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. (2.4)

• 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 ∆H°_f 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

• 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.
  • Syllabus and cross-curricular links:
  • Physics topic 2.3—conservation of mass-energy

Aims:

• Aim 4: Discuss the source of accepted values and use this idea to critique experiments.
• Aim 6: Experiments could include Hess's Law labs.
• Aim 7: Use of data loggers to record temperature changes.