Module 3: Physical Chemistry V2 Flashcards
Define what is meant by “enthalpy change”
The heat energy transferred in a reaction at constant pressure. ✓
What are the units for enthalpy change
Define what is meant by “standard states”
Physical states they are in under standard conditions ✓
State what standard conditions are
100kPa and 298K ✓
What is meant by “⦵” symbol
100kPa and 298K ✓
Define what is meant by “standard enthalpy change of reaction” and give an example for this between the reaction of carbon monoxide and nitrogen dioxide to form carbon dioxide and nitrogen monoxide.
Define what is meant by “standard enthalpy change of neutralisation” and give an example for this i.e. sulfuric acid and sodium hydroxide. Also predict the enthalpy change as being positive or negative.
Define what is meant by “standard enthalpy change of combustion” and give an example for this i.e. ethanol. Also predict the enthalpy change as being positive or negative.
Define what is meant by “standard enthalpy change of formation” and give an example for this i.e. ethanol. Also predict the enthalpy change as being positive or negative.
Define what is meant by the term “activation energy”
The minimum amount of energy needed for a reaction to start. ✓
Define what is meany by an “exothermic reaction”
Reaction that releases energy with a negative ∆H ✓
Define what is meany by an “endothermic reaction”
Reaction that absorbs energy with a positive ∆H ✓
Draw an enthalpy profile diagram for the following.
Draw an enthalpy profile diagram for the following.
A student carried out an experiment to measure the enthalpy change of combustion of methanol.The energy from the combustion of methanol was used to heat a beaker containing water.
The student’s calculated enthalpy change of combustion was more exothermic than the value in data books. explain which error could have caused this difference?
A: Some methanol had evaporated from the wick before the final weighing.
B: In the calculation, the student used the molar mass of ethanol instead of methanol.
C: There was incomplete combustion.
D: The water boiled for 5 minutes before the final temperature was taken.
B: In the calculation, the student used the molar mass of ethanol instead of methanol.
n becomes smaller, so when delta H = q/n, q is divided by a smaller number, delta H will be larger overall.
Explain why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values
Heat released to the surroundings ✓
Incomplete combustion ✓
Non-standard conditions ✓
Explain why standard enthalpy changes of neutralisation determined experimentally are less exothermic than the calculated theoretical values
Heat released to the surroundings ✓
Incomplete reaction ✓
Non-standard conditions ✓
Explain why it is not always possible to measure the enthalpy change directly from experiment
The activation energy of the reaction is too high ✓
There is a slow reaction rate ✓
More than one reaction may be taking place (and so more than one product is formed) ✓
Suggest how enthalpy change can be determined indirectly
Average bond enthalpy data ✓
or the standard enthalpy change of combustion or formation data instead. ✓
Define the term ‘average bond enthalpy’
Average enthalpy change when one mole ✓
of gaseous covalent bonds is broken ✓
Explain why bond enthalpies have positive values?
Bond breaking is endothermic ✓
Explain why bond enthalpies have negative values?
Bond forming is exothermic ✓
Explain, in terms of bond breaking and bond forming, why a reaction can be exothermic.
More energy is released by forming bonds ✓
than energy required when breaking bonds ✓
Explain, in terms of bond breaking and bond forming, why a reaction can be endothermic.
More energy is required for breaking bonds ✓
than is released by forming bonds ✓