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Flashcards in Thermodynamics: thermochemistry Deck (18)
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1

Define exothermic reactions

Exothermic reactions release heat.  ΔH < 0

2

Define endothermic reactions

Endothermic reactions absorb heat. ΔH > 0 

3

Define the standard state of a substance at a specific temperature

The standard state of a substance at temperature T is its pure and most stable form at 1 bar

4

Define the standard reaction enthalpy, ΔRH0

This is the reaction enthalpy of a given reaction. Note that it is dependent on the stoichiometric numbers, so the value for enthalpy must be quoted with the associated reaction equation.

5

Define the standard combustion enthalpy, ΔCH0

The standard combustion enthalpy is the energy required for the complete oxidation of organic compounds to H2O and CO2

6

Define standard enthalpy of formation, ΔFH0

The standard enthalpy of formation is the enthalpy required to form a substance from its elements in their standard states. 

Typically, the stoichiometric number of the product is 1

ΔFHof elements in their standard states = 0 

7

Describe how a bomb calorimetry experiment is carried out 

  •  A sample of known mass is burnt in oxygen
  • The amount of heat released during the reaction can be determined using the temperature increase of the water.
  • The calorimeter is isochoric, so we have measured ΔU

8

Describe Hess' law

The total enthalpy of a reaction is independent of the path taken. 

ΔRHis the sum of the standard enthalpies. 

 

9

State the equation needed to work out reaction enthalpy from enthalpies of formation 

10

Give Kirchoff's law in relation to working out the reaction enthalpy

Note that this only works if there are no phase transitions between T1 and T2

11

Describe how to calculate various reaction enthalpies at different temperatures 

Unknown ΔRH0 can be calculated using a known reaction enthalpy at a different temperature and the heat capacities of the reactants and products. We use the difference of molar heat capacities of products and reactants and then apply Kirchoff's law 

12

Describe phase transitions 

Pure substances transform from one phase to another at precisely defined temperatures. The enthalpies are endothermic going from low-temperature to high-temperature phases 

13

Describe, give the notation for and give an example of the solid-liquid phase transition 

Fusion. ΔfusH

Eg Ice melting

14

Describe, give the notation for and give an example of the solid-gas phase transition 

ΔSubH0

Eg dry ice

15

Describe, give the notation for and give an example of the liquid -gas phase transition 

Vapourisation. ΔVapH0

Eg water boiling

16

Describe, give the notation for and give an example of the solid-solid phase transition 

ΔTrsH0

Eg graphite-diamond 

17

Describe the Debye approximation

a = T3

The approximation is used to estimate Cin the 0 - 10K range (as this is hard to access experimentally). 

18

Describe the total heat required to heat materials 

Phase transitions need to be considered plus the heat required to heat the material