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 ∆H°, 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.
Guidance:
- Enthalpy changes of combustion (∆H°c) and formation (∆H°f )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.
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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.
- Syllabus and cross-curricular links:
- Topic 1.1—conservation of mass, changes of state
- Topic 1.2—the mole concept
Aims:
- 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).
- Aim 7: Use of databases to analyse the energy content of food.
- Aim 7: Use of data loggers to record temperature changes.
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