Gen 8 - Enthalpy & Thermochemistry Flashcards Preview

GAMSAT March 2016 > Gen 8 - Enthalpy & Thermochemistry > Flashcards

Flashcards in Gen 8 - Enthalpy & Thermochemistry Deck (43)
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What is thermochemistry ? 

The study of energy absorbed or released in chemical reactions or any physical transformations (ie. phase change)


What equation represents the enthalpy change of a reaction ? 


What does a positive enthalpy change (delta H+) represent ? 

The flow of heat into a system as a reaction occurs - E.g. An Endothermic reaction 


What does a negative enthalpy change represent ? 

Heat energy is given off to its surroundings - an exothermic reaction. 


What is the law of conservation of energy ? 

The total energy of an isolated system remains constant over time (if there is no associated external force)


What is the second law of thermodynamics ? 

The entropy or randomness of the universe increases in any real system. 

For a reaction to proceed, the entropy of the system must increase. 


What is the difference between enthalpy and entropy ? 

Enthalpy is the heat transfer that takes place at constant pressure. 

Entropy gives an idea of the randomness of a system. 


What is the first law of thermodynamics ? 

Energy can neither be created nor destroyed. 


What is delta Hf

The standard enthalpy (or heat) of formation is the change of enthalpy that would occur when 1 mole of a substance is formed from its elemental constituents, in a standard state reaction. 


What is the standard enthalpy of formation of an element in its standard state ?

Zero, as no enthalpy change is involved in their formation. 


What are the SI units for enthalpy ?



What is delta Hrxn

The standard enthalpy change for a reaction - the change of enthalpy that would occur if one mole of matter is transformed by a chemical reaction with all reactants and products under standard state. 


What is Hess's law ? 


Why is it important to define standard conditions in terms of Thermochemistry ? 

Because enthalpy changes depend on the conditions under which the reactions are carried out. 


What is standard pressure ? 

1 atmosphere, approx 1 barr


What is standard temperature with regards to thermochemistry ? 

Generally 25 ºC if unspecified 


What is the standard physical state of an element ? 

The "natural" physical state of an element under standard temperature and pressure 


What is the standard temperature for an ideal gas ? 



What is the bond dissociation energy ? 

A measure of the bond strength within a particular molecule - the standard enthalpy change in the homolytic cleavage of a chemical bond. 


Why are bond energies always positive values ? 

Because it always takes energy to break bonds apart. 


Describe bond dissociation energy and bond energy in relation to CH4 - why are they not the same ? 

The bond dissociation energies for each of the homolytic C-H bonds cleavage are all slightly different. The average of the four bond dissociation energies is known as the bond energy. 


When is bond dissociation energy and bond energy the same ? 

In all diatomic molecules because there is cleavage of only one bond. 


Define bond energy 

A measure of bond strength. The energy required to sever 1 mole of a chemical bond in a gas. 


How would you calculate Energy transferred (Q) in a thermochemical reaction ? 


When is heat absorbed or released but without a change in temperature of the surroundings ? 

During a phase change 


What is a calorie ? 

The amount of thermal energy required to raise the temperature of 1g of water by 1ºC


What is a calorie in joules ? 

1 cal = 4.184 Joules 


Describe the 3 enthalpy phase changes ? 

  1. Heat of fusion Solid --> Liquid 
  2. Heat of vaporization Liquid --> Gas
  3. Heat of sublimation Solid --> Gas


How do we calculate the heat required for a phase change ? 

Q = ML


Q = heat transferred to system

M = mass of sample 

L = Heat of transformation


What is Calorimetry ? 

The science of measuring the heat evolved or exchanged due to a chemical reaction.