Chapter 25

Thermochemical Equations

• An explosion is an exothermic reaction in which the chemical energy in the explosive is transformed into thermal energy as chemical bonds are broken and new ones are formed.

• We have seen that ∆H = Hproducts – Hreactants

• The heat of reaction, ∆H, is negative when there is an overall release of energy (exothermic) and positive when heat is absorbed (endothermic).

Thermochemical Equations

The enthalpy change obtained by burning petrol molecules is directly proportional to the number of mole of those molecules.

In general for any reaction ∆H is directly proportional to the

amount of substance Turn to page 402

Thermochemical Equations

Octane is a major component of petrol. Complete combustion of one mole of octane

molecules to form carbon dioxide and steam releases 5054 kJ.

We can write this as a thermochemical equation :

C8H18(g) + 12½O2(g) → 8CO2(g) + 9H2O(g); ∆H = -5054 kJ mol-1

If we were to burn twice as much octane, twice as much energy would be released (10 108 kJ)

Thermochemical Equations

When working with thermochemical equations you should be aware that: 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 states are converted. If we combusted octane to form water rather than steam we get a different ∆H value.

If reactions occur in reverse it has the same ∆H but the opposite sign.

Calculations Using Thermochemical Equations Calculate the heat energy released

when 50.00 mL of 0.200 M sodium hydroxide reacts with excess dilute hydrochloric acidH+(aq) + OH-(aq) → H2O(l); ∆H = 57.2 kJ mol-1

Calculations Using Thermochemical Equations Calculate the energy released 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 = -10 108 kJ mol-1

Calculations Using Thermochemical Equations What volume of methane, measured

at standard laboratory conditions, is burnt to form carbon and water in order to provide 4.00 x 104 kJ.

Your Turn

Page 405 Questions 1-3

The connection between energy and temperature change If I were to heat the exact same

volume of water and oil on a stove top would their temperatures rise at the same rate?

The increase in temperature when a given amount of energy is absorbed depends upon the material’s ability to store thermal energy in its bonds.

The connection between energy and temperature change

The amount of energy required to raise the temperature of one gram of a substance by 1°C is called the specific heat capacity.

The higher the specific heat capacity, the more effectively a material will store heat energy.

Calculations

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.

A useful formula to use in calculations such as this is: Energy (J) = specific heat capacity x mass x temperature rise

Measuring the heat released during a reaction Enthalpy changes are measured directly using an

instrument called a calorimeter. One such device is called a bomb calorimeter and

is used for reactions that involve gaseous reactants of products.

The reaction vessel within a bomb calorimeter is designed to withstand the high pressures that may be created during reactions

Figure 25.7 on page 406 compares a bomb calorimeter to a solution calorimeter.

Both calorimeters are insulated to reduce loss or gain of energy to or from the outside environment

Calorimeters

Measuring the heat released during a reaction 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 use of a calorimeter we 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

Calibration Factor The calorimeter is calibrated by using an electric

heater to release a known quantity of thermal energy and measuring the resultant rise in temperature.

The calibration factor can be calculated using the formulaCalibration factor = VIt

∆T Where

V is voltage (volts) I is current (amps) t is time (seconds) ∆T is temperature rise (°C)

Look at the worked example on page 407

Heat of Combustion

The heat of combustion of a substance is defined as the energy released when a specified amount of the substance burns completely in oxygen.

Many substances, such as wood, coal and kerosene are mixtures of chemicals and do not have a specific chemical formula or molar mass.

The energy released when they burn is therefore measured per gram or per litre, rather than per mole

Heat of Combustion

Your Turn

Page 409 Questions 4 -6 Chapter Questions Page 410 Q 7, 10, 11, 13 Pg 411 Q 16, 21, 22, 24, 26 and 27 Pg 412 Q 30, 33