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O-Level Chemical Energetics
exothermic and endothermic changes and the energy of bonds
What the syllabus expects
- Explain enthalpy change, distinguishing exothermic reactions (∆H negative) from endothermic ones (∆H positive).
- Show energy changes on energy profile diagrams, marking reaction enthalpy changes and activation energies.
- Describe the breaking of bonds as endothermic and the making of bonds as exothermic.
- Account qualitatively for the overall enthalpy change by weighing the energy taken to break covalent bonds against the energy released when they form.
How it's examined
Questions on this topic most often ask you to identify. About 2% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
For the experiment in which the temperature change is recorded as sodium carbonate is mixed with hydrochloric acid inside a Styrofoam cup, identify one possible cause of inaccuracy and propose a single change to the method that would lessen it. [2]
Show the worked answer
The Styrofoam cup is not a perfect insulator, so heat is lost to the surroundings (and through the open top), making the recorded temperature change smaller than the true value. Fitting a lid over the cup reduces this heat loss and gives a more accurate temperature change.
More O-Level Pure Chemistry topics
Experimental Chemistry · The Particulate Nature of Matter · Chemical Bonding and Structure · Chemical Calculations · Acid-Base Chemistry · Qualitative Analysis · all of O-Level Pure Chemistry