrium yield. A temperature of 400-500C° is typically used.
• Pressure - a high pressure muts be used, but it must not be so high that the workforce is put in danger or the environment threatened. A pressure of 200 atmospheres is typically used.
• Catalyst - an iron catalyst is added to speed up the rate of the reaction, allowing the equilibrium to be established faster and lower temperatures to be used. Less energy is needed to generate heat, reducing costs.

54
Q

What are the conditions for the haber process?

A

Temperature- 400-500 °C
Pressure-200 atmospheres
An iron catalyst is used

55
Q

What does le Chatelier’s principle say?

A

-that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

56
Q

Draw out an enthalpy profile diagram for exothermic reactions?

A
57
Q

Draw out an enthalpy change diagram for endothermic reactions

A
58
Q

What is activation energy?

A

-the minimum amount of energy required to break bonds

59
Q

What is the equation used to calculate enthalpy changes from average bond enthalpies?

A

Enthalpy change = (bond enthalpies of products) - (bond enthalpies of reactants)

60
Q

What does the Boltzmann distribution graph look like ?

A
61
Q

What is the effect of temperature on the Boltzmann distribution graph?

A
62
Q

What is the effect of catalysts on the Boltzmann distribution graph?

A
63
Q

Why do Br2 and I2 not exist in the gaseous state under standard conditions?
(1mark)

A

—because energy is needed to break London Forces between molecules

64
Q

Why do Br2 and I2 not exist in the gaseous state under standard conditions?
(1mark)

A

—because energy is needed to break London Forces between molecules

Chemistry Alevel OCR (9 decks)

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