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OCR Chemistry AS Level > 3.2 - Physical chemistry > Flashcards

3.2 - Physical chemistry Flashcards

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1
Q

What is Chemical Energy?

A

The potential energy in BONDS.

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2
Q

What is Enthalpy?

A

The THERMAL ENERGY that is stored in a chemical system.

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3
Q

What is a System?

What are the “Surroundings”?

A

The actual CHEMICAL REACTION (the atoms and bonds involved)

The surroundings are everything else.

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4
Q

What does the Law of the Conservation of Energy state?

A

No energy is created or destroyed. Energy only changes from one form to another.

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5
Q

What is the Enthalpy Change (▲H)?

A

The heat exchange with the surroundings during a chemical reaction, at constant pressure.

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6
Q

What is the formula for Enthalpy Change, ▲H?

A

▲H = H(products) - H(reactancts)

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7
Q

In terms of Enthalpy Change (▲H), define what an exothermic reaction is.

A

An exothermic reaction is one where the enthalpy of the products is SMALLER than the enthalpy of the reactants (the ▲H is therefore NEGATIVE)
There’s a heat LOSS.

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8
Q

In terms of Enthalpy Change (▲H), define what an endothermic reaction is.

A

An endothermic reaction is one where the enthalpy of the products is LARGER than the enthalpy of the reactants (the ▲H is therefore POSITIVE)
There’s a heat GAIN.

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9
Q

What is the Activation Energy, E(a)?

A

The MINIMUM energy required to INITIATE a reaction by breaking bonds in the reactants.

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10
Q

What are the Standard Conditions considered to be (2)?

A 
100 kPa ( 100 000 Pa / 0.986 atm)
273 K (from C to K, add 273)
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11
Q

What is the Enthalpy Change of REACTION (▲ r H ⦵)?

A 
The energy change associated with a given reaction where:
▲: change in
r: stands for "reaction"
H: enthalpy or energy in a system
⦵: means "under standard conditions"
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12
Q

What is the Enthalpy Change of FORMATION ()?

A

The energy change when 1 MOLE of a compound is formed from its CONSTITUTENT ELEMENTS under standard conditions.

e.g.: H2 (g) + (1/2) O2 (g) → H2O (l)
(▲ f H ⦵ = -286 kJ mol(-1))

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13
Q

The enthalpy change of formation for an ELEMENT in its standard state is…

A

0 kJ mol -1.

This is because according to the definition, there is NO CHANGE so NO ENERGY is released or taken in.

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14
Q

What is the Enthalpy Change of Combustion (▲ c H ⦵)?

A

The energy change that takes place when 1 MOLE of a substance is COMPLETELY combusted.

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15
Q

What is the Enthalpy Change of Neutralisation (▲ neut H ⦵)?

A

The energy change when 1 MOLE of WATER is formed in neutralisation.

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16
Q

What is 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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17
Q

What is the 3-Step Process in chemical reactions (breaking and making bonds)?

A
  1. REACTANT bonds are BROKEN. This process takes IN energy and so is ENDOthermic.
  2. Atoms REARRANGE to form PRODUCTS.
  3. PRODUCT bonds are FORMED. This RELEASES energy and so is EXOthermic.
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18
Q

How can you predict the enthalpy change of a reaction, using the average bond enthalpy data of the reactants and products?

A

▲ H = Σ(bond enthalpies of reactants) - Σ(bond enthalpies of products)

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19
Q

What are the steps to calculate the Enthalpy Change?

A
  1. Draw molecules to show ALL bonds (displayed formula)
  2. Calculate energy PUT IN to BREAK bonds
  3. Calculate energy GIVEN OUT to MAKE bonds.
  4. Calculate the Enthalpy Change and use the appropiate sign: if “IN > OUT” then its ENDO (+ve)
    if “OUT > IN” then its EXO (-ve)
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20
Q

What is Calorimetry?

A

A mathematical relationship to calculate Enthalpy Change from experimental quantitative data.

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21
Q

What is the formula for Calorimetry? Define each variable.

A

Q = mc ▲ T

Q = heat exchanged with the surroundings (J)
m = mass of substance heated or cooled (g)
c = SPECIFIC HEAT CAPACITY
▲ T = change in temperature (K)

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22
Q

What is the “Specific Heat Capacity”?

A

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

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23
Q

What are the steps to conduct a “Coffee Cup Calorimeter” experiment (4)?

A
  1. Add the measured mass of the first LIQUID reactant. Wait for the temperature to stabilise by checking each minute (usually 4 minutes)
  2. At 5 minutes, add the SECOND reactant. DO NOT take or record the temperature for the fifth minute.
  3. Monitor the temperature of the reaction mixture EVERY MINUTE for a furher 5 minutes.
  4. Plot a graph to infer the maximum temperature change generated by the reaction.
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24
Q

What are the steps to conduct a “Copper Calorimetry” experiment (5) ?

