Chapter 9 - Enthalpy Flashcards

Enthalpy changes, Measuring enthalpy changes, Bond enthalpies and Hess' law and enthalpy changes.

1
Q

What is the law of the conservation of energy?

A

Energy cannot be created or destroyed, just transferred from one form to another.

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2
Q

Define enthalpy

A

The heat content that is stored in a chemical system.

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3
Q

Define enthalpy change

A

The difference between the enthalpy of the products and the reactants.

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4
Q

Define exothermic

A

A chemical system where energy is transferred from the system to the surrounding

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5
Q

Define endothermic

A

A chemical system where energy is transferred to the system from the surrounding

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6
Q

Define activation energy

A

The minimum energy required for a reaction to take place.

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7
Q

What are the standard conditions?

A

Standard pressure - 100kPa
Standard temperature - 298K
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions

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8
Q

Define standard enthalpy change of formation

A

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

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9
Q

Define standard enthalpy change of combustion

A

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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10
Q

What is the equation used to measure an energy change?

A
Q=mcΔT
Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)
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11
Q

How is enthalpy change worked out from the energy change?

A

ΔH = Q/n
ΔH - enthalpy change (J/mol)
Q - energy change (J)
n - number of moles (mol)

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12
Q

Define standard enthalpy change of neutralisation

A

The enthalpy change when an acid and an alkali react together under standard conditions give one mole of water(H20) , with all reactants and products in their standard states.

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13
Q

What are the causes for less exothermic results than expected when working out ΔH

A

Heat loss to the surroundings
Incomplete combustion of ‘alcohol’
Evaporation of ‘alcohol’ from the wick
Non-standard conditions

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14
Q

How can heat loss be accounted for using a graph of temperature against time?

A

Extrapolate the cooling curve back to when it was added.

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15
Q

Enthalpy change of neutralisation always has the same value, what is it?

A

-57.5 kJ/mol

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16
Q

Define average bond enthalpy

A

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

17
Q

What are some properties of bond enthalpies?

A

Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value

18
Q

What kind of energy and enthalpy changes are bond formation?

A

Exothermic and releases energy

19
Q

Define the specific heat capacity

A

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

20
Q

Determination of an enthalpy change of combustion:

Describe the experiment for spirit burner

A

1) using a measuring cylinder, measure out given vol of water. Then pour it into a beaker and record the initial temperature to the nearest 0.5C.
2) add given solution to the spirit burner. Then weigh it.
3) place the spirit burner under the beaker. Then light it. Then stir the water whilst burning with a thermometer.
4) after given time, extinguish the flame. Immediately, record the maximum temperature reached by the water.
5) re weigh the spirit burner containing the methanol. Assume that the wick has not been burnt

21
Q

what is endothermic in terms of bonds

A

where energy is required to break bonds

22
Q

what is exothermic in terms of bonds

A

where energy is released when bonds are formed

23
Q

Formula for calculating enthalpy changes from average bond enthalpies

A

Delta r H = the sum of bond enthalpies in reactants - the sum of bond enthalpies in products

24
Q

The enthalpy change for any element in its standard state is

A

0

25
Q

Limitation of calculating the enthalpy change of reaction from average bond enthalpies

A

average bonds enthalpies are averages and are not exact values so your values may differ

Average bond enthalpies measure bonds in gaseous molecules, so if chemicals or in other state values will differ

26
Q

Determination of an enthalpy change of reaction, the experiment.

A
  • use polystyrene cups as offer insulation against heat lost to the surroundings
  • Record temperature change using thermometer
  • place reaction chemicals inside
  • Heat energy calculated using mcΔT
27
Q

Define the standard enthalpy change of reaction?

A

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

28
Q

Methods for reducing heat loss in the determination of an enthalpy change of a reaction?

A

Adding a lid to the polystyrene cup

Adding insulation around the polystyrene cup

29
Q

Methods for reducing heat loss in the determination of an enthalpy change of combustion?

A

Adding a lid to the beaker

Using draft shields around the apparatus