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Essential ideas
- In chemical transformations energy can neither be created nor destroyed (the first law of thermodynamics).
- The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.
- Energy is absorbed when bonds are broken and is released when bonds are formed.
5.1 Heat and temperature
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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. |
5.1 Enthalpy and enthalpy change
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Understandings:
∆H values are usually expressed under standard conditions, given by ∆H°, including standard states. Guidance: 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 Exothermic and endothermic reactions
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Understandings:
Chemical reactions that involve transfer of heat between the system and the surroundings are described as exothermic or endothermic. Applications and skills: Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic. |
5.1 Calculating ΔH using q = mcΔT
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Understandings:
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. |
5.1 Calculating enthalpy changes
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Understandings:
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. |
5.1 Standard enthalpy change of formation
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Applications and skills:
Calculation of ∆H for reactions using ∆H°f data. |
5.1 Standard enthalpy changes of formation and combustion
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This video covers the standard enthalpy changes of formation and combustion.
Please note that the standard conditions are now 298 K and 100.0 kPa. |
5.2 Enthalpy cycles
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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. |
5.2 Hess's law example
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Applications and skills:
Application of Hess’s Law to calculate enthalpy changes. Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes. |
5.3 Average bond enthalpies
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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. |
5.3 Bonding in ozone and oxygen
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Applications and skills:
Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere. This video goes into more detail about the energy and wavelength required to break the bonds in ozone and oxygen. |
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Applications and skills:
Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere. This video gives the equations for the formation and destruction of ozone. |