Enthalpy Flashcards
what is enthalpy?
measure of the heat energy in a chemical system
equation for enthalpy change?
enthalpy of products - enthalpy of reactants
is a postive enthalpy endo or exothermic ?
endothermic
what is activation energy?
minimum energy required for a reaction to take place
what are the standard conditions?
25 degrees celcius ( 298 k)
100kpa
1moldm3
standard state
what is the standard enthalpy of reaction?
enthalpy change that accompanies a reaction in the molar quantities under standard conditions and elements in standard states
what is standard enthalpy of formation?
enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products at standard states
what is standard enthalpy of combustion?
enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions and all reactants and products at standard states
enthalpy change of neutralisation?
enthalpy change when an acid and a base reacts to form one mole of water under standard conditions and reactants and products at its standard states
what is specific heat capacity?
energy required to raise temperature of 1g of a substance by 1K
What is the formula for energy change?
q=m x c x change in temperature
q = energy change
m = mass
c = specific heat capacity = 4.18
t = change in temperature
why might enthalpy of combustion not be as expected?
1) heat loss to surrounding
2) incomplete combustion of methanol
3) evaporation of methanol from wick. Weigh burner as soon as possible after extinguishing flame.
4) non standard conditions
how to reduce heat loss to surroundings?
use a plastic cup made out of polystyrene foam
what is average bond enthalpy?
energy required to break one mole of a specified type of bond in gas molecule
are bond enthalpies endothermic?
bond enthalpies are always endothermic as energy is required to break bonds therefore have a positive enthalpy value
what is the limitation of bond enthalpies?
the actual bond enthalpy may vary depending on chemical environment of a bond
is bond breaking endothermic or exothermic?
endothermic as energy is required to break bonds so has positive enthalpy
is bond making endothermic or exothermic?
exothermic as energy is released when bonds are formed therefore has a negative enthalpy
what is the formula for enthalpy change form average bond enthalpies?
sum of bond enthalpies in reactants - sum of bond enthalpies in products
what is hess law?
if a reaction can take place by 2 routes and the starting and finishing conditions are the same , the total enthalpy change is the same for each route
what is the formula for enthalpy change of formation?
sum of enthalpy change in products - sum of enthalpy change in reactants
what is the formula for enthalpy change of combustion?
sum of enthalpy change of combustion of reactants - sum of enthalpy change of combustion of products
what is the formula for enthalpy change of combustion?
sum of enthalpy change of combustion of reactants - sum of enthalpy change of combustion of products
