Key Terms Flashcards
ΔH
‘Delta H’ symbol used to represent enthalpy change
Enthalpy
A measure of the energy stored in (or heat content of) a system. Given the symbol H.
Enthalpy level diagram
A diagram showing:
- amount of energy contained in both the reactants and the products for a reaction
- therefore the difference between them
- also called an enthalpy profile diagram
Specific heat capacity
Amount of energy required:
- to raise the temperature 1 gram of a substance
- by one degree Kelvin (or Celcius)
Heat
Energy transferred from one body to another due to a temperature difference
Exothermic reaction
- a chemical reaction
- during which heat energy is given out to the surroundings
- ΔH for the reaction is negative
Calorimetry
A technique used for measuring the amount of heat energy absorbed or given out by a reaction
First law of thermodynamics
A law stating:
- increase in the internal energy of a system
- to the equal amount of energy added by heating the system
- minus the amount of energy lost as a result of the work done by the system
- it is sometimes stated in the form ‘energy cannot be created or destroyed; it can only change form’.
Enthalpy change (for a reaction)
- heat energy exchange between the reacting system and its surroundings at constant pressure
- given the symbol ΔH.
Endothermic reaction
- chemical reaction during which heat energy is taken in from the surroundings
- ΔH for the reaction is positive
Hess’s Law
- a rule stating that the overall enthalpy change for a reaction
- is independent of the route the reaction takes
Bond dissociation enthalpy
The energy required to break a particular bond
Enthalpy cycle
- a diagram showing a reaction taking place by 2 or more alternative routes
- all of which involve the same enthalpy change, according to Hess’s law
- also called a Hess cycle
Hess cycle
- a diagram showing a reaction taking place by 2 or more alternative routes
- all of which involve the same enthalpy change, according to Hess’s law
- also called an enthalpy cycle
Bomb calorimeter
- a type of calorimeter that virtually eliminates heat loss
- and so provides very accurate for enthalpies of combustion
Standard enthalpy of formation
- enthalpy change when 1 mole of a substance is formed from its elements
- under standard conditions (298K and 100kPa)
- with all the substances being in their standard states
Thermodynamics
- a branch of study concerned with the transfer of heat and work
- conversion between heat and other forms of energy
- in its modern form it is based on 4 laws, the zeroth, first, second and third laws of thermodynamics
Standard enthalpy of atomisation
- enthalpy change when 1 mole of gaseous atoms of a substance is formed from the substance in its standard state
- under standard conditions (298K and 100kPa)
Standard state
State a substance is under standard conditions (298K and 100kPa)
Standard enthalpy of neutralisation
- enthalpy change when 1 mole of water is produced by the reaction of acids and a base
- under standard conditions (298K and 100kPa)
- with all the substances being in their standard states
Standard enthalpy of combustion
- enthalpy change for a reaction under standard conditions (298K and 100kPa)
- with all substances being in their standard states
Standard enthalpy of combustion
- enthalpy change when 1 mole of a substance undergoes complete combustion
- under standard conditions (298K and 100kPa), with all
Mean bond enthalpy
- average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds
- precisely, it is the average enthalpy change for breaking one mole of a particular bond
- in a range of different compounds in the gas phase.
