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AP Chemistry 6.4 Heat Capacity and Calorimetry Study Notes

AP Chemistry 6.4 Heat Capacity and Calorimetry Study Notes - New Syllabus Effective fall 2024

AP Chemistry 6.4 Heat Capacity and Calorimetry Study Notes.- New syllabus

AP Chemistry 6.4 Heat Capacity and Calorimetry Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Calculate the heat q absorbed or released by a system undergoing heating/cooling based on the amount of the substance, the heat capacity, and the change in temperature.

Key Concepts: 

  • Energy Transfers
  • Calorimetry Calculations

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry Exam Questions MCQs and FRQs

6.4.A.1 Heat Transfer and Calorimetry:

1. Basic Concepts of Heat Transfer:

There are three ways in which heat is transferred:

i. Conduction: Heat travels through a solid by particle collisions, hot to cold locations. Example: Metal spoon warming in hot water.
ii. Convection: Heat is transferred in liquids and gases by the actual motion of the fluid, with warm portions moving upwards and cold portions moving downwards. Example: Rising hot air from a heater.

iii. Radiation: Heat is transferred by electromagnetic waves (such as infrared waves), with no medium required. Example: The Sun’s heat.

2. Specific Heat Capacity:

Specific Heat Capacity: The quantity of heat energy needed to increase 1 kg of a substance by 1°C. It is different for each material and tells us how much energy is required to alter the temperature.

Heat Transfer Equation:

q

– ( q ): Heat energy (J)
– ( m ): Mass (kg)
– ( c ): Specific heat (J/kg°C)
– ( Delta T ): Temperature change (°C or K)

This formula determines the heat energy required to alter a substance’s temperature.

3. Calorimetry:

Principle: Calorimetry is a measurement of heat transfer in physical or chemical change, relying on the principle that heat gained or lost by a substance is equal to the heat lost or gained by the surroundings.

Methods:
i. Constant Pressure Calorimetry: Measures constant pressure heat (e.g., coffee cup calorimeter).
ii. Constant Volume Calorimetry: Measures constant volume heat (e.g., bomb calorimeter).

Measurement of Heat Transfer: Heat transfer is determined with q=mcΔT, where ( q ) is heat,( m ) is mass, ( c ) is specific heat, and ( Delta T ) is change in temperature.

Latent Heat: Heat that is used to change phase with no change in temperature:
– Latent heat of fusion: Melt a solid.
– Latent heat of vaporization: Evaporate a liquid.

4. Latent Heat and Phase Changes:

Latent Heat is the heat required to alter a substance’s phase without altering its temperature:
– Latent Heat of Fusion: Heat to melt a solid to a liquid.
– Latent Heat of Vaporization: Heat to vaporize a liquid to a gas.

Heat Transfer During Phase Changes:
– Melting: Heat is absorbed to transform solid to liquid.
– Boiling: Heat is absorbed to transform liquid to gas.

During phase changes, temperature remains constant as all heat goes into the phase change.

6.4.A.2 First Law of Thermodynamics: Conservation of Energy:

1. Concept of Energy:

Energy is a capability to do work or effect change, coming in different forms. It cannot be created or destroyed but converted according to the Law of Conservation of Energy.

Energy Forms:
1. Kinetic Energy: Motion energy.
2. Potential Energy: Energy of being in place.
3. Thermal Energy: Energy of heat.
4. Chemical Energy: Energy contained within chemical bonds.
5. Electrical Energy: Energy in electron motion.
6. Nuclear Energy: Energy derived from atomic nuclei.
7. Radiant Energy: Energy derived from light and electromagnetic waves.

Energy Conservation: Energy may be converted from one form to another but total energy is conserved.

2. Internal Energy and Work:

Internal Energy (U) is the total energy within a system, including kinetic and potential energy of molecules.

Relationship:

  • First Law of Thermodynamics:
    ΔU=Q−W\Delta U = Q – W
    • Heat (Q): Energy added or removed from the system.
    • Work (W): Energy used or done by the system.

In short:

  • Heat increases internal energy.
  • Work done by the system decreases internal energy.
  • Total energy change is the sum of heat and work.

6.4.A.3 Effect of Specific Heat Capacity on Temperature Change:

1.Specific Heat Capacity:

Definition: Specific heat capacity ( c ) refers to the amount of heat needed to raise the temperature of 1 kg of a substance by 1°C (or 1 K).

Effect on Temperature Change:
– High specific heat: A substance with a high specific heat needs more heat to change its temperature (like water).
– Low specific heat: A substance with a low specific heat heats up or cools down quickly (such as metals).

The relationship can be expressed with the formula

, where ( q ) represents heat energy, ( m ) is mass, ( c ) is specific heat, and ( Delta T ) is the change in temperature.

2. Heat and Temperature Relationship:

Heat transfer results in a change in temperature, but the extent of the effect depends upon the material’s specific heat capacity:

– High specific heat: Material warms or cools gradually (e.g., water).
– Low specific heat: Material warms up or cools rapidly (e.g., metals).

The equation is represented by 

, where ( q ) is heat, ( m ) is mass, ( c ) is specific heat, and ( Delta T ) is temperature change.

