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A level chemistry
A level chem Module 3
Enthalpy Changes
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Cards (20)
Enthalpy
The
heat
stored in a
chemical system
Exothermic
(
-ΔH
)
A product releases heat into the surroundings, therefore feels hotter
Exothermic
reaction profile
Endothermic
(+ΔH)
A
product
absorbs heat from its
surroundings
, therefore feels
cooler
)
Endothermic
reaction profile
Activation energy
The energy required to start a chemical reaction
Standard conditions
298K
and
100kPa
Standard states
Physical states under standard conditions
Enthalpy change of reaction
The
enthalpy change
when the
reaction
occurs in the
molar quantities
shown in the
chemical equation
under
standard conditions
Enthalpy change of formation
The energy required to form one mole of a
gaseous
compound is formed from its
elements
under
standard conditions
Enthalpy change of combustion
The energy required to completely
burn
one mole of a
gaseous compound
under
standard
conditions
Enthalpy change of neutralisation
The energy change of a reaction of an
acid
and a
base
associated with the formation of
1 mole
of
water
, it is measured per
mole
of
water
produced
Equation to measure enthalpy change
Equation to measure
enthalpy change
Q=MCΔT where
Q=
Heat energy
(J)
M=
Mass of solution
(g)
c =
Specific heat capacity
ΔT =
Change in temp
(K)
Average bond enthalpy
The
breaking
of
1
mole of bonds in
gaseous
molecules
Making bonds is
Exothermic
Breaking bonds is
Endothermic
Hess
cycle
-An
alternative
route
is used
-Products
and
reactants
are the
same
-Energy of initial reaction
is
equal
to
energy of alternative route
reaction
Why can't enthalpy of formation be calculated directly?
The
reaction
may produce
multiple compounds
Which direction are the arrows pointing in a hess cycle of formation?
Up
(
forming
the
products
)
Which direction are the arrows pointing in a hess cycle of combustion?
Down