Chapter 4 - Energetics Flashcards
quote hess’ law
The enthalpy change in a reaction at constant pressure is independent of the route taken.
what is an endothermic reaction
overall absorbs heat from the surroundings
the energy released breaking bonds is less than the energy required to form new bonds
temp of surroundings decreases
positive enthalpy change because the systems overall energy increases
what is an exothermic reaction
overall releases heat to the surroundings
the energy released breaking bonds is greater than the energy required to form new bonds
temp of surroundings increases
negative enthalpy change because the systems overall energy decreases
what are standard conditions
298K temp
1 atm or 101kPa pressure
substance is in its most stable state
list the different types of standard enthalpy changw
standard enthalpy of reaction
enthalpy of formation
enthalpy of combustion
enthalpy of hydration
enthalpy of neutralisation
define enthalpy of formation
must be word for word
Energy change when 1 mole of the compound is formed from its constituent elements under standard conditions
define enthalpy of combustion
word for word
Energyreleasedwhen1 mole of a substanceiscompletely burnedinoxygenunderstandard conditions
define enthalpy of neutralisation
Energy released when 1 mole of water is formed in the neutralisation between an acid and an alkali under standard conditions
define bond energy
Energy absorbed to break 1 mole of a covalent bond between 2 atoms in the gaseous state.
how can you find the standard enthalpy change of a reaction using enthalpies of formation
enthalpy change of products - enthalpy change of reactants
how can you find the standard enthalpy change of a reaction using enthalpies of combustion
enthalpy change of reactants - enthalpy change of products
how can you find the standard enthalpy change of reaction using bond enthalpies
total bond enthalpy of bonds broken (reactants) - total bond enthalpy of bonds made (products)
why is using hess cycles to find the enthalpy of reaction more accurate than using bond enthalpies
bond enthalpies are an average of the enthalpy change when forming kr breaking the bond, the actual values of these quantities vary depending on the conditions and molecular environment of the molecule
why is the enthalpy of combustion always complete combustion
it is impossible to measure the enthalpy of incomplete combustion because you cannot control the reaction that occurs
there are 3 different reactions that may occur in combustion and 2 of them occur in incomplete and you cannot measure how much of each occurs hence it is impossible
