Mr. Strickland's Chemistry     pi-sigma

Topic 5: Energetics/Thermochemistry

 

5.1 Measuring energy changes

Essential idea: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their 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 tranfer of heat between the system and the surroundings are described as endothermic or exothermic.
  • The enthalpy change (ΔH) for chemical reactions is indicated in kJ mol-1.
  • ΔH values are usually expressed under standard conditions, known as ΔHθ, including standard states.

 

Applications & Skills:

  • Calculation of the heat change when the temperature of a pure substance is changed using q = mcΔT.
  • A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.

 

Guidance:

  • Enthalpy changes of combustion (∆Hc°) and formation (∆Hf°) should be covered.
  • Consider reactions in aqueous solution and combustion reactions.
  • 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.
  • The specific heat capacity of water is provided in the data booklet in section 2.
  • 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.
  • 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.

 

International-mindedness:

  • 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.

 

Theory of knowledge:

  • 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?

 

Utilization:

  • Determining energy content of important substances in food and fuels.

 

5.2 Hess's law

Essential idea: In chemical transformations energy can neither be created nor destroyed (the first law of thermodynamics).

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 & Skills:

  • Application of Hess's law to calculate enthalpy changes.
  • Calculation of ΔH reactions using ΔHfθ data.
  • Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.

 

Guidance:

  • Enthalpy of formation data can be found in the data booklet in section 12.
  • An application of Hess's Law is ΔH reaction = Σ(ΔHfθ products) - Σ(ΔHfθ reactants).

 

International-mindedness:

  • 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.

 

Theory of knowledge:

  • 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?

 

Utilization:

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

 

Topic 5.3 Bond Enthalpies

Essential idea: Energy is absorbed when bonds are broken and is released when bonds are formed.

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.

 

Guidance:

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

 

International-mindedness:

  • 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.

 

Utilization:

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