Energetics Flashcards
What do all chemical reactions involve
A conversion of energy from one form to another
Exothermic
Negative enthalpy change
Produce heat and cause temperature of surrounding to increase
Endothermic
Positive enthalpjy change cause temperature of surroundings to decrease
What is enthalpy change
Change in heat energy at constant pressure
What happens when hydrogen burns
Produces water vapour but it’s difficult to store and transport because it’s an explosive has
What does breaking bonds require
Energy (endothermic)
What does forming bonds release
Energy (endothermic )
What happens during endothermic reaction
- Energy is put into reactants to break bonds between atoms - enthalpy increases
- Energy is released as new bonds form to give the product molecules , enthalpy decreases
More enthalpy increases than decreases so it’s endothermic
What happens during exothermic reaction
Energy is put into reactants to break all of bonds between atoms , enthalpy increases
Energy is released as new bonds form to give product molecules
More energy is released in forming bonds than is put in to make bonds making it exothermic
Why are experimental values for enthalpy changes different to theoretical
Heat loss unless a well insulated calorimeter is used , heat is lost to surrounding air and from the water to air , energy used to heat the calorimeter and the water
Incomplete combustion unless a good oxygen supply is available , doesn’t release as much energy as complete combustion
How can you calculate an enthalpy change experimently
Calorimetry Equation = mass of water x specific heat capacity (4.2) x change in temperature This gives an answer in joules Convert to kj/mol for enthalpy change Add negative sigh if heat is given off
What is bond disassociation energy from
When one mole of a particular bond between two atoms is formed a certain amount of energy is released as heat , in order to break one mole of the same bind the same amount of energy must be supplied as heat
What is the sequence of An exothermic reaction
Energy is out into reactants to break bonds between atoms - enthalpy increases
Energy is released as new bonds form to give product molecules - enthalpy decreases
Sequence of endothermic reaction
Energy put into reactants to break all of the bonds between atoms - enthalpy increases . Energy is released as new bonds form to give product molecules , enthalpy decreases
