Year 12 Chemistry: Chapter 25 Energy from Chemical Reactions
25.1 Thermochemical Equations
∆H is equal to the difference in chemical energy, or enthalpy, between reactants and
products.
∆H = ∆Hproducts - ∆Hreactants
The heat of reaction, ∆H, is negative when there is an overall release of energy
(exothermic reaction) and positive when heat is absorbed (endothermic reaction).
∆H is directly proportional to the amount of substance
Energy
Octane is a major component of petrol. Complete combustion of one mole of octane
molecules to form carbon dioxide and steam releases 5054 kJ. This information can be
written as a thermochemical equation:
C8H18(g) + 12 O2(g)  8CO2(g) + 9H2O(g);
∆H= -5054 kJmol-1
Time
If you were to burn twice as much octane, twice the energy (10 108 kJ) would be
released.
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g); ∆H= -5054 kJmol-1
Note:
 The coefficients of the reactants indicate the amounts, in mole, of each
substance that react to give the specified heat energy change.
 States of reactants and products must be specified, since energy changes occur
when solids are converted to liquids or liquids to gases. Eg, the evaporation of
water.
H2O(l)  H2O(g); ∆H= +44 kJmol-1
The combustion of octane to form water has a different ∆H value from the earlier
equation where steam was produced.
C8H18(g) + 12 O2(g)  8CO2(g) + 9H2O(l); ∆H= -5450 kJmol-1
 If a reaction occurs in reverse, it has the same magnitude of ∆H but the opposite
sign.
H2O(l)  H2O(g); ∆H= +44 kJmol-1
H2O(g)  H2O(l); ∆H= -44 kJmol-1
Calculations using thermochemical equations
1. Calculate the heat energy released when 50.00 mL of 0.200 M sodium hydroxide
reacts with excess dilute hydrochloric acid.
H+(aq) + OH-(aq) H2O(l); ∆H= -57.2 kJmol-1
1 mol of NaOH releases _________ of energy.
n(NaOH) = c x V
Therefore 1 mol = 57.2 kJ what does 0.0100 mol = x
2. Calculate the energy releases when 250.0 g of petrol burns completely in a car
engine. Assume petrol is mainly octane and burns according to the equation:
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g); ∆H= -5054 kJmol-1
2 mol of C8H18 releases ___________ of energy.
nC8H18) = n
M(C8H18) =
M
Therefore 2 mol = 5054 kJ what does 2.139 mol = x
3. What volume of methane, measured at standard laboratory conditions, is burnt to
form carbon dioxide and water in order to provide 4.00 x 104 kJ of energy?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l); ∆H= -890 kJmol-1
1 mol of CH4 releases __________ of energy.
Let x mol of CH4 release 4.00 x 104 kJ.
By proportion:
Since 1 mol of gas occupies a volume of 24.5 L at SLC, the volume occupied by 44.94 mol
= 44.94 x 24.5 = 1.10 x 103 L
Questions: 1, 2, 3, 4, 7, 8, 9, 10 & 21.
Year 12 Chemistry: Chapter 25 Calorimetry
25.2 The connection between Energy and Temperature Change
The increase in temperature of a substance when a given amount of energy is absorbed
depends upon the materials ability to store thermal energy in its bonds.
The amount of energy required to raise the temperature of one gram of a substance by
1C is called the specific heat capacity of the substance.
The higher the specific heat capacity, the more effectively a material will store heat
energy.
Energy (J) = SHC x mass of substance (g) x temperature rise (C)
Or
Energy (J) = SHC x volume of substance (mL) x density of substance x
temperature rise (C)
*note: The density of water varies with temperature. At 5C water’s density
is 1.0000 g mol-1
Example: Calculate the energy required to heat 120 mL of water for a cup of coffee to
boiling point if the initial water temperature is 20.0C.
Generally 1 mL = 1 g
*note: don’t forget to calculate the temperature rise
25.3 Measuring the heat released during a reaction
Enthalpy changes are measured directly using an instrument called a calorimeter. A
bomb calorimeter is used for reactions that involve gaseous reactants or products. A
solution calorimeter is used for reactions in aqueous solutions.
When a reaction takes place in a calorimeter, the heat change causes a rise or fall in the
temperature of the contents of the calorimeter. Before the calorimeter can be of use,
you must first determine how much energy is required to change the temperature within
a calorimeter by 1C. This is known as the calibration factor.
The thermal energy released when an electric current passes through the heater can be
calculated from the formula:
Energy = voltage (volts) x current (amps) x times (seconds)
E = VIt
Once the calorimeter has been calibrated, we can measure the temperature change
caused by a reaction occurring in the calorimeter. The calibration factor is then used to
determine what energy change is responsible for this temperature change.
Example: A bomb calorimeter was calibrated by passing 1.50 A through the electric
heater for 50.1 s at a potential difference of 6.05 V. The temperature of the water in
the calorimeter rose by 0.387C.
∆H for the equation:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
was determined by burning 8.58 x 10-4 mol of methane gas in the calorimeter. The
temperature rose from 20.241C to 20.891C.
Step 1: Determine the calibration factor of the calorimeter.
E = VIt
Since the energy made the temperature rise by 0.387C, the energy required to raise
the temperature by 1C = energy
temp rise
Step 2: Calculate the energy change during the reaction.
∆Temperature =
Energy change in the calorimeter = calibration factor x ∆Temperature
Energy change in the calorimeter =
Step 3: Calculate ∆H for the equation.
From the equation we require the heat change for the reaction of 1 mol of CH 4, we have
8.58 x 10-4 mol of CH4. 8.58 x 10-4 caused an energy change of
Therefore as the reaction is a combustion it is an ________________ reaction.
Therefore the ∆H =
Heat of Combustion
The heat of combustion of a substance is defined as the energy released when a
specified amount (eg 1 g, 1 L, 1 mol) of the substance burns completely in oxygen.
Choosing the best Fuel
A wide range of fuels are used. Factors that need to be considered when selecting a fuel
for a particular purpose include:
 Energy released per unit mass or unit volume
 Availability and cost of fuel
 Technology to convert chemical energy to useful work
 Ease of transport
 Hazards to people associated with its use or waste products
 Hazards to the environment associated with its use or waste products.
Questions: 5, 12, 16, 27, 29 & 36.