Thermochemistry Overview
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Questions and Answers

What is the specific heat of water?

  • 0.444 J/g·°C
  • 0.129 J/g·°C
  • 0.720 J/g·°C
  • 4.184 J/g·°C (correct)

Which substance has the highest specific heat amongst the listed options?

  • Ethanol
  • Iron
  • Water (correct)
  • Diamond

What does the heat capacity of an apparatus depend on?

  • The mass of the individual components
  • The heat required to warm it by 1 °C
  • The surface area of the apparatus
  • The composition of the materials used (correct)

Why are absolute energy values difficult to calculate in thermodynamics?

<p>Only differences in energy states are relevant (B)</p> Signup and view all the answers

Which of the following statements about specific heat is correct?

<p>It is the same for all samples of a homogeneous substance (A)</p> Signup and view all the answers

What is the heat capacity of the apparatus described?

<p>75.5 kJ/°C (A)</p> Signup and view all the answers

What is a necessary condition for studying thermochemistry in a calorimeter?

<p>The heat capacity must be predetermined (A)</p> Signup and view all the answers

How does changing one part of a calorimeter affect its heat capacity?

<p>It can either increase or decrease the heat capacity (A)</p> Signup and view all the answers

What is the total volume of the mixed solutions in the coffee-cup calorimeter during the acid-base neutralization reaction?

<p>50.00 mL (A)</p> Signup and view all the answers

What is the specific heat assumed for the mixed solutions in the calorimeter?

<p>4.184 J/g·°C (B)</p> Signup and view all the answers

Which of the following represents the heat capacity of the calorimeter?

<p>27.8 J/°C (C)</p> Signup and view all the answers

What does a negative enthalpy change (∆H) indicate about a reaction?

<p>The reaction releases heat (B)</p> Signup and view all the answers

What is the final temperature after mixing 1.00 L of Ba(NO3)2 solution with 1.00 L of Na2SO4 solution?

<p>28.1 °C (C)</p> Signup and view all the answers

What will be the predominant observation when BaSO4 forms in the reaction between Ba(NO3)2 and Na2SO4?

<p>A white precipitate (C)</p> Signup and view all the answers

When calculating the heat released from the acid-base neutralization, what must be considered?

<p>Both the calorimeter and solution heat capacities (D)</p> Signup and view all the answers

What is the thermochemical equation for the neutralization of acetic acid and sodium hydroxide?

<p>CH3COOH + NaOH → CH3COONa + H2O + ∆H = -56.9 kJ (C)</p> Signup and view all the answers

What is the molar heat of combustion of naphthalene as calculated from the given experiment?

<p>-5.14 × 10³ kJ/mol (B)</p> Signup and view all the answers

What is the primary function of a coffee-cup calorimeter?

<p>To measure heat transferred at constant pressure (A)</p> Signup and view all the answers

Why are coffee-cup calorimeters not suitable for reactions that generate gases?

<p>Gases can escape the system and heat goes unaccounted for (A)</p> Signup and view all the answers

Which of the following is a characteristic of bomb calorimeters?

<p>They maintain constant volume conditions (A)</p> Signup and view all the answers

What specific heat capacity value is commonly used for diluted aqueous solutions in calorimetry?

<p>4.184 J/g·°C (B)</p> Signup and view all the answers

In the equation 𝑞𝑟𝑥𝑛 = −[(𝑠𝑠𝑜𝑙 ∙ 𝑚𝑠𝑜𝑙 ∙ ∆𝑡𝑠𝑜𝑙 ) + (𝐶𝑐𝑎𝑙 ∙ ∆𝑡𝑐𝑎𝑙 )], what do the terms represent?

<p>Heat exchange of the solution and calorimeter (A)</p> Signup and view all the answers

Which factor is NOT accounted for in constant-pressure calorimetry when using a coffee-cup calorimeter?

<p>Heat from dissolved ionic compounds (B)</p> Signup and view all the answers

What would be the consequence of using a coffee-cup calorimeter for a gas-producing reaction?

<p>Useful heat data would be lost (D)</p> Signup and view all the answers

What is the main reason why bond enthalpy values in tables are often presented as averages?

