energetics Flashcards
how do some chemical reactions cause the temperature of the surroundings to heat up?
release heat energy so exothermic
how do some chemical reactions cause the temperature of the surroundings to fall?
take in heat energy so endothermic
what is enthalpy change?
the heat energy of reactions measured under constant pressure
what are standard conditions?
298 K and 100kPa
what is the standard enthalpy of combustion?
the enthalpy change when one mole of a substance is burned completely in oxygen with all reactants and products in their standard states and measured in their standard conditions
what is the standard enthalpy of formation?
the enthalpy change when one mole of a compound is formed from its element with all reactants and products in their standard states and measured under standard conditions
what is the first law of thermodynamics?
energy can neither be created or destroyed, only transferred from one form to another
what is Hess’s law?
the enthalpy change of a reaction depends only on its initial and final states and is independent of the route by which the reaction occurs
ΔH= ∑ ΔH of all steps
what is the equation for ΔHr when using ΔHc?
ΔHr= ∑ΔHc reactants - ∑ΔHc products
REMEMBER CRISP
what is the equation for ΔHr when using ΔHf?
ΔHr= ∑ΔHc products - ∑ΔHc reactants
REMEMBER FLIPPER
what is calorimetry?
how the heat energy being transferred between the reaction system and the surroundings is measured
what is the equation used to measure heat transfer?
q= m x c x ∆T
then use ∆Hc=q/moles
why would an experimentally obtained value be lower than the data book value?
not taken account the energy to warm the glass/equipment, some heat energy lost to surroundings, and incomplete combustion
why are bond enthalpies not the same?
environment of the bond changes its strength, also depends on the structure of rest of molecule.
so mean bond enthalpies are more useful
what is mean bond enthalpy?
enthalpy to break one mole of bonds averaged over a range of compounds
