Energetics Flashcards
what is an enthalpy change
amount of heat energy taken in or given out at a constant pressure
what is an exothermic reaction and give some examples
- energy transferred from the system to the surroundings
- products are at a lower energy than reactants
- e.g combustion, neutralisation
what is an endothermic reaction and give some examples
- energy is transferred from the surroundings to the system
- products are at a higher energy than the reactants
- e.g thermal decomposition
what are the standard conditions for an enthalpy change
- 100 KPa pressure
- 298K temperature
- 1 mol dm-3 concentation
define standard enthalpy of combustion
the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
define standard enthalpy of formation
the enthalpy change when one mole of a substance is produced from its elements under standard conditions
what errors can occur in a calorimetry practical
- energy transfer from surroundings (usually loss)
- approximation in specific heat capacity of solution
- reaction or dissolving may be incomplete or slow
what is Hess’s law
the total enthalpy change for a reaction is independent of the route by which the reaction takes place
what formula is used when dealing with formation data
△H = △H products - △H reactants
what formula is used when dealing with combustion data
△H = △H reactants - △H products
what does Hess cycle for enthalpy of formation look like
box = elements in standard states
* arrows go from box towards reactants and products
what does Hess cycle for enthalpy of combustion look like
box = combustion products
* arrows go from reactants and products towards the box
