topic 8: energetics 1 Flashcards
define enthalpy change
the heat energy change in a reaction at constant pressure. kj mol-1
what are the standard conditions
100kPa (1 atm) specified temperature (normally 298 K)
explain the energy change in an exothermic reaction
energy is released from the reactants leaving the products with less energy. the surrounding temperature increases
explain the energy change in an endothermic reaction
energy needs to be supplied to the reactants from the surroundings so the products have more energy and the surroundings become colder
define standard enthalpy change of reaction
the enthalpy change when the reaction occurs in the molar quantities shown in the chemical reaction, under standard conditions
define standard enthalpy chnage in formation
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions
define standard enthalpy change of combustion
the enthalpy change when 1 mol of a substance is completely burned in oxygen, under standard conditions
define standard enthalpy change of neutralisation
the enthalpy chnage when an acid and an alkali react together, under standard condition, to form 1 mole of water
describe the use of a calorimeter
a flammible liquid is burned and the enthalpy is recorded by heating water that can have it’s temperature measured. idealy, all energy from liquid is transfered to the water
what is Hess’s law
the totla energy change of a reaction is always the same, no matter which route is taken
Enthalpy chnage of reaction =
Sum of bond enthalpies of reactants - Sum of bond enthalpies of products
define bond enthalpy
the amount of energy required to break 1 mole of a type of bond of bond in a molecule in the gas phase
why aren’t mean bond enthalpies exact
similar bonds in different mollecules aren’t the same. they have different bond enthalpies due to other factor in the mollecule
