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Chemistry A Level > 3.2 Physical chemistry > Flashcards

3.2 Physical chemistry Flashcards

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1
Q

What is enthalpy?

A

Enthalpy is the total energy stored in a chemical system.

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2
Q

What is the enthalpy change?

A

The enthalpy change of a reaction is the heat given out or taken in as the reaction proceeds

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3
Q

What is an exothermic reaction?

A

A reaction which gives out energy

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4
Q

What is an endothermic reaction?

A

A reaction which takes in energy

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5
Q

Give three examples of exothermic reactions

A
  • Acids + alkalis
  • Combustion
  • Respiration
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6
Q

Give three examples of endothermic reactions

A
  • Photosynthesis
  • Electrolysis
  • Melting
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7
Q

Why do exothermic reactions increase temperature?

A

Bonds are formed releasing energy

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8
Q

Why do endothermic reactions decrease temperature?

A

Bonds are broken using energy

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9
Q

What is the activation energy?

A

The amount of energy needed for effective collisions between reactant particles, and thus breaking bonds

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10
Q

What is the bond energy?

A

The specific amount of energy needed to break a covalent bond

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11
Q

Why are these bond energies averages?

A

The actual bond energy depends on the rest of the chemical the bond is in

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12
Q

What is the equation for enthalpy change?

A

Enthalpy change = Enthalpy in (break bonds) - Enthalpy out (make bonds)

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13
Q

What is the enthalpy change of neutralisation?

A

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

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14
Q

What is the enthalpy change of formation?

A

The enthalpy change when 1 mole of a compound is formed from its elements in their standard state

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15
Q

What is the enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance reacts completely with oxygen

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16
Q

What three things do you need to know when calculating heat change?

A
  • how much substance there is
  • what substance it is
  • the temperature change of the substance
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17
Q

What is the equation for heat change (q)?

A

Heat change (q)= mass (m)x specific heat capacity (c) x temperature change (ΔH)

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18
Q

What is the equation for enthalpy change (ΔH)?

A

q/moles in reaction

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19
Q

What is calorimetry?

A

Quantitative study of chemical energy in a chemical reaction

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20
Q

What are the two ways of carrying out calorimetry?

A
  • Coffee cup calorimeter

- Spirit Burner

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21
Q

How do you carry out a coffee cup calorimeter?

A
  1. Add a measured mass of the first reactant to styrofoam cup
  2. Take temperature until stable using thermometer
  3. Add second reactant
  4. Monitor temperature
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22
Q

How do you carry out a spirit burner calorimeter?

A
  1. Weigh the spirit burner containing the test liquid
  2. Add a known volume of water to the copper can
  3. Using the thermometer, measure the temperature of the water
  4. Light the burner
  5. Continuously stir the water
  6. After a few minutes, extinguish the flame of the spirit burner
  7. Immediately reweigh burner and calculate the mass of the fuel burned
  8. Measure the final temperature of the water
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23
Q

What are some errors that can occur during an enthalpy change of combustion reaction that results in a result different from the databook?

A
  • Some energy produced is not transferred to the water but e.g. to the air
  • Incomplete combustion may occur
  • The conditions are not standard
  • Some of the alcohol or water may evaporate
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24
Q

What do bond enthalpies tell you?

A

How much energy is needed to break each different bond

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25
Q

How does a typical chemical reaction occur?

A
  1. Reactant bonds are broken. This process takes in energy and is endothermic
  2. Atoms rearrange to form products
  3. Product bonds are formed. This releases energy and is an exothermic change
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26
Q

What happens in an endothermic and exothermic reaction?

A

In an endothermic reaction, more energy is needed to break bonds than is released when making bonds so overall the reaction takes in energy

In an exothermic reaction, more energy is released when bonds are made than is taken in when bonds are broke so overall the reaction releases energy

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27
Q

What is the definition for the average bond enthalpy?

A

The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission.

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28
Q

How is the enthalpy change for a reaction calculated from average bond enthalpy?

A

sum of bond enthalpies of reactants- sum of bond enthalpies of products

29
Q

What does Hess’ Law state?

A

The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place

30
Q

What is an enthalpy cycle?

A

An enthalpy cycle shows alternative routes between reactants and products

31
Q

How do you work out enthalpy change of combustion and formation?

A

Combustion: Products - reactants
Formation: Reactants- products

32
Q

What are the intermediates for enthalpy change of combustion?

A

CO2 + H20

33
Q

What are the intermediates for enthalpy change of formation?

A

The elements involved in the reaction

34
Q

How does the rate change during the reaction?

A
  • Concentrations of reactants are highest at the start of the reaction when the rate is at its fastest
  • The rate slows down as reactant concentrations decrease
  • When any reactant has a concentration of zero the reaction stops and the rate is zero
35
Q

What are the factors affecting the rate of a chemical reaction? (5)

A
  • Temperature
  • Pressure, for gaseous reactants only
  • Concentration
  • Surface area
  • Catalyst
36
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

37
Q

How does concentration affect the rate of reaction?

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 (successful collisions)
  • Collisions will be more frequent so more collisions will occur in a certain amount of time
  • Rate of reaction increases
38
Q

How does pressure affect the rate of reaction?

A
  • When the pressure of a gas is increased the molecules are pushed closer together
  • The same number of molecules occupies a smaller volume
  • For a gaseous reaction, increasing the pressure is the same as increasing the concentration
  • More collisions are likely to occur with sufficient energy
  • Therefore rate of reaction increases
39
Q

How can rate of reaction be calculated by monitoring changes in physical quantities?

A
  • the concentration of a reactant or product (e.g. using a titration)
  • gas volume of products (e.g. using a gas syringe to collect gas formed)
  • mass of substances formed or decreasing mass of reactants (monitoring a reaction on a balance)
40
Q

How can you work out the rate of reaction from a graph?

