5. Energetics/thermochemistry

5. Energetics/thermochemistry

5.1 Measuring energy changes

Nature of science:

• Fundamental principle - conservation of energy is a fundamental principle of science.

• Making careful observations - measurable energy transfers between systems and surroundings.

Understandings:

• Heat is a form of energy.

• Temperature is a measure of the average kinetic energy of the particles.

• Total energy is conserved in chemical reactions.

• Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.

• The enthalpy change (△H) for chemical reactions is indicated in kJ mol⁻¹.

• △H values are usually expressed under standard conditions, given by △H⁰, including standard states.

Applications and skills:

• Calculation of the heat change when the temperature of a pure substance is changed using q=mc△T.

• A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.

5.2 Hess's Law

Nature of science:

• Hypotheses - based on the conservation of energy and atomic theory, scientists can test the hypothesis that if the same products are formed from the same initial reactants then the energy change should be the same regardless of the number of steps.

Understandings:

• The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.

Applications and skills:

• Application of Hess’s Law to calculate enthalpy changes.

• Calculation of △H reactions using △H⁰f data.

• Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.

5.3 Bond enthalpies

Nature of science:

• Models and theories - measured energy changes can be explained based on the model of bonds broken and bonds formed. Since these explanations are based on a model, agreement with empirical data depends on the sophistication of the model and data obtained can be used to modify theories where appropriate.

Understandings:

• Bond-forming releases energy and bond-breaking requires energy.

• Average bond enthalpy is the energy needed to break one mol of a bond in a gaseous molecule averaged over similar compounds.

Applications and skills:

• Calculation of the enthalpy changes from known bond enthalpy values and comparison of these to experimentally measured values.

• Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.

• Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.