Energetics
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2 types of reaction
Endothermic Exothermic
Enthalpy change
Heat energy change measured under conditions of constant pressure
standard enthalpy change symbol
ΔH°
What are standard conditions for standard enthalpy changes
100kPa And a stated temp eg 298 Kelvin
Enthalpy of formation
ΔfH is the enthalpy change when one mole of compound is formed from its constituent elements in their standard states under standard conditions (298K and 100 kPa).
enthalpy of combustion
ΔcH is the enthalpy change when one mole of substance in its standard state is burnt in excess oxygen under standard conditions.
Exothermic definition
Heat energy given out Reactants have more energy than products 🔼H = neg
Endothermic def
Heat given in Reactants have less energy 🔼H = positive
Enthalpy change units
Kj/mol Kjmol^-1
Axis of Enthalpy change diagrams
X axis = progress of reaction Y axis = energy
Activation energy on a energy profile graph
From highest point to reactants
Making bonds is …
Exothermic Releases energy -ve 🔼H
Breaking bonds is …
Endothermic Req energy 🔼H = positive
Whne is a reaction exothermic
When energy rel from bond making is greater than energy taken in from bond breaking
calorimetry equation
q = m x c x ΔT
what is calorimetry
Calorimetry is the measurement enthalpy changes in chemical reactions
q ?
heat change (j)
m?
mass (g) of water whereby 1cm3 = 1g
c?
specifc heat capacity 4.18
ΔT
change in temp K
After calc q you …
convert to kj divide by number of mol
hess law states
enthalpy change is independent of the route taken
how to calc enthalpy change of combustion
arrows point down toward co2 and h2o at bottom enthalpy chage = reac - proud
State the meaning of the term average bond Enthalpy for the _____ bond
bond Enthalpy change to break 1 mol of _____ bonds