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A Level Chemistry > Energetics > Flashcards

Energetics Flashcards

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1
Q

What is enthalpy change?

A

The heat energy change at constant pressure

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2
Q

What is the enthalpy change in an

a) endothermic
b) exothermic

Reaction?

A

a) positive

b) negative

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3
Q

At what temperature does standard enthalpy change occur at?

What pressure?

A

298K

100kPa

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4
Q

What is standard enthalpy of combustion?

A

The enthalpy change when 1 mole if a substance
Is completely burnt in Oxygen
Under standard conditions, all substances in standard states

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5
Q

What is standard enthalpy of formation?

A

The enthalpy change when 1 mole of a compound
is formed from its elements
Under standard conditions, all substances in standard states

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6
Q

What is the enthalpy of formation for an element

A

Zero by definition

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7
Q

State Hess’ Law

A

The enthalpy change in a reaction is independent if the route taken, and depends only in the initial and final states

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8
Q

What does
q=mc🔺t
Stand for?

A

q= heat energy (J)
m= mass of water (g; density of water is 1g/cm3)
c=specific heat capacity of water (4.18JK-1g-1)
🔺t= change in temperature

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9
Q

The q in 🔺H=q/n is in what unit?

A

kJ

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10
Q

What is the method for Calorimetry?

A
  • Mass of spirit burner and fuel with cap on is recorded
  • a known vol. of water is placed in a beaker and temp is recorded
  • light fuel to heat water - ensure it is close to beaker and protected by heat-proof guards to minimise heat loss. Stir water.
  • when temp has increased aptly, extinguish flame by placing lid on flame
  • keep monitoring temp and record highest reached
  • re-weigh spirit burner with lid on and calculate the mass of fuel burnt
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11
Q

What are the two reasons the value obtained maybe be less than that in the data book?

A

Heat loss to surroundings

Incomplete combustion

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12
Q

Describe the method for calorimetry using two solutions

A

Pour aqueous reactant into polystyrene cup and record temp every minute for 3 mins to ensure solution is at room temperature
On 4th minute, add the other reactant and stir. Do not record temp on 4th min
Record temperature every minute after that up to 10 mins.

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13
Q

Why do you stir the solution in calorimetry?

A

To ensure the reactants mix thoroughly fit complete reaction

To ensure temperature is even throughout solution

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14
Q

Why would values be slightly inaccurate in laboratory calorimetry with two solutions?

A

Heat loss to surroundings

Incomplete reaction

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15
Q

Define ‘mean bond enthalpy’

A

Enthalpy required to break 1 mole of covalent bonds averaged over a range of compounds

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16
Q

Why is mean bond enthalpy not 100% accurate?

A

The values are average values from a range of compounds

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17
Q

What is the equation for the enthalpy change of a reaction (bond enthalpy)?

A

🔺H= (bonds broken) - (bonds formed)

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18
Q

Define activation energy

A

The minimum amount of energy required for the reaction to occur

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19
Q

Why increase the frequency of collisions

A

Increasing frequency of collisions increases rate of reaction but the percentage of successful collisions will remain the stay

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20
Q

Define rate of reaction

A

Change in concentration
————————————-
Time

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21
Q

How can you increase frequency of collisions

A

Increasing concentration of reagents
Increasing surface area
Increasing pressure
Increasing temperature

22
Q

How does increasing concentration increase rate

A

It increasing number of particles in a given volume and so increases frequency of collisions, therefore there are more successful collisions in a given time

23
Q

When is the rate at its maximum

A

At the start of the reaction as Reagents are used up and their concentration falls

24
Q

Two facts about the graph which shows how concentration varies over time

A

Gradient is negative at the start of reaction

Gradient falls to zero at the completion of the reaction

25
Q

How would a graph for a reaction with 2x moles of reactants and higher concentration compare to a standard reaction

A

It would be twice the height, with a steeper gradient due to the increased rate of reaction

26
Q

How would the graph of a reaction with a lower concentration but same moles compare to that of a standard reaction

A

Same height, with a less steep gradient due to the decreased rate of reaction

27
Q

How would a graph of a reaction with half the moles, carried out at a higher temperature with a catalyst compare to a standard reaction?

A

It would be half the height, with a steeper gradient due to its increased rate

28
Q

How is surface area increased for a solid?

A

Break it into smaller pieces

29
Q

How does increasing the surface area increase rate?

A

More collisions can occur therefore there are more successful collisions in a given time

30
Q

How can pressure be increased without changing the temperature

A

Using a smaller reaction vessel

31
Q

How does volume affect pressure

A

A larger volume creates a lower pressure

A smaller volume creates a higher pressure

32
Q

Other than decreasing volume, how else can pressure be increased?

A

Increasing the number of molecules in the same volume reaction vessel

33
Q

What does an increase in pressure mean?

A

That there are more particles in. A given volume, therefore there are more successful collisions in a given time

34
Q

What factor has the biggest l effect on rate

Why

A

Temperature

It increases number of collisions and percentage of successful collisions in a given time

35
Q

How does increasing temperature increase percentage of successful collisions

A

Particles have more energy at a high temperature so move faster and so move collisions occur and there are more successful collisions in a given time

36
Q

Why do particles have different energies

A

The collisions are random and so energy is transferred randomly

37
Q

What does the Maxwell-Boltzmann curve show

A

The distribution of the energies of particles

Number of molecules(y) vs. Energy (x)

38
Q

What is the peak of the curve?

A

Most probable energy

39
Q

What does the area under the Maxwell-Boltzmann curve show

A

Total number of particles in the system

40
Q

What changes and what stays the same in a Maxwell-Boltzmann curve when the temperature is increased?

A

Total area underneath stays the same - total number of particles stays the same
At a higher temperature the curve shifts to the right and the peak is lower
MANY more particles have energy>Ea

41
Q

Why can a small temperature increase result in a large increase in rate

A

Many more particles have energy>Ea

42
Q

What happens to a Maxwell-Boltzmann curve if the temperature is decreased

A

Curve shifts left and peak is higher

Much fewer particles have energy>Ea

43
Q

What kind of curve should be drawn on a rate/time graph?

A

Exponential

44
Q

Define catalyst

A

A substance that speeds up the rate of reaction

But is not used up

45
Q

How does a catalyst work?

A

By providing an alternative route which has a lower Ea than the uncatalysed route

46
Q

How does a catalyst affect enthalpy change

A

It has no effect so 🔺H is equal for both reactions

47
Q

How does a catalyst increase rate?

A

More molecules are able to react as Ea is lowered

48
Q

If a question asks about the behaviour of particles in an equilibrium what must you reference

A

Kinetics, rate, and the time taken to reach equilibrium

49
Q

If a question asks about a catalyst what must always be included in the answer

A

How the number of successful collisions in a given time is affected and what this means for the rate

50
Q

What does area under the curve represent

A

Total number of molecules IN SAMPLE

51
Q

Explain the process that causes some molecules in this sample to have very low energy

A

Collisions cause some molecules to lose energy

52
Q

If asked to draw a Maxwell Boltzman curve what measurement must you take?

A

Must level off less than 5mm from x axis

A Level Chemistry (11 decks)

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