<p>Different hybridizations and structural environments affect bond strength. (B)</p> Signup and view all the answers

Which carbon atom's bond enthalpy in molecule A is influenced by electronegative fluorine atoms?

<p>C1 (C)</p> Signup and view all the answers

In molecule B, which carbon atoms are expected to have different bond enthalpies with oxygen?

<p>C2, C3, C4, and C5 (D)</p> Signup and view all the answers

What happens to bond enthalpy values when only one type of bond exists between two elements?

<p>They can be considered precise values. (B)</p> Signup and view all the answers

How does hybridization influence bond enthalpy in the context of molecule A?

<p>Different hybridizations can lead to varying bond strength and energy requirements. (C)</p> Signup and view all the answers

What structural role do electronegative atoms, like fluorine, play in determining bond enthalpy in molecule A?

<p>They withdraw electron density, thus affecting bond energy. (C)</p> Signup and view all the answers

Which carbon atom's bond in molecule A has the highest expected bond enthalpy and why?

<p>C1, due to its connection to electronegative atoms. (D)</p> Signup and view all the answers

Why does bond enthalpy differ even if two carbon atoms share the same hybridization?

<p>Structural surroundings make a significant difference. (A)</p> Signup and view all the answers

What is the relationship between the heat absorbed by the surroundings and the heat lost by the system in a bomb calorimeter?

<p>The heat lost by the system is equal to the negative of the heat absorbed by the surroundings. (B)</p> Signup and view all the answers

In a bomb calorimeter, which element primarily acts as the heat sink?

<p>The water (B)</p> Signup and view all the answers

How does the combustion of a substance in a bomb calorimeter affect its surrounding temperature?

<p>The temperature of the surroundings increases. (B)</p> Signup and view all the answers

What equation would you use to calculate the heat absorbed by the surroundings in a bomb calorimeter?

<p>𝑞𝑟𝑥𝑛 = −(𝑞𝑤𝑎𝑡𝑒𝑟 + 𝑞𝑐𝑎𝑙) (C)</p> Signup and view all the answers

What is the importance of the specific heat capacity of water in a bomb calorimeter setup?

<p>It influences how much heat the surrounding elements can absorb. (B)</p> Signup and view all the answers

When conducting an experiment to measure ∆𝑈𝑐𝑜𝑚𝑏, what must be known about the calorimeter?

<p>The heat capacity of the calorimeter. (A)</p> Signup and view all the answers

If the temperature in a bomb calorimeter increases significantly during combustion, what can be inferred about the heat produced?

<p>The combustion reaction was exothermic. (B)</p> Signup and view all the answers

What does the term ∆𝑈𝑐𝑜𝑚𝑏 represent in calorimetry?

<p>The change in internal energy during combustion. (C)</p> Signup and view all the answers

What is the value of ΔH° for the reaction Ca2+(aq) + 2 OH–(aq) + CO2(g) → CaCO3(s) + H2O(l)?

<p>-259.6 (B)</p> Signup and view all the answers

Flashcards

State Function

A property that depends only on the initial and final states of a system, not on the path taken to reach those states.

Thermodynamics and Energy Changes

Thermodynamics focuses on changes in energy (like heat flow) rather than absolute energy values, as these are difficult to determine accurately.

Specific Heat (s)

The amount of heat required to raise the temperature of one gram of a substance by 1 °C. It is a specific property of a substance.

Molar Specific Heat

The amount of heat needed to raise the temperature of one mole of a substance by 1 °C. It is specific to a substance per mole.

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Heat Capacity (C)

The amount of heat required to raise the temperature of an entire object (or apparatus) by 1 °C. It depends on the materials and their proportions.

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Calorimeter

A device used to measure heat changes in chemical or physical processes.

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Exothermic Reaction

A reaction that releases heat to the surroundings, causing the temperature of the surroundings to increase.

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Heat Capacity of a Calorimeter

The amount of heat needed to raise the temperature of the entire calorimeter by 1 °C. It's important to determine this before studying the reaction.

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Thermochemical Equation

A balanced chemical equation that includes the enthalpy change (heat released or absorbed) for the reaction.

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Enthalpy Change (∆H)

The amount of heat released or absorbed during a chemical reaction at constant pressure.

