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Chemistry > 1.4 Energetics > Flashcards

1.4 Energetics Flashcards

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1
Q

What is enthalpy change?

A

Enthalpy change (∆H) is the heat energy transferred in a reaction at constant pressure. The units of enthalpy change are kJ mol-1

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2
Q

What are the standard conditions?

A

100kPa pressure and a stated temperature.

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3
Q

Is enthalpy change (∆H) negative or positive for an exothermic reaction? Why?

A

Negative.

In an exothermic change, energy is transferred from the system (chemicals) to the surroundings.
The products have less energy than the reactants.

(see page 36 in the revision guide)

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4
Q

Is enthalpy change (∆H) negative or positive for an endothermic reaction? Why?

A

Positive.

In an endothermic change, energy is transferred from the surroundings to the system (chemicals).
They require an input of heat energy
e.g. thermal decomposition of calcium chloride.
The products have more energy than the reactants.

(see page 36 in the revision guide)

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5
Q

What is bond enthalpy?

A

Bond enthalpy is the energy required to break bonds.

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6
Q

What is the mean bond enthalpy?

A

The mean bond enthalpy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.

These values are positive because energy is required to break a bond.

The definition only applies when the substances start and end in the gaseous state.

(see page 36 in revision guide to see how to calculate this)

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7
Q

How do you calculate the enthalpy change of a reaction?

A

Enthalpy change of reaction =
Total energy absorbed - Total energy released

(see page 37 in the revision guide for example)

q=mc/\T

What are the units?

(see page 39 in the revision guide)

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8
Q

What is the standard enthalpy change of formation (∆fH)?

A

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

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9
Q

What is standard enthalpy of combustion (∆cH) ?

A

The enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions.

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10
Q

How can you find enthalpy changes using calorimetry? And how in a solution?

A

(see page 38)

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11
Q

What does Hess’s Law state?

A

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

(see page 40 and 41 in the revision guide for example)

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12
Q

What are 2 common oxidation exothermic reactions?

A
  • Combustion of fuels

- Oxidation of carbohydrates e.g. glucose in respiration

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13
Q

Draw a standard exothermic reaction graph, the measurement of energy output against the progress of the reaction.

A

(see page 1 in the chemrevise revision guide)

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14
Q

Draw a standard endothermic reaction graph, the measurement of energy output against the progress of the reaction.

A

(see page 1 in the chemrevise revision guide)

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15
Q

What are standard conditions for an enthalpy change reaction?

A
  • temp: 298K (room temperature 25º)
  • pressure: 100kPa
  • Solutions at 1moldm-3
  • All reactants and products being in their standard states.
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16
Q

Give the equation for the enthalpy of formation of MgCl2

A

Mg(s) + Cl2(g) -> MgCl2(s)

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17
Q

Give the equation for the enthalpy of formation of Fe2O3

A

2Fe(s) + 1.5O2(g) -> Fe2O3(s)

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18
Q

What is the enthalpy of formation for any element?

A

0 kJmol-1

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19
Q

Give the equation for the enthalpy of combustion of CH4

A

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

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20
Q

What are the products of incomplete combustion?

A
  • soot (carbon)
  • carbon monoxide (CO)
  • water (H2O)

It will be less exothermic than complete combustion.

21
Q

Draw the symbol that represents standard conditions.

A

(see page 1 in the chemrevise revision guide)

22
Q

Give the equation to calculate the energy change of a reaction.

A

energy change = mass of solution x heat capacity x temperature change

Q = mc∆T

Q = (J)
m = (g)
c = (Jg-1K-1)
∆T = (K)
23
Q

Give the method of the experiment for measuring the enthalpy change for a reaction.

A

(see page 2 in the chemrevise revision guide)

24
Q

What happens to the results of the experiment if the reaction is slow in the calorimeter experiment? How do we solve this?

A

If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction.

To counteract this we take readings at regular time intervals and extrapolate the temperature curve/liner back to the time the reactants were added together.

We also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature. If the two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used.

25
Q

What are 5 errors with the calorimetry experiment?

A
  1. Energy transfer from surroundings (usually loss)
  2. Approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.
  3. Neglecting the specific heat capacity of the calorimeter - we ignore any energy absorbed by the apparatus.
  4. Reaction or dissolving may be incomplete or slow.
  5. Density of solution is taken to be the same as water.
26
Q

Give the equation for calculating ∆H.

A

∆H = q/mol

∆H = (kJmol-1)

27
Q

Answer example 1 on page 3 in the chemrevise revision guide.

A

correct?

28
Q

Answer example 2 on page 3 in the chemrevise revision guide.

A

correct?

29
Q

What is the heat capacity of water?

A

4.18Jg-1K-1

30
Q

Answer example 3 on page 4 in the chemrevise revision guide.

A

correct?

31
Q

What are 6 errors in the experiment for measuring enthalpies of combustion using calorimetry?

A
  1. Energy losses from calorimeter.
  2. Incomplete combustion of fuel.
  3. Incomplete transfer of energy.
  4. Evaporation of fuel after weighing/
  5. Heat capacity of calorimeter not included.
  6. Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment.
32
Q

What is the equation used to calculate the ∆H of a formation reaction using a Hess diagram?

A

∆H reaction = ∑∆fH products - ∑∆fH reactants

33
Q

Draw a standard Hess cycle for a formation reaction.

A

(see page 6 in the chemrevise revision guide)

34
Q

Answer example 4 on page 6 in the chemrevise revision guide.

A

correct?

35
Q

Answer example 5 on page 6 in the chemrevise revision guide.

A

correct?

36
Q

What is the equation used to calculate the ∆H of a combustion reaction using a Hess diagram?

A

∆H reaction = ∑∆cH products - ∑∆cH reactants

37
Q

Draw a standard Hess cycle for a combustion reaction.

A

(see page 6 in the chemrevise revision guide)

38
Q

Answer example 6 on page 6 in the chemrevise revision guide.

A

correct?

39
Q

Answer example 7 on page 7 in the chemrevise revision guide.

A

correct?

40
Q

Draw a reaction profile for the bond energies in an exothermic reaction.

A

(see page 7 in the chemrevise revision guide)

41
Q

Draw a reaction profile for the bond energies in a an endothermic reaction.

A

(see page 7 in the chemrevise revision guide)

42
Q

What is the equation used to calculate the ∆H of a mean bond enthalpy using a Hess diagram?

A

∆H reaction = ∑ bond energies broken - ∑bond energies made

43
Q

Draw a standard Hess cycle for a mean bond enthalpy reaction.

A

(see page 7 in the chemrevise revision guide)

44
Q

Answer example 8 on page 8 in the chemrevise revision guide.

A

correct?

45
Q

Answer example 9 on page 8 in the chemrevise revision guide.

A

correct?

46
Q

Answer example 10 on page 8 in the chemrevise revision guide.

A

correct?

47
Q

What is the trend in enthalpies of combustion in a homologous series such as alkanes or alkenes?

A

There is a constant rise in the size of the enthalpies of combustion as the number of carbon atoms increases.

As one goes up the homologous series there is a constant amount and type of extra bonds being broken and made, so the enthalpy of combustion increases by a constant amount.

48
Q

Why is calculated enthalpy data larger than experimental enthalpy data?

A

Because there will be a significant heat loss in the experimental values. There will also be incomplete combustion which will lead to less energy being released.

Chemistry (36 decks)

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