M3 Enthalpy Changes Q&A
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Explain, in terms of bond breaking and forming, why a particular reaction is exothermic.
• Breaking bonds requires/absorbs energy • Forming bonds releases energy • (In an exothermic reaction) more energy is released than required • OR: more energy is released by forming bonds than energy required by breaking bonds
Explain, in terms of bond breaking and forming, why a particular reaction is endothermic.
• Breaking bonds requires/absorbs energy • Forming bonds releases energy • (In an endothermic reaction) more energy is required than released • OR: more energy is required by breaking bonds than energy released by forming bonds
Why is the value of a standard enthalpy change of combustion, calculated from a calorimetry experiment, different to its data book value?
• Incomplete combustion • Heat loss • Non-standard conditions • Evaporation of water (or alcohol if liquid alcohol being combusted)
Suggest how the experiment design could be improved to improve the accuracy of the value of the standard enthalpy change of combustion?
• Burn in plentiful supply of oxygen • Improve insulation, e.g. draft shield, use copper can in place of beaker, use bomb calorimeter • Use standard conditions • Add lid to beaker • Use 3 DP balance (if masses in question given to 2 DP) or digital thermometer • Heat for longer to decrease % uncertainty in mass / temperature measurements
Why is the value of an enthalpy change, calculated using mean bond enthalpies, different to its data book value?
• Mean bond enthalpies are average values; they do not measure the exact value of the bond • Bonds have different strengths in different environments
Why is it not possible to measure the enthalpy change of formation directly, for the following reaction: N2(g) + ½O2(g) –> N2O(g)
• Activation energy is too high • Other products may be formed, e.g. NO, NO2