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Heat Capacity

The amount of heat required to raise the temperature of a substance by one degree Celsius.

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Specific Heat

The amount of heat required to raise the temperature of one gram of a substance by one degree Celsius.

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Heat of Neutralization

The enthalpy change (∆H) associated with the reaction of an acid and a base to form salt and water.

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Heat Transfer in a Bomb Calorimeter

In a Bomb Calorimeter, the heat released from a combustion reaction (qrxn) is absorbed by the surroundings, which is primarily the water and the calorimeter itself.

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Calorimeter Heat Capacity (Ccal)

The heat capacity of the calorimeter (Ccal) represents the amount of heat required to raise its temperature by 1 degree Celsius.

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Heat absorbed by Water (qwater)

The heat absorbed by the water in the calorimeter is calculated as the product of its mass (mw), specific heat (sw), and the change in temperature (Δt).

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Overall Heat Capacity (Coverall cal)

The overall heat capacity of the calorimeter, including the water, represents the total amount of heat required to raise the temperature of the entire system by 1 degree Celsius.

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Calculating Combustion Enthalpy (ΔUcomb)

ΔUcomb, the change in internal energy during combustion, is calculated as the negative of the heat absorbed by the surroundings (water and calorimeter).

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Using the Bomb Calorimeter Equation

The equation qrxn = -(qwater + qcal) relates the heat released in the reaction to the heat absorbed by the water and the calorimeter.

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Simplified Calculation Using Overall Heat Capacity

If the overall heat capacity of the calorimeter is given, the equation simplifies to: qrxn = -Coverall cal * Δt.

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Applying the Bomb Calorimeter Equation

The equation is applied to experimental data to calculate the heat of combustion (ΔUcomb) for specific substances like sucrose, hexane, and toluene.

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Constant Pressure Calorimetry

A type of calorimetry where reactions occur at constant pressure, often using a simple setup like a coffee-cup calorimeter. This setup allows for the measurement of enthalpy change (∆H).

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Coffee-Cup Calorimeter

A simple calorimeter made from two Styrofoam coffee cups. It's used to measure the heat change of reactions at constant pressure. The temperature change of the solution is measured to determine the heat exchanged.

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Limitations of Coffee-Cup Calorimeter

Coffee-cup calorimeters are not suitable for reactions that produce gases. The gases would escape, taking some heat with them, leading to inaccurate measurements.

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Heat Exchange in Constant Pressure Calorimetry

The heat exchanged in a reaction at constant pressure is calculated using the equation: 𝑞𝑟𝑥𝑛 = −(𝑞𝑠𝑜𝑙 + 𝑞𝑐𝑎𝑙 ), where 𝑞𝑟𝑥𝑛 is the reaction's heat, 𝑞𝑠𝑜𝑙 is the solution's heat, and 𝑞𝑐𝑎𝑙 is the calorimeter's heat.

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Specific Heat of Solution

The amount of heat required to raise the temperature of 1 gram of the solution by 1 degree Celsius. For dilute solutions, it's often approximated by the specific heat of water (4.184 J/°C·g).

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Closed System vs Open System

In a closed system (bomb calorimeter), energy is contained within a sealed vessel. In an open system (coffee-cup calorimeter), energy can be lost or gained through the environment.

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Applications of Constant Pressure Calorimetry

Constant pressure calorimetry is useful for studying the thermochemistry of aqueous reactions that don't involve gases, particularly in fields like chemistry and biochemistry.

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Enthalpy of Reaction (ΔH°rxn)

The change in heat energy during a chemical reaction at constant pressure. It represents the difference between the enthalpy of products and reactants.

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Standard Enthalpy of Formation (ΔHf°)

The enthalpy change associated with the formation of one mole of a compound from its elements in their standard states (at 25°C and 1 atm).

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Hess's Law

The total enthalpy change for a reaction is independent of the pathway taken. It's the sum of enthalpy changes for individual steps.

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Calculating ΔH°rxn with ΔHf°

You can calculate the enthalpy change of a reaction using the standard enthalpy of formation values for products and reactants: ΔH°rxn = ΣnΔHf°(products) - ΣnΔHf°(reactants), where 'n' is the stoichiometric coefficient.

