enthalpy reactions Flashcards
describe an endothermic reaction
- reactions that absorb energy from surroundings
- products have more energy than reactants
- positive enthalpy change
describe an exothermic reaction
- reactions that release energy to the surroundings
- products have less energy than reactants
- negative enthalpy change
describe an exothermic reaction profile diagram
- reactants higher than products
- arrow from reactants to products going downwards
describe an endothermic reaction profile diagram
- products higher than reactants
- arrow from reactants to products going upwards
describe an endothermic reaction in terms of breaking/making bonds
more energy is required to break bonds than is released in making bonds
describe an exothermic reaction in terms of breaking/making bonds
more energy is released in making bonds than is required to break bonds
what is bond dissociation enthalpy?
energy needed per mole to break the attractive forces in bonds in gaseous molecules
what is mean bond enthalpy?
the average enthalpy change when 1 mole of bonds are broken
how is enthalpy change calculated using mean bond enthalpies?
reactants - products
describe how calorimetry works to calculate enthalpy change
- fuel is burned to raise water temperature by a specific amount
- energy from fuel is transferred to water
- lid prevents heat loss
- windshield prevents draught in flame
- polystyrene cup prevents heat loss
what is the equation for energy transfer?
q = mcdeltaT
what is the mass in a calorimetry calculation?
all of the liquids involved
how is enthalpy change calculated?
energy change (q) / moles
what are the limitations of using average bond enthalpies?
- actual bond enthalpy varies depending on the chemical environment
- all species must be gaseous
define the term bond enthalpy
energy required to break one mole of a covalent bond
