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Energy changes in chemical reactions

The amount of energy is constant (law of conservation of energy)
-energy lost = energy gained
system= substances involved in reaction
surroundings= environment outside the test tube

in a closed system- constant volume – change in internal energy
in an open system – constant pressure – enthalpy
= change in internal energy + work done on the surrounding (change in volume
of gases)

enthalpy = indication of energy content
- heat content = H


Exothermic
Endothermic
(formation, combustion)
(decomposition)
Change in H -
+
( the energy total enthalpy of the product < total
total enthalpy of the
content of
enthalpy of the reactant
product >total enthalpy
the
of the reactant
substance)
Temp of the Increase
Decreases
mixture
Bonding
Forming bonds
Breaking bonds
(the energy given out to form bonds> (the energy taken to
energy taken to break bonds)
break bonds > energy
released to form bonds)



Examples
1
...
cracking of
2
...
precipitation
2
...
calcium oxide and water
decomposition
( e
...
calcium
carbonate)
Application Instant heat pack
Instant cold packs
- salts dissolved in solvent
- ammonium
- calcium chloride and water
nitrate dissolve in
- keeping warm and muscular
water
fatigue
- treating sport
injuries and
reducing swelling









Ch
...
pressure of one atmosphere
2
...
concentration of 1 mole dm3
4
...
of moles of sub
...
heat loss to the surrounding (convection, conduction, evaporation)
2
...
Of the metal can and the thermometer were not taken into
account
3
...
the experiment was not carried out under standard conditions

most accurate: bomb calorimeter







standard enthalpy change of neutralization =

when one mole –water is produced- from neutralization between an acid
and an alkali- under standard conditions
- always negative

# strong acid + alkali
- more or less the same amount of energy released
# weak
- less heat
- some energy is used for complete ionization of the acid and alkali

how to determine the enthalpy change
- directly measure the temp change of the reaction mixture
- heat released = heat absorbed

# expanded polystyrene cups – minimize heat loss
lid – reduce heat loss by convection
cotton wool- reduce heat by conduction

amount of heat released
= mass of water x specific heat cap x temp change of the mixture

enthalpy change = - heat released/ no
...
heat loss to the surrounding
2
...
Of the expanded polystyrene cup and the thermometer X taken
into account
3
...
Of the reaction mixture X the same as water
4
...
the experiment was not carried out – under standard conditions


standard enthalpy change of formation =
- the enthalpy change - when one mole of substance forms from its
constituent elements – in their standard states – under standard
conditions
- can be +/-
- formation of water- negative
- formation of Nitrogen dioxide – positive
- formation of elements in their standard states – 0-no heat change
e
...
H2 ( l) > H2(l)

difficult to determine the changes
- some formation reactions are highly exothermic
- constituent elements are in different physical states , cannot mix well in a
calorimeter




side reactions – formation of side products



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