9) Enthalpy Flashcards

1
Q

Define enthalpy H

A

the heat content that is stored in a chemical system

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2
Q

What does the law of the conservation of energy state?

A

energy cannot be created or destroyed

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3
Q

Define exothermic reaction

A

a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (∆H is negative)

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4
Q

Define endothermic reaction

A

a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (∆H is positive)

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5
Q

Standard pressure?

A

100 kPa

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6
Q

Standard temperature?

A

298K / 25 degrees C

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7
Q

Standard concentration?

A

1 mol per dm^3

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8
Q

Standard state?

A

physical state of a substance under standard conditions

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9
Q

Define activation energy Ea

A

the minimum energy required to start a reaction by the breaking of bonds

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10
Q

Define standard enthalpy change of reaction ∆rH⊖

A

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

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11
Q

Define standard enthalpy change of formation ∆fH⊖

A

the enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

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12
Q

Define standard enthalpy change of combustion ∆cH⊖

A

the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

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13
Q

Define standard enthalpy change of neutralisation ∆neutH⊖

A

the enthalpy change that accompanies the reaction of an acid by a base to form 1 mole of H2O (l) under standard conditions, all reactants and products being in their standard states

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14
Q

What is special about ∆neutH⊖?

A

it is the same for all neutralisation reactions

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15
Q

Give an equation to determine heat energy, q in Joules

A
q = mc∆T
m = mass of surroundings
c = specific heat capacity
∆T = temperature change of the surroundings (T(final) - T(intial))
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16
Q

Define specific heat capacity

A

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

17
Q

Give a method (5) for the determination of enthalpy change of combustion

A
  1. using a measuring cylinder, measure out 150cm3 of water into a beaker and record the initial temperature of the water
  2. add methanol (or other fuel) to the spirit burner and weigh
  3. place the burner under the beaker, light and burn the methanol whilst stirring the water with a thermometer
  4. after about 3 mins, extinguish flame and record maximum temperature reached by the water
  5. re-weigh the spirit burner
    Then use q = mc∆T with experimental data
18
Q

Name 4 factors of experimental determination of enthalpy change of combustion which compromise accuracy

A
heat loss to surroundings
incomplete combustion of methanol
evaporation of methanol from the wick
non-standard conditions
(all but the last would lead to a ∆cH⊖ value that is less exothermic than expected)
19
Q

How could heat loss to surroundings and incomplete combustion of methanol be minimised when determining enthalpy change of combustion?

A

use of draught screens

oxygen input

20
Q

How could evaporation of methanol from the wick be minimised when determining enthalpy change of combustion?

A

weigh burner as soon as possible after extinguishing the flame
some spirit burners have a cover

21
Q

Why is a plastic cup of polystyrene foam helpful to determine ∆rH?

A

light weight
cheap
waterproof
insulating

22
Q

How can a cooling curve be determined?

A
  1. record temperature of solution every 30 seconds until constant
  2. add an excess of solid powder, stirring and recording the temperature every 30 seconds until the temperature has fallen for several mins
  3. plot a graph of temperature against time and extrapolate the cooling curve back to when the solid was added
23
Q

What is the purpose of a cooling curve?

A

gives a corrected / more accurate value for ∆T without heat loss

24
Q

Define average bond enthalpy

A

the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species

25
Q

What is a limitation of average bond enthalpies?

A

calculated from bond enthalpies in different chemical environments

26
Q

Energy is always required to _. Bond enthalpies are always _ and have _ enthalpy values.

A

break bonds
endothermic
positive

27
Q

Energy is _ therefore, _ is endothermic and ∆H is _

A

required to break bonds
bond breaking
positive

28
Q

Energy is _ therefore, _ is exothermic and ∆H is _

A

released when bonds form
bond making
negative

29
Q

Equation for ∆rH?

A

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

30
Q

Define Hess’ Law

A

if a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route

31
Q

What does Hess’ Law enable?

A

enthalpy changes to be determined indirectly

32
Q

When using enthalpy changes of formation, ∆rH = ?

A

∑∆fH (products) - ∑∆fH (reactants)

33
Q

When using enthalpy changes of combustion, ∆rH = ?

A

∑∆fH (reactants) - ∑∆fH (products)