6.4.A.4  Effect of Heating and Cooling on System Energy:

1. Energy Transfer:

Energy Transfer: How Heat Modifies the System’s Energy

Heat modifies a system’s energy by adding or removing its internal energy:

– Heat added: Adds to internal energy, increasing temperature or causing phase change.
– Heat removed: Removes from internal energy, reducing temperature or causing phase change.

The First Law of Thermodynamics: ΔU=q−w

Change in the internal energy Delta U  is equal to heat added( q ) minus work done by the system ( w ).

2. Internal Energy:

Internal Energy: Relationship Between Heat and Internal Energy Changes

Heat directly affects the internal energy of a system:
– Heat added: Increases the internal energy (raises temperature or causes phase change).
– Heat removed: Decreases the internal energy (lowers temperature or causes phase change).

6.4.A.5  Specific Heat Capacity vs. Molar Heat Capacity in Energy Calculations: 

PropertySpecific Heat CapacityMolar Heat Capacity
DefinitionThe amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).The amount of heat required to raise the temperature of 1 mole of a substance by 1°C (or 1 K).
UnitJ/kg·°C or J/kg·K\text{J/kg·°C or J/kg·K} J/mol·°C and J/mol·K\text{J/mol·°C or J/mol·K}
Formula for Energy Calculationq=mcΔTq = mc\Delta Tq=nCmΔTq = nC_m \Delta T
Where

m = mass,

c = specific heat capacity,

T= temperature change

n= number of moles,

cm= molar heat capacity,

T= temperature change

Used forCalculating heat for a given mass of a substance.Calculating heat for a given amount (moles) of a substance.
ExampleWater’s specific heat is 4.18 J/g·°C, meaning 4.18 J is needed to raise 1 g of water by 1°C.Water’s molar heat capacity is 75.3 J/mol·°C, meaning 75.3 J is needed to raise 1 mole of water by 1°C.

Key Difference:

  • Specific heat capacity refers to the energy required for a specific mass, while molar heat capacity refers to the energy required for a specific amount (moles) of a substance.

6.4.A.6  Energy Changes in Chemical Systems: Heating, Phase Transitions, and Reactions:

1. Energy Changes in Heating, Cooling, and Phase Transition:

Heating and Cooling: Energy Changes Due to Temperature Differences

Energy is added (heating) or taken away (cooling) in order to alter the temperature of a substance, calculated by:

q=mcΔT

Where:
– ( q ) = heat energy (J)
– ( m ) = mass (kg)
– ( c ) = specific heat capacity
– ( Delta T ) = temperature change (°C or K)

Phase Transitions: Energy Required for Melting, Freezing, and Boiling

i. Melting (Solid to Liquid): Energy is taken in (latent heat of fusion).
ii. Freezing (Liquid to Solid): Energy is released (same as latent heat of fusion).
iii. Boiling (Liquid to Gas): Energy is taken in (latent heat of vaporization).
iv. Condensation (Gas to Liquid): Energy is released.

Latent heat is energy required for phase change without change in temperature.

2. Energy Changes in Chemical Reactions:

Chemical Reactions: Changes in Energy

– Exothermic Reactions: Energy is given out, because products have higher bond energies than reactants.
– Example: Combustion.

– Endothermic Reactions: Energy is  taken in, since reactants possess higher bond energies than products.
– Example: Photosynthesis.

Exothermic: Energy is a product.
Endothermic: Energy is a reactant.

6.4.A.7 Energy Flow in Calorimetry: Exothermic vs. Endothermic Dissolution:

1. Exothermic vs. Endothermic Dissolution:

i. Exothermic Dissolution:
– Energy is released as the substance dissolves.
– Example: NaOH in water (heats solution).

2. Endothermic Dissolution**:
– **Energy is absorbed** as the substance dissolves.
– **Example**: Ammonium nitrate in water (cools solution).

In calorimetry, the change in heat (\( q = mc\Delta T \)) assists in establishing whether the dissolution is exothermic (heat release) or endothermic (heat absorption).

OLD Content

Heat Capacity and Calorimetry  

  • Heat capacity (C): heat absorbed per degree (J/C or J/K)
    • Extensive property: depend on amount of substance
  • Specific heat capacity (cp): heat capacity per gram (J/C g or J/K)
    • Amount of heat required to change one gram of a substance temperature by one degree C or K
      • Every substance has its own specific heat capacity
    • Cp of water is 4.18 J/C g → requires 4.184 J (1 cal) of energy to heat a gram by one degree
  • Molar heat capacity: heat capacity per mol (K/C mol or K/K mol)
  • Specific and Molar heat capacity are intensive properties: independent of the amount (of substance)

Heat Transfer Equations

  • qA = -qB → heat lost = – heat gained
    • qsystem = -qsurroundings; qsolution = -qsurroundings
  • Questions involving specific heat → the amount of heat (J) gained/lost by a sample (q) can be determined by the formula: q = mcpΔT or ncpΔT
    • M = mass
    • C = specific heat
    • ΔH = -q
    • Questions involving two substances: Do two mcats → mcΔT = – (mcΔT)
      • Ex:
    •  
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