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M3:Chatper 9=> Enthalpy > 9.1 - 9.4 > Flashcards

9.1 - 9.4 Flashcards

1
Q

Define enthalpy

A
  • the heat content that is stored in a chemical system
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2
Q

Define the standard enthalpy change of atomisation

A
  • the enthalpy change that takes place when 1mole of gaseous atoms forms from the elements in its standard state and standard condition
    Na(s) —> Na(g)
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3
Q

Define standard enthalpy change of combustion

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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4
Q

Define standard enthalpy change of formation

A
  • the enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard a states under standard conditions
    C(s) + O2(g) —> CO2(g)
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5
Q

Define standard enthalpy change of hydration

A
  • the enthalpy change that takes place when 1 mole of isolated gaseous ions is dissolved in water forming 1 mole of aqueous ions under standard conditions
    Na+(g) —> Na(aq)
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6
Q

Define standard enthalpy change of neutralisation

A
  • the enthalpy change that takes accompanies the reaction of an acid by a base to form 1 mole of H2O(l), under standard conditions, with all reactants and products in their standard states
    1/2 H2SO4(aq) + NaOH(aq) —> Na2SO4(aq) + H2O(l)
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7
Q

Define standard enthalpy change of reaction

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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8
Q

Define standard enthalpy change of solution

A
  • the enthalpy change that takes place when 1 mole of a compound is completely dissolved in water under standard conditions
    NaCl(s) —> NaCl(aq)
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9
Q

Define enthalpy change

A
  • is the heat change in a reaction at constant pressure
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10
Q

What are the units of enthalpy change

A
  • kJmol^-1
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11
Q

What are the standard conditions for enthalpy change

A
  • 298K
  • 100kPa
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12
Q

What are endothermic reactions

A
  • reactions that absorb energy from the surroundings
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13
Q

Are endothermic reactions positive of negative

A
  • positive
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14
Q

Describe how an energy profile of an endothermic reaction would look

A
  • products higher that reactants
  • positive ^H
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15
Q

What are exothermic reactions

A
  • reactions that release energy to the surroundings
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16
Q

Are exothermic reactions positive or negative

A
  • neagtive
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17
Q

Describe how an energy profile of an endothermic reaction would look

A
  • products lower than reactants
  • negative ^H
18
Q

Where do you label activation energy on an energy profile diagram

A
  • from the reactants to the top of the curve
19
Q

What is an example of an endothermic reaction

A
  • thermal decomposition of calcium carbonate
20
Q

What is an example of an exothermic reaction

A
  • combustion of ethane
21
Q

When are bonds broken and made

A
  • during a reaction
22
Q

What is needed to break bonds and so is bond breaking exothermic or endothermic

A
  • energy is needed to be absorbed
  • so it is endothermic positive ^H
23
Q

What happens when making bonds and so is bond breaking exothermic or endothermic

A
  • energy is released
  • so it is exothermic, negative ^H
24
Q

What is bond dissociation enthalpy

A
  • the energy needed per mole to break attractive forces in bonds
25
Q

What type of molecule does bond dissociation happen in

A
  • ONLY gaseous compounds
26
Q

Complete the sentence:
Bonds of the same type do net all have..

A
  • the same amount of energy
27
Q

How do you work out the mean bond enthalpy

A
  • work out the energy need to break all four
  • then divide by 4
28
Q

What is the formula for enthalpy change

A

Reactants — products

29
Q

What is calorimetry

A
  • a experiment used to work out the enthalpy change of combustion
30
Q

How is heat energy lost to the surroundings

A

1) flame may flicker in the wind causing heat to go to the sides of beaker
2) heat may be absorbed by the tripod
3) heat may escape from the water

31
Q

What do we do to ensure that heat doesn’t escape from the water

A
  • put a lid on top of the breaker
32
Q

How do we work out the energy transferred

A
  • q=mc^T
  • q= heat energy lost or gained in J
  • m= mass of water in g
  • c= specific heat capacity 4.18
  • t= k
33
Q

100g of water was heated from 23 degrees celcius to 57 degrees celcius by 1.8g of ethanol. Calculate the energy transferred and whence the enthalpy change of the fuel

A

Enthalpy= 364.4kJmol^-1

34
Q

Explain a calorimetry experiment of solutions

A

1) acid is added to a polystyrene cup
2) then you measure the temperature
3) then add alkali/solid and stir
4) measure the temperature change

35
Q

Why is calorimetry of solutions done in a polystyrene cup

A
  • to prevent heat loss
36
Q

25.0cm^3 of 1 moldm^-3 of HCl had a temperature of 20 degrees celcius. 25cm^3 of 1 moldm^-3 of NaOH was added and this raised the temperature to a maximum of 26 degrees celcius. Calculate the enthalpy of neutralisation for HCl

A
  • Enthalpy= -50.16kJmol^-1
37
Q

State Hess’s Law

A
  • if a reaction can take place by more than one route and the initial and final conditions are the same, the total energy change is the same for each route
  • the total enthalpy change of a reaction is independent of the route taken
38
Q

Why is Hess’s law so useful

A
  • as it allows us to work out the enthalpy change that you can’t find by doing experiments
39
Q

What do you do if you are giving formation data for Hess’s cycles

A
  • draw the arrows POINTING UP
40
Q

What do you do if you are giving combustion data for Hess’s cycles

A
  • draw arrows pointing DOWN

M3:Chatper 9=> Enthalpy (1 decks)

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