Ch 5 Thermochem AP practice Questions for test
Thermal Energy and Phase Changes
Transfer of Thermal Energy to Substances:
In an experiment involving thermal energy transfer, we evaluate which of several scenarios allows for the greatest mass of the material to undergo a phase change. The scenarios considered are:
(A) Melting ice at 0°C to form liquid water at 0°C.
(B) Boiling liquid acetone at 56°C to form acetone vapor at 56°C.
(C) Increasing the temperature of liquid glycerin from 20°C to 60°C.
(D) Increasing the temperature of solid copper from 20°C to 100°C.
Analysis of Phase Changes:
Melting has a latent heat associated with it, typically significant.
Boiling also requires significant energy input due to latent heat.
Increasing temperature without phase change may not require as much thermal energy as phase changes.
Chemistry Demonstration - Freezing Observation
Classroom Experiment:
A teacher mixed two pure solid powders and placed the beaker on wood with a wet surface.
The bottom of the beaker froze to the wood surface, prompting students to form claims about the observation.
Claims & Justifications:
(A) An exothermic chemical change occurred because heat flowed from the contents to the room.
(B) An exothermic physical change occurred as the contents and board lost heat to the room.
(C) An endothermic physical change occurred since the freezing process absorbs heat.
(D) An endothermic chemical change occurred as heat was absorbed, reducing temperature.
Combustion and Bond Dissociation Energy
Combustion Reaction:
Evaluating bond dissociation for a combustion reaction, represented generally as:
(A) The energy associated with breaking bonds must be examined.
(B) Additional understanding of bonds formed and energy released is necessary.
**Key Facts:
More bonds formed in the products than broken in the reactants indicates exothermic behavior.
The sum of bond dissociation energies between products and reactants must be compared.**
Reaction Enthalpies
Enthalpy Calculations:
Calculate standard enthalpy change for the reaction of three moles of acetylene (C2H2) producing one mole of benzene (C6H6).
Data: ΔH f of C2H2 = 230 kJ/mol, ΔH f of C6H6 = 83 kJ/mol.
Standard enthalpy change can be calculated as follows:
→ Choices available to determine enthalpy changes include:
(A) -607 kJ, (B) -147 kJ, (C) +19 kJ, (D) +773 kJ.
Heat Conduction in Calorimetry
Combustion in Calorimeter:
When an organic compound undergoes combustion, consider the heat transfer to water:
A solid organic compound combusted completely in calorimetry will transfer heat to water, increasing water's temperature.
Choice Determination: Evaluate best estimates of the heat of combustion.
Specific Heat Calculation Example:
A cashew (1.0 g) burned in 1000 g of water; water changed from 20.0°C to 25.0°C.
A marshmallow (3