A
  1. Measure the starting mass of the FUEL.
  2. Add a known mass of water to a copper calorimeter.
  3. Mount the copper calorimeter OVER the fuel and take the starting temperature of the water.
  4. Combust the fuel for a few minutes and take the final temperature of the water.
  5. Take the mass of the unused fuel and calculate the mass of the fuel burnt.
25
Q

What is a Bomb Calorimeter and why is it used over the other methods of Calorimetry?

A
  • It’s a sophisticated piece of equipment that MINIMISES HEAT LOSS as much as possible.
  • It uses PURE oxygen, to ensure COMPLETE combustion takes place.
26
Q

How can we measure Enthalpy Change INDIRECTLY?

A

Hess’ Law

27
Q

What is Hess’ Law?

A

Hess’ Law states that the enthalpy change in a chemical reaction is INDEPENDENT of the route it takes.

e.g. if A + B → C, it’s the same as A + B → D then D→C

28
Q

What is an Enthalpy Cycle?

A

A pictorial representation showing alternative routes between reactants and products.

29
Q

What is the formula of the Rate of Reaction?

State the units.

A

Rate = Change in Concentration / Time

Units: mol dm^-3 s^-1

30
Q

What are the 5 MAIN FACTORS affecting the rate of a chemical reaction?

A
  1. TEMPERATURE
  2. PRESSURE (gaseous reactants only)
  3. CONCENTRATION
  4. SURFACE AREA
  5. CATALYSTS
31
Q

Explain how increasing the temperature increases the rate of a chemical reaction.

A
  1. INCREASING TEMPERATURE = INCREASES K.E. OF THE MOLECULES
  2. That means the MOLECULES MOVE FASTER.
  3. So that means the molecules COLLIDE MORE OFTEN WITH ENERGY in excess of the ACTIVATION ENERGY
32
Q

Explain how increasing the pressure increases the rate of a chemical reaction.

A
  1. At HIGHER PRESSURES, the molecules are CLOSER TOGETHER.
  2. The same number of molecules are in a SMALLER VOLUME
  3. That means that they’re MORE LIKELY to collide with energy in excess of the ACTIVATION ENERGY
33
Q

Explain how increasing the concentration increases the rate of a chemical reaction.

A
  1. MORE molecules are in the SAME VOLUME.
  2. There’s a GREATER CHANCE of COLLISIONS.
  3. So there’s MORE FREQUENT COLLISIONS.
34
Q

Explain how increasing the Surface Area increases the rate of a chemical reaction.

A

There’s MORE AREA for the reaction to take place.

35
Q

Explain how using a catalyst increases the rate of a chemical reaction.

A

It provides an ALTERNATE PATH for the reaction to proceed through.
This path has a LOWER ACTIVATION ENERGY, so less energy is required to take place, so the reaction initiates faster.

36
Q

A graph of a Factor of Rates of Reaction (e.g. concentration) is plotted against time.

What is the GRADIENT of this graph.

A

The RATE OF REACTION.

37
Q

What is a CATALYST?

A

A substance that INCREASES THE RATE OF REACTION, WITHOUT being used up in the process.

38
Q

How does a catalyst work without being used up in the process?

A
  1. The catalyst may react to form an INTERMEDIATE.

2. The catalyst is later REGENERATED (to not undergo any permanent change).

39
Q

What are the 2 types of catalysts?

A
  1. HOMOGENEOUS

2. HETEROGENOUS

40
Q

What is a HOMOGENEOUS catalyst?

A

A catalyst that is in the SAME PHASE as the reactants.

e.g. a liquid catalyst mixed with liquid reactants.

41
Q

What is a HETEROGENEOUS catalyst?

A

A catalyst that is in a DIFFERENT PHASE from the reactants.

e.g. gaseous reactants that is passed over a solid catalyst.

42
Q

How are catalysts beneficial in terms of its economic importance?

A

Catalysts LOWER THE ENERGY DEMANDS.
So this:
- REDUCES THE COSTS
- HELPS the ENVIRONMENT because less energy is being used.

43
Q

Explain how the Haber Process greatly benefits from the catalyst being used.

A

N2 (g) + 3H2 (g) -→ 2NH3 (g)

A lot of energy is required to break the TRIPLE BOND in Nitrogen. This contributes to a HIGH ACTIVATION ENERGY.

But Iron is the reaction’s catalyst and this WEAKENS the Nitrogen Triple Bond and LOWERS the Activation Energy and therefore reduces the costs (both environmental and financial).

