Chapter 9 Enthalpy Flashcards

1
Q

What is ENTHALPY?

A

Enthalpy (H) is the measure of the heat energy in a chemical system.

The chemical system refers to the atoms, molecules, or ions making up the chemicals.

  • (sometimes thought of as the energy stored within bonds)
  • enthalpy cannot be measured but enthalpy changes can be.
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2
Q

What is an enthalpy change?

A

In a chemical reaction, the reactants and products are likely to have different enthalpies.
The difference in the enthalpies = enthalpy change (∆ H)

∆ H = H (products) - H (reactants)
- can be positive OR negative, depending on whether the products contain more or less energy than the reactants

  • can be determined by measuring the energy transfer between the system and the surroundings
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3
Q

What is the law of conservation of energy?

A

Energy can not be created or destroyed.

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4
Q

Energy transfer??

A

Energy transferred from the system to the surroundings = EXOthermic

Energy transferred from the surroundings to the system = ENDOthermic

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5
Q

Exothermic?

A

Energy transferred from the system to the surroundings.
- temperature of the surroundings increases as they gain energy

∆ H is NEGATIVE

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6
Q

Endothermic?

A

Energy transferred from surroundings into system.
- the temperature of the surroundings decrease as they lose energy

∆H is POSITIVE

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7
Q

What is activation energy?

A

The minimum energy required for a reaction to take place.

  • reactions with small activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings
  • very large activation energies may present such a large energy barrier that a reaction may take place very slowly or not at all
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8
Q

What are the standard conditions?

A
  • enthalpy change = kJ/mol
  • standard pressure = 100 kPa
  • standard temp. = 298 K
  • standard concentration = 1mol/dm^3
  • standard state = data table
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9
Q

What is a standard enthalpy change?

A

∆H (standard sign)
- enthalpy change under standard conditions

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10
Q

Define standard enthalpy change of reaction?

A

The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

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11
Q

Define enthalpy change of formation?

A

The standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound if formed from its elements under standard conditions, with all reactants and products in their standard states.

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12
Q

Define enthalpy change of combustion?

A

The standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

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13
Q

Define the standard enthalpy change of neutralisation?

A

The standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l), under standard conditions, with all reactants and products in their standard states.

Involves the reaction of H+ (aq) with OH- (aq) to form 1 mole of H2O (l).

  • the value of enthalpy change of neutralisation is the SAME for ALL neutralisation reactions.
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14
Q

What is 0K in degrees Celsius?

A
  • 273 degrees Celsius
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15
Q

At what temperature does ice melt on the Kelvin scale?

A

273 K = (0 degrees Celsius)

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16
Q

At what temperature does water boil at on the Kelvin scale?

A

373 K = (100 degrees Celsius)

17
Q

How do you calculate an energy change?

A
  • mass (of surroundings) = GRAMS (g)
  • specific heat capacity = Joules per gram per Kelvin
  • temperature change = KELVIN (K)
18
Q

What is specific heat capacity?

A

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

  • (good conductors of heat have SMALL SHC values)
  • (insulators of heat have LARGE SHC values)
  • water SHC = 4.18 J/g/K
19
Q

Calculating an energy change formula??

A

Q = mc∆T

Q = heat energy = JOULES (J)

20
Q

Determination of enthalpy change of combustion PRACTICAL??

A

1) measure out water, pour in beaker, and record initial temp.

2) add methanol (alcohol) to the spirit burner and weigh the spirit burner

3) place the spirit burner under the beaker, light it, and burn the methanol (alcohol) whilst stirring the water (w/ thermometer)

4) after a few minutes extinguish the flame, and record the MAX temperature reached by the water

5) reweigh the spirit burner (assume the wick hasn’t been burnt)

Calculation…

1) calculate energy change
2) calculate amount in mol. of alcohol burnt
3) calculate standard enthalpy change of combustion (complete of 1 mol.)

21
Q

What is the accuracy of the experimental enthalpy change of combustion value?

A

Not accurate due to…

  • HEAT LOSS to the surroundings other than water
  • INCOMPLETE combustion of methanol
  • EVAPORATION of alcohol from the wick
  • NON-STANDARD conditions

(All but the last of these reasons would lead to the value of enthalpy change of combustion being LESS exothermic than expected).

22
Q

Determination of enthalpy change of reaction??

A

1) calculate energy change in the solution in kJ

2) calculate the amount (mol.) of reactant (not in excess) that reacted

3) calculate the enthalpy change of reaction (kJ/mol)

23
Q

Determination of enthalpy change of neutralisation??

A

1) calculate the energy change in the solution in kJ

2) calculate the amount (mol.) of acid and base that reacted

3) calculate the enthalpy change of reaction (kJ/mol)

24
Q

What is average bond enthalpy?

A

The energy required to break one mole of a specified type of bond in a gaseous molecule.

  • energy is always required to break bonds
  • bond enthalpies are always endothermic (+ POSITIVE value)
25
Q

What are the limitations of average bond enthalpies?

A

The actual bond enthalpy can vary depending on the chemical environment of the bond.

  • it is calculated from the actual bond enthalpies in different chemical environments
26
Q

Bond breaking??

A

Energy is required to break bonds = ENDOTHERMIC

∆ H = POSITIVE

27
Q

Bond making??

A

Energy is released when bonds formed = EXOTHERMIC

∆ H = NEGATIVE

28
Q

What does the different between the energy required for bond breaking and the energy released in bond making determine?

A

Whether an overall reaction is EXO or ENDO

29
Q

How do you determine enthalpy changes from average bond enthalpies?

A

= ∑ (bond enthalpies in reactants) - ∑ (bond enthalpies in products)

30
Q

What is Hess’ Law?

A

If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, then the total enthalpy change is the SAME for either route.

  • comes from the idea of conservation of energy

Route 1 : A + B
Route 2 : C

A + B = C

31
Q

Indirect determination of enthalpy changes?? (Reaction)

A

You can work out the standard enthalpy change of any reaction from the standard enthalpy changes of formation of the reactants and products.

1) construct the enthalpy cycle between the reactants, products, and their elements (arrows)

2) add enthalpy change of formation value and calculate the unknown enthalpy change

32
Q

Indirect determination of enthalpy changes?? (Combustion)

A

‘It would be impossible to measure the enthalpy change of this reaction directly since C and H form so many compounds so the product would be formed alongside many others.’

  • the enthalpy changes of combustion of the reactants and the product CAN be measured directly… so you use that.

1) construct the enthalpy cycle between reactants, products, and their common combustion products (CO2 and H2O)

2) add enthalpy change of combustion values and calculate unknown

33
Q

What are the 2 rules that can help with enthalpy cycle calculations?

A

(1) Using enthalpy changes of formation
= enthalpy change of reaction = enthalpy change of formation of products - enthalpy change of formation of reactants

(2) Using enthalpy changes of combustion
= enthalpy change of reaction = enthalpy change of combustion of reactants - enthalpy change of combustion of products