Chemistry

Key unit competency:

To be able to design an experimental procedure to verify the enthalpy chang-es in a chemical reaction

Learning objectives

• Define heat of reaction, standard enthalpy change of combustion, enthalpy of neutralisation, enthalpy of solution, enthalpy of hydration and lattice enthalpy

• Describe an experimental procedure in determination of heat of combustion

• Explain the relationship between quantity of heat produced and mass of substance in combustion reaction

• State Hess’s law of constant heat summation

• State and explain the factors that affect the magnitude of lattice energy

• Describe bond breaking as endothermic and bond making as exothermic

• Develop practical experimental skills about enthalpy changes of reactions, interpreting results and drawing valid conclusions.

• Carry out practical activities to determine enthalpy change of reactions (enthalpy change of combustion of ethanol, enthalpy change of neutralization).

• Calculate the enthalpy change of combustion, neutralization and dissolution from experimental data

• Deduce how Hess’s law is applied to Born-Haber cycle.

• Construct Hess’s energy cycles and Born-Haber cycles from data obtained experimentally or provided.

• Calculate the enthalpy changes of reactions using Hess’s law.

• Use the standard bond energy to determine the standard enthalpy of reaction.

• Relate the heat of hydration and lattice energy to heat of solution.

• Respect of procedure during experiments of combustion and neutralization.

• Appreciate the contributions of other scientists such as Hess, Born and Haber’s work.

14.1. Definition of standard enthalpy of different reactions

In thermodynamics, it is shown how energy, work, and heat are related. Every chemical reaction occurs with a concurrent change in energy. Before to embark the explanation of these chemical changes, some key terms have to be defined as follows.

(i) Enthalpy change (ΔH)

In thermodynamics, the heat of reaction also known as enthalpy of reaction is the change in the enthalpy (H) of a chemical reaction that occurs at a constant pressure. Enthalpy, H is a state function used to describe the heat changes that occur in a reaction under constant pressure. It is a state function as it is derived from pressure, volume, and internal energy, all of which are state functions. The enthalpy is a measurement of the amount of energy per mole either released or absorbed in a reaction.

When a reaction is taking place in an open container, a quantity of heat which is proportional to the quantity of the matter present, will be released or absorbed.

The flow of heat is the enthalpy change noted ΔH. The units of ΔH are kJ/mol or kcal/mol.

(ii) Thermochemical equation

A thermochemical equation is a balanced equation that includes the amount of heat exchanged (produced or absorbed).

The rules of enthalpy change of reaction:

a) The enthalpy change of a reaction is proportional to the amount of reactants that are involved in the reaction.

b) Enthalpy of combustion

The enthalpy of the combustion of a substance (element or compound) ΔHoc, is the enthalpy change which occurs when one mole of a substance undergoes complete combustion with oxygen in excess at 298 K and 1 atm.

c) Enthalpy of neutralization

The standard enthalpy of neutralization, ΔHon is the enthalpy change which occurs when one gram equivalent of an acid is neutralized by one gram equivalent of a base to produce a salt and water under the standard conditions of temperature and pressure.

The equation of the neutralization reaction is: H+(aq) + OH- (aq)→H2O(l)

14.2. Relationship between temperature and heat

14.3. Experimental methods for finding the standard enthalpy of combustion reactions

14.4. Experimental methods for finding the standard enthalpy of neutralisation reactions

14.5. Hess’s law or Law of constant heat summation

14.12. Calculating enthalpy change of reaction using average bond enthalpies