Thermal Chemistry Flashcards

1
Q

Why is sign of enthalpy value different from sign of temperature change?
(Exothermic reaction has temperature rise)

A

Enthalpy measures chemical energy, and as heat energy increases, chemical energy must decrease

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2
Q

Draw a labelled diagram for measuring enthalpy change. (3 marks)

A

Diagram to show insulated / polystyrene container with reagents (1)
Lid (1)
Thermometer (1)

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3
Q

Why Mg+Cl2 -> MgCl2 cannot be used to measure the enthalpy change?

A

Cannot measure enthalpy change as solution is required to measure temperature change

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4
Q

Why is average bond enthalpy used?

A

Average used since each individual bond will be in a different environment and therefore have a different strength

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5
Q

How to draw reflux setup? (3 marks)

A

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)

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6
Q

Reasons for reflux? (3 marks)

A

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

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7
Q

How to improve yield of ester? (2 marks)

A

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)

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8
Q

Draw and label suitable apparatus for measuring enthalpy change of combustion of methanol. (2 marks)

A

liquid in spirit burner with flame (1)

thermometer in water and beaker (1)

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9
Q

Improvements when experimental value is lower than literature one. (4)

A
• 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)
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