Measuring Energy Changes Measuring Energy Changes
• total energy of the universe is a constant
• law of conservation of energy
– if a system loses energy, it must be gained by the
surroundings, and vice versa
• when examining energy changes in a chemical
reactions, we divide “the universe” into two parts
Temperature vs. Heat
• temperature
– a measure of the average kinetic energy of the
particles regardless of the amount
• heat (q)
– one of many forms of energy
– the transfer from a warmer body to a cooler body
as a result of a temperature gradient
– measures the total energy in a given substance
(amount does matter)
• 50 ml
water
100 ml
water
• 100 C
100C
100ml of water
contains twice the
heat of 50 ml.
Enthalpy
H
• the amount of heat used or released (change) in a
system
• you cannot measure the actual enthalpy of a
substance directly
– but you can measure an enthalpy CHANGE
because of energy it takes in or releases
Enthalpy (Heat) of a Reaction
• the amount of energy absorbed or given off
in a chemical reaction
• H = ∑  Hproducts − ∑  Hreactants
Exothermic reactions
• heat energy is given out by the reaction
hence the surroundings increase in
temperature (feels hot)
• occurs when bonds are formed
– new products are more stable and extra energy is
given off
• Hproducts &lt; Hreactants
– H is negative
• examples
– combustion of fuels
– respiration
– neutralization reactions (acid reacts with
something)
ENTHALPY
activation energy
energy necessary
to get the
reaction going
energy given
out, ∆H is
negative
reactants
products
REACTION CO-ORDINATE
H2 + Cl2  2HCl
Energy taken in
to break bonds.
H, H, Cl, Cl
(Atoms)
Energy given out when
energy
H-H, Cl-Cl
Reactants
Overall energy
change, H
H-Cl, H-Cl
Products
H2 + Cl2  2HCl
Energy in
= +678kJ
H, H, Cl, Cl
(Atoms)
Energy out = -862kJ
energy
H-H, Cl-Cl
Reactants
Overall energy
change,
H = -184kJ
H-Cl, H-Cl
Products
Endothermic reactions
• heat energy is taken in by the reaction
mixture hence the surroundings decrease in
temperature (feels cold)
• occurs when bonds are broken
– the reactants were more stable (bonds are
stronger)
• overall, took energy from the surroundings
• Hreactants &lt; Hproduct
H is positive
• examples
– photosynthesis
activation energy
energy necessary
to get the
reaction going
ENTHALPY
products
reactants
REACTION CO-ORDINATE
Energy taken in
and now stored
in the bonds.
∆H is positive
Summary Table
Exothermic
reactions
Endothermic
reactions
Energy is given out to
the surroundings
Energy is taken in from
the surroundings
∆H is negative
∆H is positive
Products have less
energy than reactants
Products have more
energy than reactants
Calculation of enthalpy change
Heat Capacity/Specific Heat
• the amount of energy a substance absorbs
depends on:
– mass of material
– temperature
– kind of material and its ability to absorb or
retain heat
• specific heat
– the amount of heat required to raise the
temperature of 1 gram of a substance 1 oC (or 1
Kelvin)
19
Specific Heat (c) values for Some
Common Substances
Substance
J/g (K)
Water (liquid)
4.184
Water (steam)
2.080
Water (ice)
2.050
Copper
0.385
Aluminum
0.897
Ethanol
2.44
0.127
or
kJ/kg (K)
if multiply
by 1000
Heat energy change/transfer
q = m c T
• q = change in heat (same as H if
pressure held constant)
• m = mass in grams or kilograms
• c = specific heat in J/g (K) or kJ/kg (K)
(or Celsius which has same increments as Kelvin)
• T = temperature change
Heat Transfer Problem 1
Calculate the heat that would be required to heat an aluminum
cooking pan whose mass is 402.50 grams, from 20.5oC to
201.5oC. The specific heat of aluminum is 0.902 J/g (oC)
q = mcT
= (402.50 g) (0.902 J/g (oC))(181.0oC)
= 65,712.955 J
= 65,710 J with correct sig. figs.
only 4 sig.
figs.
Heat Transfer Problem 2
On complete combustion, 0.18g of hexane raised the
temperature of 100.5g water from 22.5&deg;C to 47.5&deg;C.
Calculate its enthalpy of combustion in kJ/mole of
hexane.
– Heat absorbed by the water…
• q = mcT
• q = 100.5 (4.18) (25.0) = 10,500 J which is same as 10.5 kJ
– Moles of hexane burned = mass / molar mass
= 0.18 g / 86 g/mol
= 0.0021 moles of hexane
– Need to find heat energy / mole
= 10.5 kJ/ 0.0021 mol
= 5000 kJ mol -1 or 5.0 x 103 kJ/mol
hexane is C6H14