Download Thermal Energy from Chemical Reactions

Thermal Energy
from Chemical
Reactions
Thermochemical Equations
• An equation with the amount of energy
produced or absorbed
– C8H18(l) + 12½O2(g)  8CO2(g) + 9H2O (g) ΔH =
– 5054kJ mol –1
• If you burnt twice as much octane, twice the
amount of energy is produced
– 2C8H18(l) + 25O2(g)  16CO2(g) + 18H2O(g) ΔH
= – 10108kJ mol –1
Thermochemical Equations
• The coefficients of the reactants indicates
the number of moles
• The equation states that 2 moles of C8H18
reacting with 25 moles of O2 produces
10108 kJ
• States must be specified since change of
state can produce or needs energy
Thermochemical Equations
• The combustion of Octane can also produce
liquid water
– C8H18(l) + 12½O2(g)  8CO2(g) + 9H2O (l) ΔH =
– 5450kJ mol –1
• This is because the evaporation of water
absorbs energy
• H2O (l)  H2O (g) ΔH = + 44kJ mol–1
Thermochemical Equations
• A Reaction that occurs in reverse has the
same size ΔH but reversed sign
• H2O (l)  H2O (g) ΔH = + 44kJ mol–1
• H2O (g)  H2O (l) ΔH = – 44kJ mol–1
Calculations Involving Thermal
Equations
• Involves Stoichiometry to determine how
many moles are reacting
• multiplying the ΔH by the number of moles
and dividing by the coefficient from
equation
Calculations Involving Thermal
Equations
• Example, how much energy would 11g of
C3H8 generated if burnt in O2
– C3H8(l) + 10O2(g)  3CO2(g) + 4H2O (l) ΔH = –
2220kJ mol –1
11
n(C3H8) 
 0.25mol
44
0.25
energy 
x 2220  555kJ
1
Connection Between Energy and
Temperature Change
• Objects heat up at different rates
• This is expressed by the Specific Heat
Content
• Values are given in a table of values
Specific Heat Content
• The amount of energy needed to raise the
temperature of 1g of a substance by 1°C
• The higher the specific heat, the more
effectively the substance will store heat
• Has the unit Jg–1°C –1
Temperature
Energy needed
Specific
X mass (g) X
=
to heat
Heat
Rise (°C)
Specific Heat Content
• For Water this would become
• 4.184 X mass of water X temperature rise
• 1 g = 1mL for water density = 1 g mL-1
Enthalpy
• The energy in a chemical bond
• During a chemical reaction where chemical
bonds are broken then formed there will be
a change in enthalpy
• If energy (products) < energy (reactants)
– Energy change is negative
– Energy will be released
– Exothermic Reaction
Measuring Heat Released During
a Reaction (Enthalpy Change)
• Measured by a Calorimeter
• When a reaction takes place, the heat
change causes a rise or fall in temperature
in the contents of the calorimeter.
• Before use calorimeter must be calibrated
• Find out how much energy is needed to
change the temperature by 1°C (Calibration
Factor)
Electric Heater
for Calibration
Bomb Calorimeter
Thermometer
Electric Heater
to ignite sample
Stirrer
Pressurised
Vessel
Oxygen
under
pressure
Insulated Container
Water
Sample in Crucible
Electric Heater
for Calibration
Solution Calorimeter
Stirrer
Thermometer
Solution of one
Reactant
Insulated Container
Glass Bulb
Containing Second
Reactant
Calibration Factor
• Energy = Voltage X Current X Time
• E = VIt
– Voltage is measured in Volts
– Current is measured in Amps
– Time is measured in seconds
Calculate Energy Change During
Reaction
Energy
Calibration
X
=
Change
Factor
Temperature
Change
Calculate ΔH
• This is the change of energy for 1 mole
• If reaction produced heat ΔH = negative
• If reaction absorbed heat ΔH = positive
If change in heat = 30kJ for 0.25mol
ΔH = 30 X 0.25 = 120 kJ mol-1
Heat of Combustion of a
Substance
• The energy released when a specified
amount (1g, 1L or 1 mol) of the substance
burns completely in Oxygen.
• Substances that are mixtures of chemical
are measured in terms of grams or litres
Which Fuel is Best
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Factors to be considered
Energy released per unit of mass
Availability
Cost
Ease of transport
Hazards associated with waste products