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(C) Enthalpy / Hess / Activation / Energy (Types of Enthalpy (∆cH⁰…
(C) Enthalpy / Hess / Activation / Energy
Types of Enthalpy
∆cH⁰ (
Combustion
):
The enthalpy change when
one mole of a substance
completely reacts with oxygen
under standard conditions, where each substance is in their standard state
∆fH⁰ (
Formation
)
Enthalpy change when
one mole of a substance
is made from its
constituent elements
under standard conditions, where each substance is in their standard state
∆neutH⁰ (
Neutralisation
)
Enthalpy change when
one mole of water
is produced when an
acid and a base react
under standard conditions, where each substance is in their standard state
∆rH⁰ (
Reaction
)
Enthalpy change of a reaction in the
molar quantities shown
in a
standard reaction
under standard conditions, where each substance is in their standard state
STANDARD Q
Measuring enthalpy change
(Step-by-Step process)
Measuring energy equation
:
q = mc∆T
q = energy produced
m = mass of substance heated
c = specific heat capacity (usually 4.18)
∆T = temperature change
e.g. 5000J
Convert q
into
kilojoules
(divide by
1000
)
e.g. 5kJ
Measure
number of moles
of
fuel
burnt
(Mass / Mr)
2/50 = 0.04mol
FINALLY
measuring enthalpy change:
q/moles
e.g. 5/0.04 = 125kJmol⁻¹
Hess
Hess cycle
Different route
from the reactants to products
Hess' Law
The
enthalpy change
accompanying a chemical change is
independent of the route
by which the chemical change occurs.
Activation energy
Minimum energy
required for a
reaction to take place
by
breaking bonds
Standard conditions
100kPa
298K
1moldm⁻³