44
Q

What is the Boltzmann Distribution?

A
  • shows the DISTRIBUTION of MOLECULAR ENERGIES at a particular temperature, often shown as a graph.
45
Q

Describe the distribution of the energy that the molecules have in a Boltzmann Distribution graph.

A

Some molecules are FAST and have HIGH energy.
Some molecules move SLOWLY and have LOW energy.
The majority of molecules have an AVERAGE energy.

46
Q

What are the 4 key features of the Boltzmann Distribution graph? In 2 of these features, explain how the graph expresses those features.

A
  1. The AREA under the graph = TOTAL NUMBER of molecules
  2. NO MOLECULES have ZERO energy (curve starts from origin)
  3. There is NO MAXIMUM ENERGY for a molecule (curve does not touch energy axis)
  4. Only molecules with an energy GREATER than the Activation Energy (Ea) can react in a reaction.
47
Q

At higher temperatures, how does the Boltzmann distribution curve change?

A

It FLATTENS and SHIFTS TO THE RIGHT.

The peak moves to a higher energy with a lower height.

48
Q

In terms of the Boltzmann Distribution, how does increasing the temperature increase the rate of reaction?

A
  1. As temperature increases, K.E. increases.
  2. A greater proportion of molecules have energies in EXCESS of (Ea)
  3. So the rate of reaction INCREASE.

(looking at a graph of two temperatures, where T(2) is at a higher temperature, the height of the curve is larger than T(1) so more molecules exceed the E(a).)

49
Q

In terms of the Botlzmann Distribution, how does the use of a catalyst increase the rate of a reaction?

A
  1. Catalysts LOWER the Activation Energy.
  2. But they DO NOT CHANGE the distribution of the energy within molecules.
  3. It allows MORE PARTICLES to have energies within EXCESS of E(a).
  4. So there’s more successful collisions so the rate of reaction INCREASES.
50
Q

When something is in equilibrium what does that mean?

A

There is no observable change.

But the system is DYNAMIC (in constant motion).

As fast as the reactants are converted into products, the products are being converted back into reactants.

51
Q

What are the 2 conditions of a chemical system being in dynamic equilibrium?

A
  1. The CONCENTRATIONS of the reactants and the products remain CONSTANT.
  2. The RATE of the FORWARD REACTION is the SAME as the rate of the REVERSE REACTION.
  3. The system is ISOLATED; no materials are being added or taken away and no external conditions are being altered.
52
Q

What is the POSITION of EQUILIBRIUM?

A

The EXTENT of a reaction at equilibrium.

53
Q

What are the 2 key conditions where equilibrium has been estabilished?

A
  1. The concentrations of the reactants and products have become STABLE.
  2. The FORWARD and REVERSE rates are the same.
54
Q

What is Le Chatelier’s Principle?

A
  • a principle used to predict the EFFECT OF CHANGE
  • When a system is in dynamic equilibrium and experiences a change, the position of equilibrium will SHIFT to MINIMISE the change.
55
Q

What are the 3 factors that affect the Position of Equilibrium?

A
  1. The CONCENTRATIONS of the REACTANTS and PRODUCTS
  2. PRESSURE (ONLY for GASES)
  3. TEMPERATURE
56
Q

What is the effect of concentration on equilibrium?

A

a + b ⇌ c + d
- if we increase the concentration of “d”:

a + b ⇌ c + D

The system will try to OPPOSE the change.
It will MOVE TO THE LEFT to try and form more reactants to balance the reaction rates.

57
Q

What is the effect of PRESSURE on equilibrium?

A

(!) ONLY an effect of equilibrium if GASES are present.

2A (g) + B (g) ⇌ 3C (g) + D (g)

Reactants: 3 moles - LOWER pressure
Products: 4 moles - HIGHER pressure

INCREASING the PRESSURE causes the system to move to the side with the LOWER PRESSURE.
So in this case, it will move to the LEFT (towards reactants).

58
Q

What is the effect of TEMPERATURE on equilibrium?

A

-depends on the enthalpy sign:

A + B ⇌ C + D
▲ H = -100 kJ mol^-1
Products: + 100 kJ mol^-1
Reactants: -100 kJ mol^-1

  • The FORWARD and BACKWARD reactions have the SAME MAGNITUDE but DIFFERENT SIGNS.
  • INCREASING the TEMPERATURE causes the system to move to the LOWER temperature (endothermic)
  • So it will move to the REACTANTS side.
59
Q

What is the effect of a catalyst on equilibrium?

A
  • DOES NOT ALTER the position of the equilibrium.

- SPEEDS UP Forward and Backward reactions EQUALLY.

OCR Chemistry AS Level (6 decks)

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