5 / 15 Energetics

5.1.NoS1 Fundamental principle—conservation of energy is a fundamental principle of science. (2.6)

5.1.NoS2 Making careful observations—measurable energy transfers between systems and surroundings. (3.1)

5.1.U1 Heat is a form of energy.

5.1.U2 Temperature is a measure of the average kinetic energy of the particles.

5.1.U3 Total energy is conserved in chemical reactions.

5.1.U4 Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.

5.1.U5 The enthalpy change (∆H) for chemical reactions is indicated in kJ mol^{-1}.

5.1.U6 ∆H values are usually expressed under standard conditions, given by ∆H°, including standard states.

5.1.AS1 Calculation of the heat change when the temperature of a pure substance is changed using 𝑞 = 𝑚c∆𝑇.

5.1.AS2 A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.

5.1.G1 Enthalpy changes of combustion (∆H°_{c} ) and formation (∆H°_{f} )should be covered.

5.1.G2 Consider reactions in aqueous solution and combustion reactions.

5.1.G3 Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standard state, but 298 K is commonly given as the temperature of interest.

5.1.G4 The specific heat capacity of water is provided in the data booklet in section 2.

5.1.G5 Students can assume the density and specific heat capacities of aqueous solutions are equal to those of water, but should be aware of this limitation.

5.1.G6 Heat losses to the environment and the heat capacity of the calorimeter in experiments should be considered, but the use of a bomb calorimeter is not required.

5.1.IM1 The SI unit of temperature is the Kelvin (K), but the Celsius scale (°C), which has the same incremental scaling, is commonly used in most countries. The exception is the USA which continues to use the Fahrenheit scale (°F) for all non-scientific communication.

5.1.ToK1 What criteria do we use in judging discrepancies between experimental and theoretical values? Which ways of knowing do we use when assessing experimental limitations and theoretical assumptions?

5.1.Uz1 Determining energy content of important substances in food and fuels.

5.1.Aims1 Aim 6: Experiments could include calculating enthalpy changes from given experimental data (energy content of food, enthalpy of melting of ice or the enthalpy change of simple reactions in aqueous solution).

5.1.Aims2 Aim 7: Use of databases to analyse the energy content of food.

5.1.Aims3 Aim 7: Use of data loggers to record temperature changes.

5.2.NoS 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. (2.4)

5.2.U1 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.

5.2.AS1 Application of Hess’s Law to calculate enthalpy changes.

5.2.AS2 Calculation of ∆𝐻𝐻 reactions using ∆H°_{f} data.

5.2.AS3 Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.

5.2.G1 Enthalpy of formation data can be found in the data booklet in section 12.

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5.2.IM1 Recycling of materials is often an effective means of reducing the environmental impact of production, but varies in its efficiency in energy terms in different countries.

5.2.ToK1 Hess’s Law is an example of the application of the Conservation of Energy. What are the challenges and limitations of applying general principles to specific instances?

5.2.Uz1 Hess’s Law has significance in the study of nutrition, drugs, and Gibbs free energy where direct synthesis from constituent elements is not possible.

5.2.Aims1 Aim 4: Discuss the source of accepted values and use this idea to critique experiments

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5.2.Aims3 Aim 7: Use of data loggers to record temperature changes.

5.3.NoS 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. (2.2)

5.3.U1 Bond-forming releases energy and bond-breaking requires energy.

5.3.U2 Average bond enthalpy is the energy needed to break one mol of a bond in a gaseous molecule averaged over similar compounds.

5.3.AS1 Calculation of the enthalpy changes from known bond enthalpy values and comparison of these to experimentally measured values.

5.3.AS2 Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.

5.3.AS3 Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.

5.3.G1 Bond enthalpy values are given in the data booklet in section 11.

5.3.G2 Average bond enthalpies are only valid for gases and calculations involving bond enthalpies may be inaccurate because they do not take into account intermolecular forces.

5.3.IM1 Stratospheric ozone depletion is a particular concern in the polar regions of the planet, although the pollution that causes it comes from a variety of regions and sources. International action and cooperation have helped to ameliorate the ozone depletion problem.

5.3.Uz1 Energy sources, such as combustion of fossil fuels, require high ΔH values.

