3.2.1 Enthalpy Changes Flashcards
enthalpy
measure of heat energy in chemical system
enthalpy change equation
enthalpy change of bonds broken - enthalpy change of bonds formed
where is heat energy transferred to when enthalpy change occurs
between the system and the surroundings
what is the system
the chemicals - reactants and products
what is the surroundings
the apparatus, laboratory
everything that’s not the chemicals
universe =
system + surroundings
exothermic enthalpy change
from system to surroundings
enthalpy change is negative
surroundings heat up as they gain energy
endothermic enthalpy change
from surroundings to system
enthalpy change is positive
temperature of surroundings decrease as they lose energy
activation energy
minimum energy required for reaction to take place
standard conditions
100kPa
298K/ 25 degrees
1 mol/dm3
standard enthalpy change of a reaction definition
enthalpy change that accompanies a reaction in the molar quantities show in chemical equation under standard conditions with all reactants and products in there standard states and under standard conditions
enthalpy change of formation
enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products in there standard states and under standard conditions
enthalpy change of combustion
enthalpy change when one mole of substance reacts completely with oxygen under standard conditions with all reactants and products in there standard states and under standard conditions
enthalpy change of neutralisation
energy change that accompanies reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in there standard states and under standard conditions
what 3 quantities measure energy change of surroundings
mass, specific heat capacity and temperature change
specific heat capacity definition
energy required to raise the temperature of 1g of substance by 1K
calculating energy change
q = mc∆T
what does c equal for water
4.18 J g-1 K-1
average bond enthalpy
energy required to break one mole of a specific type of bond in a gaseous molecule
are bond enthalpies exo or endothermic
always endothermic so ∆H is always positive
is bond breaking exo or endo
endo
is bond making exo or endo
exo
enthalpy change of reaction formulae
sum of bond enthalpies in reactants - sum of bond enthalpies of products
what does Hess’ law state
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
enthalpy change using Hess cycles
construct enthalpy cycle
add ∆H values
calculate unknow
clockwise= anticlockwise
∆H
q(in kj) / n( moles of limiting reagent
hess’ law definition
if reaction can take place by more than one route and initial and final conditions are the same the total enthalpy change is the same for each route
how do you know if standard conditions are used
correct state at room temperature
Q- energy in J
m (in grams of solution that changes temp) x c x ∆T
∆H for hess law
(sum of all ∆fH products) - (sum of all ∆fH reactants)
how to work out bond enthalpies
enthalpy of reactants add enthalpy of products
if negative more energy is released so exothermic if positive its endothermic
