enthalpy changes and hess law Flashcards
what is enthalpy (H)
measure of the heat change in a chemical system
what is a chemical system
the atoms, molecules, or ions making up the chemicals
how to calculate enthalpy change
H(products) - H(reactants)
describe an exothermic reaction
- enthalpy change is negative
- temp of surroundings increase–> gain energy
-enthalpy of reactants more than enthalpy of products
describe an endothermic reaction
-enthalpy change is positive
- temp of surroundings decrease–> loses energy
-enthalpy of reactants less than enthalpy of products
what is activation energy
the minimum amount of energy require for the reaction to take place
how does activation energy affect rate of reaction
large Ea–> slow or do not occur
- large energy requirement is a large energy barrier
small Ea–> very rapid
- energy needed to break bonds readily available from surroundings
what are the standard conditions
-100kPa
- 298K
- 1moldm-3
- standard state
what is the definition of Enthalpy change of reaction
the enthalpy
change when the number of moles of reactants as specified in the
balanced equation react together
what is the definition of Enthalpy change of formation
is the
enthalpy change when 1 mole of the compound is formed from
its elements under standard conditions (298K and 100kpa), all
reactants and products being in their standard states
what is the definition of Enthalpy change of combustion
the enthalpy change that occurs when one mole of a substance is
combusted completely in oxygen under standard conditions.
(298K and 100kPa), all reactants and products being in their
standard states
what is the definition of Enthalpy change of neutralisation
the enthalpy change when solutions of an
acid and an alkali react together under standard conditions to produce 1 mole of water
what is the equations for calculating enthalpy change using calorimetry
energy change = mass of solution x heat capacity x temperature change
enthalpy change= energy change/moles
what is Hess’ law
Hess’s law states that total enthalpy change for a reaction is
independent of the route by which the chemical change takes place
