IB Chemistry IA idea · Energy, fuels and thermodynamics
IB Chemistry IA idea: how good are average bond enthalpies?
Average bond enthalpies are everywhere in exams; find out how well they predict real combustion energies.
These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.
Example research question
How well do average bond enthalpies predict the enthalpy of combustion (kJ mol⁻¹) of the alkanes from C₅ to C₁₀ compared with values derived from NIST formation data?
Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Chemistry IA guide.
Variables
- You change
- Number of carbon atoms in a homologous series of alkanes or alcohols, 6 to 10 members
- You measure
- Difference between the enthalpy of combustion predicted from bond enthalpies and the data value (kJ mol-1 and percent)
- You keep the same
- Same bond-enthalpy table
- State of fuel and products (gas or liquid)
- Same data source
- Standard conditions
Method
Build the combustion equation for each compound, count bonds, calculate the predicted enthalpy, and compare with enthalpy of combustion derived from NIST formation data.
Data analysis
Plot predicted against actual enthalpy and plot the error against chain length; discuss where bond averages fail; give uncertainty in tabulated values.
How it works
You change Chain length (carbon atoms) and measure Predicted combustion enthalpy (kJ mol⁻¹) in Alkanes C₅ to C₁₀ (data), keeping Physical state, Data source, Products assumed the same. Expect a graph of Enthalpy from bond enthalpies / kJ mol⁻¹ against Enthalpy from NIST data / kJ mol⁻¹: Points near a line of gradient 1 mean good prediction.
Make it yours
Repeat for alcohols or compare gas and liquid fuels.
Safety
Database only.
Data sources
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