A

Work out the gradient by doing change on y-axis / change on x axis (do this by drawing a tangent)

41
Q

What is a catalyst?

A

A catalyst is a substance that increases the rate of reaction without being used up in the process

42
Q

How does a catalyst work?

A

A catalyst works by increasing the rate of a chemical reaction without being used up:

  • A catalyst may react to form an intermediate
  • The catalyst is later regenerated, so it does not undergo any permanent change
43
Q

How does a catalyst increase rate of reaction?

A

A catalyst lowers the activation energy of the reaction by providing an alternative route for the reaction to follow. This alternative route has a lower energy

44
Q

What are homogeneous catalysts?

A

Homogeneous catalysts are in the same phase as the reactants, for example, a liquid catalyst being mixed with liquid reactants

45
Q

What are heterogeneous catalysts?

A

Heterogeneous catalysts are in a different phase from the reactants. An example of this would be a solid catalyst used in liquid reactants

46
Q

Explain the economic importance of catalysts

A
  • Catalysts help to lower the energy demands of processes so they reduce costs and help the environment
  • Less fossil fuels need to be burnt to generate the required energy, so this means lower CO2 emissions
  • Improve air quality (catalytic converters)
  • Whole branches of chemistry are based on the development of successful catalytic processes
47
Q

What is the Boltzmann distribution?

A

The Boltzmann distribution is a graph which shows the distribution of molecular energies in a gas at constant temperature

48
Q

What are the important features of the Boltzmann distribution?

A
  • The area under the curve is equal to the total number of molecules in the sample
  • There are no molecules in the system with zero energy
  • There is no maximum energy for a molecule, the curve does not touch the energy axis
  • Only the molecules with an energy greater than the activation energy, Ea are able to react
49
Q

What happens to the Boltzmann distribution as temperature increases?

A
  • At higher temperatures, the kinetic energy of all the molecules increases.
  • The Boltzmann distribution flattens and shifts to the right.
  • The number of molecules in the system does not change so the area under the curve remains the same
50
Q

How does temperature affect rate of reaction?

A
  • As the temperature of the reaction is increased, the rate of reaction increases.
  • This is because more collisions are taking place in a certain length of time because the molecules are moving faster and have more kinetic energy
  • A higher proportion of molecules have the energy that is greater than the activation energy- more successful collisions
51
Q

What happens to the Boltzmann distribution when a catalyst is added?

A

The activation energy is lowered so a larger proportion is shaded. More particles will automatically be above the activation energy barrier and have enough energy to react successfully

52
Q

What is a reversible reaction?

A

A reaction that can take place in either the forward or the reverse direction

53
Q

What is dynamic equilibrium?

A

When a system is in dynamic equilibrium, it is in constant motion. As fast as the reactants are being converted into products, the products are being converted back into the reactants.

54
Q

When is a chemical system in dynamic equilibrium?

A
  • the concentrations of the reactants and the products remain constant
  • the rate of the forward reaction is the same as the rate of the reverse reaction
55
Q

What can the position of equilibrium be altered by?

A
  • Concentrations of the reactants and products
  • Pressure in reactions involving gases
  • Temperature
56
Q

What is Le Chatelier’s principle?

A

This principle states that when a system is in dynamic equilibrium and is subjected to a change, the position of equilibrium will shift to minimise that change.

57
Q

How does concentration affect the position of equilibrium?

A
  • Increasing the concentration of a reactant will cause the equilibrium to shift in the direction that decreases this reactant’s concentration (i.e. the right hand side)
  • Increasing the concentration of the product causes the equilibrium to shift in the direction that decreases the product’s concentration (i.e. the left)
58
Q

How does pressure affect the position of equilibrium?

A
  • Increasing the total pressure causes the position of equilibrium to move to the side with fewer gas molecules
  • Decreasing the total pressure of the system causes the position of equilibrium to shift to the side with greater number of gas molecules
59
Q

How does temperature affect the position of equilibrium?

A
  • Increasing the temperature causes the position of equilibrium to move in the direction that decreases it by taking IN heat i.e. moving in the endothermic direction
  • Decreasing the temperature causes the position of equilibrium to move in the direction that increases it by RELEASING heat i.e. moving in the exothermic direction
60
Q

Does a catalyst have an effect on the position of equilibrium?

A

A catalyst does not alter the position of equilibrium because it speeds up the rate of the forward and reverse reactions equally

61
Q

What is the Haber process?

A

The Haber process is when nitrogen and hydrogen react to form ammonia

62
Q

What are the ideal conditions for the Haber process?

A
  • High pressure because the forward reaction produces fewer gas molecules
  • Low temperature because the forward reaction is exothermic
63
Q

What are the drawbacks for the theoretical conditions for the Haber process?

A
  • Low temperature would mean very low rate of reaction

- High pressure is very hard to maintain cost wise and there are also serious safety implications

64
Q

What are the conditions in the modern ammonia plant?

A
  • A temperature of about 400-500 degrees which is high enough for the reaction to proceed at an appropriate rate but also producing an acceptable yield
  • A pressure of 200atm because it is high but not high enough to endanger the workers
  • Iron catalyst to speed up the rate of reaction, allowing the equilibrium to be established faster
65
Q

What is Kc?

A

Kc is the equilibrium constant and gives a measure of where the equilibrium lies

66
Q

How is Kc calculated?

A

Kc= products/reactants

67
Q

Wha does a Kc value of 1 indicate?

A

The position of Kc is halfway between reactants and products

68
Q

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

A
  • The reaction favours the products

- The products on the right hand side predominate at equilibrium

69
Q

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

A
  • The reaction favours the reactants

- The reactants on the left hand side predominate at equilibrium

Chemistry A Level (6 decks)

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