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Standard State

The reference conditions for thermodynamic calculations: 25°C (298K) and 1 atm pressure.

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Decomposition Reaction

A chemical reaction where a compound breaks down into two or more simpler substances.

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Hydrogenation Reaction

A chemical reaction involving the addition of hydrogen (H2) to a molecule, usually an unsaturated compound, to create a saturated one.

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Standard Heat Change of a Reaction

The enthalpy change for a reaction occurring under standard conditions (25°C and 1 atm).

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Bond Enthalpy

The average energy required to break one mole of a specific type of bond in a gaseous molecule, measured under standard conditions.

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Why are Bond Enthalpies Average?

Because different molecules have varying bond strengths due to factors like hybridization and surrounding atoms, bond enthalpy tables list average values across various compounds.

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Hybridization's Impact on Bond Enthalpy

The hybridization of an atom affects bond strength and therefore its enthalpy. Different hybridizations lead to different bond lengths and electron distributions.

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Electron Density Influence on Bond Enthalpy

Electron density around a bond can be influenced by electronegative atoms nearby, affecting bond strength and enthalpy. The more electronegative atoms nearby, the weaker the bond.

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Structural Environment's Impact on Bond Enthalpy

Even with the same hybridization, the atoms surrounding a bond can influence its strength and enthalpy. This is due to differences in electron distribution and interactions.

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Unique Bond Enthalpy

Certain molecules, like diatomic molecules, have a single, specific bond type, so their bond enthalpy values are more precise and not averages.

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Bond Enthalpy as an Average

Bond enthalpies are averages due to variations in bond strengths across different molecules with the same bond type.

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Study Notes

Thermochemistry

  • Thermochemistry is the study of thermal energy changes during chemical reactions.
  • Energy is released or absorbed during chemical reactions.
  • Tracking energy transfer requires defining boundaries (system, surroundings, universe).
  • The system is the chemical reaction, while the surroundings are the environment around the reaction.
  • The universe encompasses both the system and surroundings.
  • The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted. The total amount of energy in the universe is constant.

Energy Conversions Associated with Chemical Reactions

  • Chemical reactions often involve the transfer of thermal energy (heat).
  • This heat transfer occurs due to temperature differences between the system and surroundings.
  • Heat flow is the most common energy transfer encountered in chemical reactions.

Types of Systems

  • Open systems: exchange energy and mass.
  • Closed systems: exchange only energy.
  • Isolated systems: exchange neither energy nor mass.

Units of Energy

  • 4.184 Joules (J) = 1 calorie (cal)
  • 1000 calories (cal) = 1 kilocalorie (kcal) or 1 Calorie (Cal)
  • 1 kilojoule (kJ) = 1000 joules (J)

Thermodynamic Quantities

  • Thermodynamic quantities have a magnitude and a sign.
  • The magnitude indicates the amount, while the sign indicates the direction of flow from the system's perspective.

Calorimetry

  • Specific heat (s) is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius (°C).
  • Molar specific heat is the amount of heat required to raise the temperature of one mole of a substance by one degree Celsius (°C).
  • Heat capacity (C) is the amount of heat required to raise the temperature of an object by one degree Celsius (°C).
  • Calorimetry is used to measure heat transfer in chemical reactions or physical processes.
  • Constant pressure calorimeters (coffee-cup calorimeters) are used to measure heat transfer in reactions involving liquids.
  • Bomb calorimeters are used to measure heat in combustion reactions.

Standard Enthalpy of Formation and Reaction

  • Standard enthalpy of formation (ΔHf°) is the enthalpy change associated with the formation of one mole of a compound from its constituent elements in their standard states at a specified temperature.
  • Standard enthalpy of reaction (ΔHr°) is the enthalpy change for a reaction when all reactants and products are in their standard states.
  • Standard states are defined temperature and pressure (e.g., 298 K and 1 atm).
  • Methods for calculating enthalpy changes exist using bond enthalpies or standard enthalpies of formation.

Hess's Law

  • Hess's law allows calculating enthalpy changes of a reaction by summing enthalpy changes of multiple related reactions.

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