9. Enthalpy Flashcards
What is enthalpy (H)?
The measure of the heat energy in a chemical system
What is the formula for enthalpy change?
deltaH = H(products) - H(reactants)
Define enthalpy change of reaction
the enthalpy change accompanying a reaction in the molar quantities shown under standard conditions, with reactants and products in their standard states
Define standard enthalpy change of formation
the enthalpy change that takes place when one mole of a compound is formed from its elements in their standard states, under standard conditions
Define enthalpy change of combustion
the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in standard states
Define enthalpy change of neutralisation
the energy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in their standard states
What is the equation for enthalpy change of neutralisation?
H+ (aq) + OH- (aq) => H2O (l)
What is the system and surroundings ?
system: chemicals (reactants and products)
surroundings: apparatus, lab, everything not the chemical system
How do we measure an enthalpy change through experiment?
measuring energy transfer between system and surroundings
Define average bond enthalpy
the average energy change when 1 mole of gaseous covalent bonds is broken
When using enthalpies of formation and a hess’s cycle, how do you calculate delta Hr ?
sum of enthalpies of products — sum of enthalpies of reactants
When using enthalpies of combustion and a hess’s cycle, how do you calculate delta Hr ?
sum of enthalpies of reactants — sum of enthalpies of products
Why might the calculated enthalpy change of combustion be different from the standard?
the actual bond enthalpies are different to the mean values
the substance was not fully combusted
conditions were not standard
heat energy lost to the environment
specific heat capacity of apparatus ignored
Why do bond enthalpies have positive values?
bond breaking is endothermic
Is bond forming endothermic or exothermic?
exothermic