5.3.Aims1 Aim 6: Experiments could be enthalpy of combustion of propane or butane.

5.3.Aims2 Aim 7: Data loggers can be used to record temperature changes

5.3.Aims3 Aim 8: Moral, ethical, social, economic and environmental consequences of ozone depletion and its causes.

15.1.NoS Making quantitative measurements with replicates to ensure reliability—energy cycles allow for the calculation of values that cannot be determined directly. (3.2)

15.1.U1 Representative equations (eg M+(g) M+(aq)) can be used for enthalpy/energy of hydration, ionization, atomization, electron affinity, lattice, covalent bond and solution.

15.1.U2 Enthalpy of solution, hydration enthalpy and lattice enthalpy are related in an energy cycle.

15.1.AS1 Construction of Born-Haber cycles for group 1 and 2 oxides and chlorides

15.1.AS2 Construction of energy cycles from hydration, lattice and solution enthalpy. For example dissolution of solid NaOH or NH4Cl in water.

15.1.AS3 Calculation of enthalpy changes from Born-Haber or dissolution energy cycles.

15.1.AS4 Relate size and charge of ions to lattice and hydration enthalpies

15.1.AS5 Perform lab experiments which could include single replacement reactions in aqueous solutions.

15.1.G1 Polarizing effect of some ions producing covalent character in some largely ionic substances will not be assessed.

15.1.G2 The following enthalpy/energy terms should be covered: ionization, atomization, electron affinity, lattice, covalent bond, hydration and solution.

15.1.G3 Value for lattice enthalpies (section 18), enthalpies of aqueous solutions (section 19) and enthalpies of hydration (section 20) are given in the data booklet

15.1.IM1 The importance of being able to obtain measurements of something which cannot be measured directly is significant everywhere. Borehole temperatures, snow cover depth, glacier recession, rates of evaporation and precipitation cycles are among some indirect indicators of global warming. Why is it important for countries to collaborate to combat global problems like global warming?

15.1.Uz1 Other energy cycles—carbon cycle, the Krebs cycle and electron transfer in biology

15.1.Aims1 Aim 4: Discuss the source of accepted values and use this idea to critique experiments.

15.1.Aims2 Aim 6: A possible experiment is to calculate either the enthalpy of crystallization of water or the heat capacity of water when a cube of ice is added to hot water.

15.1.Aims3 Aim 7: Use of data loggers to record temperature changes. Use of databases to source accepted values.

15.2.NoS Theories can be superseded—the idea of entropy has evolved through the years as a result of developments in statistics and probability. (2.2)

15.2.U1 Entropy (S) refers to the distribution of available energy among the particles.The more ways the energy can be distributed the higher the entropy.

15.2.U2 Gibbs free energy (G) relates the energy that can be obtained from a chemical reaction to the change in enthalpy (ΔH), change in entropy (ΔS), and absolute temperature (T).

15.2.U3 Entropy of gas>liquid>solid under same conditions.

15.2.AS1 Prediction of whether a change will result in an increase or decrease in entropy by considering the states of the reactants and products

15.2.AS2 Calculation of entropy changes (ΔS) from given standard entropy values (Sº).

15.2.AS3 Application of ∆G° = ∆H° - T∆S° in predicting spontaneity and calculation of various conditions of enthalpy and temperature that will affect this.

15.2.AS4 Relation of ΔG to position of equilibrium.

15.2.G1 Examine various reaction conditions that affect ΔG.

15.2.G2 ΔG is a convenient way to take into account both the direct entropy change resulting from the transformation of the chemicals, and the indirect entropy change of the surroundings as a result of the gain/loss of heat energy.

15.2.G3 Thermodynamic data is given in section 12 of the data booklet.

15.2.IM1 Sustainable energy is a UN initiative with a goal of doubling of global sustainable energy resources by 2030.

15.2.ToK1 Entropy is a technical term which has a precise meaning. How important are such technical terms in different areas of knowledge?

15.2.Aims1 Aim 1, Aim 4, Aim 7: Use of databases to research hypothetical reactions capable of generating free energy.

15.2.Aims2 Aim 6: Experiments investigating endothermic and exothermic processes could be run numerous times to compare reliability of repetitive data and compare to theoretical values.