Heat/enthalpy of reaction (O-level chemistry)

Calculations involving heat in reactions

The reaction involves energy change; exothermic reactions such as burning firewoods produce heat while endothermic reactions such as the dissolution of ammonium chloride in water absorb heat.

The amount of heat absorbed or released in a reaction depend on

• Amounts of the reactants
•  Temperature and
• The pressure at which a reaction is carried out.

When molar quantities are involved at 298K and 1 atmosphere, the resultant heat changes are referred to as standard heat/enthalpy changes given a symbol ∆H (delta H).

Enthalpy changes (∆H) (for exothermic reactions carry a negative sign because heat is lost from the system while enthalpy changes (∆H) for endothermic reaction carry a positive sign because heat is gained by the system.

Heat is measured in joules ( J)

Each heat change is identified by names; the common ones are:

1. The heat of combustion or enthalpy of combustion of a substance is the heat change when 1 mole of a substance is burnt completely in oxygen.

Experiment to determine enthalpy of combustion A given mass (M₁g) of a substance of molecular mass, Mr, is burnt completely in excess oxygen. Heat liberated raises the temperature of (Mw g) of water through a temperature change of θ⁰

Experimental depends on whether the fuel is a liquid or a solid.

An experimental method for finding enthalpy of combustion a liquid fuel

The figure below shows a simple method for obtaining approximate value for the enthalpy of combustion of a fuel.

Example 1

When 23g of ethanol completely burnt, 13600KJ of heat was produced.

Calculate the molar heat of combustion of ethanol (C = 12,  H = 1,  O = 16)

solution

Formula mass of ethanol (CH₃CH₂OH) = 12 x 2  +  6  +  16 = 46

23g of ethanol liberate 13600KJ

2. Enthalpy of neutralization

This refers to enthalpy change for the formation 1 mole of water from hydrogen and hydroxide ions

H⁺(g) + ^–OH(aq) → H₂O(l)

Measurement of standard enthalpy of neutralization

The heat released when a known amount of water is formed is found by measuring the temperature produced in a calorimeter and its contents.

NB. A vacuum flask is used to minimize heat losses

Know volume of standard acid (V₁) and alkali (V₂) are added to calorimeter, and temperature change θ⁰C is noted. The number of moles of water formed, M_w, is calculated

Example 2

250 cm³ of 0.40M NaOH were added to 250cm³ of 0.40M HCl in the calorimeter. The temperature of the two solutions was 17.5⁰C and rose to 20.1⁰C

Calculate the enthalpy of neutralization assuming that the specific heat capacities of a solution are the same as that of water = 4180J kg^-1 K^-1.

For Revision question and answers please download PDF

Sponsored by The Science Foundation College + 256 753 80 27 09

Compiled by Dr. Bbosa Science