Thermal Chemistry Flashcards
Why is sign of enthalpy value different from sign of temperature change?
(Exothermic reaction has temperature rise)
Enthalpy measures chemical energy, and as heat energy increases, chemical energy must decrease
Draw a labelled diagram for measuring enthalpy change. (3 marks)
Diagram to show insulated / polystyrene container with reagents (1)
Lid (1)
Thermometer (1)
Why Mg+Cl2 -> MgCl2 cannot be used to measure the enthalpy change?
Cannot measure enthalpy change as solution is required to measure temperature change
Why is average bond enthalpy used?
Average used since each individual bond will be in a different environment and therefore have a different strength
How to draw reflux setup? (3 marks)
Labels on diagram to show vertical condenser (unsealed at top) (1)
Water in and out of condenser in correct direction (1)
Heat source below flask with reagents (1)
Reasons for reflux? (3 marks)
The reaction is slow
Allows time for equilibrium to be established to allow complete oxidation occur
Stops reactants / products boiling away (as vapour is flammable/toxic) and prevent yield from being reduced
Esp for -COOH: only partial oxidation occurs and aldehyde formed
How to improve yield of ester? (2 marks)
Reflux for longer (1)
Reaction is slow/ needs time to establish equilibrium (1)
or
Add extra methanol / ethanoic acid (1)
To allow more of the acid to react/ push the equilibrium to RHS (1)
Draw and label suitable apparatus for measuring enthalpy change of combustion of methanol. (2 marks)
liquid in spirit burner with flame (1)
thermometer in water and beaker (1)
Improvements when experimental value is lower than literature one. (4)
• Record the steady temperature of the water before adding ammonium nitrate (1) • ΔT can be calculated accurately (1) or • Record the temperature at timed intervals after adding ammonium nitrate (1) • Graph can be extrapolated (1) or • Place lid on cup / insulate cup (1) • Prevent heat loss (1) or • Crush the ammonium nitrate (1) • Solid dissolves more quickly (1)
