Podcast
Questions and Answers
Which of the following statements best describes the relationship between work and heat in the context of energy transfer?
Which of the following statements best describes the relationship between work and heat in the context of energy transfer?
- Heat is a more efficient form of energy transfer than work because it directly increases molecular motion.
- Work is always converted into heat due to the inefficiency of energy transfer processes.
- Work involves changing the position of an object, while heat involves changing the temperature of an object. (correct)
- Work and heat are equivalent forms of energy and are interchangeable under all conditions.
The total energy of an isolated system is not conserved during a chemical reaction because some energy is always lost to the surroundings as heat.
The total energy of an isolated system is not conserved during a chemical reaction because some energy is always lost to the surroundings as heat.
False (B)
Explain how the concepts of kinetic and potential energy relate to the movement of a cyclist on a hilly road.
Explain how the concepts of kinetic and potential energy relate to the movement of a cyclist on a hilly road.
As a cyclist ascends a hill, kinetic energy is converted into potential energy. Conversely, descending converts potential energy back into kinetic energy.
A reaction is considered ______ if the products have a lower heat content than the reactants, resulting in the release of heat.
A reaction is considered ______ if the products have a lower heat content than the reactants, resulting in the release of heat.
Match the following terms related to thermochemistry with their definitions:
Match the following terms related to thermochemistry with their definitions:
What does it mean for enthalpy to be a state function?
What does it mean for enthalpy to be a state function?
Hess's Law can only be applied to reactions that occur in a single step.
Hess's Law can only be applied to reactions that occur in a single step.
Describe how Hess's Law can be used to determine the enthalpy change for a reaction that is difficult to measure directly.
Describe how Hess's Law can be used to determine the enthalpy change for a reaction that is difficult to measure directly.
When applying Hess's Law, if a reaction is reversed, the sign of ΔH must be ______; if the coefficients in a reaction are multiplied by a factor, the ΔH must also be multiplied by ______.
When applying Hess's Law, if a reaction is reversed, the sign of ΔH must be ______; if the coefficients in a reaction are multiplied by a factor, the ΔH must also be multiplied by ______.
Match each type of enthalpy change with its corresponding process:
Match each type of enthalpy change with its corresponding process:
What is the standard state of an element when determining the standard enthalpy of formation?
What is the standard state of an element when determining the standard enthalpy of formation?
The standard enthalpy of formation for an element in its standard state is always a non-zero value due to inherent atomic energy.
The standard enthalpy of formation for an element in its standard state is always a non-zero value due to inherent atomic energy.
Explain how to calculate the standard enthalpy change (∆H°) for a reaction using standard enthalpies of formation (∆fH°) of reactants and products.
Explain how to calculate the standard enthalpy change (∆H°) for a reaction using standard enthalpies of formation (∆fH°) of reactants and products.
In the equation ∆H° = ∑[∆fH° (products)] - ∑[∆fH° (reactants)], the symbol ∑ represents the ______ of the terms that follow, and it's essential to account for ______ when applying the equation.
In the equation ∆H° = ∑[∆fH° (products)] - ∑[∆fH° (reactants)], the symbol ∑ represents the ______ of the terms that follow, and it's essential to account for ______ when applying the equation.
Given the following data:
What is the value of $\Delta_rH^o$ for the following reaction: $C_6H_{12}O_6(S) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(g)$?
$\Delta_fH^o$ ($C_6H_{12}O_6(s)$) = -1273 kJ/mol
$\Delta_fH^o$ ($CO_2(g)$) = -393.5 kJ/mol
$\Delta_fH^o$ ($H_2O(g)$) = -241.8 kJ/mol
$\Delta_fH^o$ ($O_2(g)$) = 0 kJ/mol
Given the following data:
What is the value of $\Delta_rH^o$ for the following reaction: $C_6H_{12}O_6(S) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(g)$?
$\Delta_fH^o$ ($C_6H_{12}O_6(s)$) = -1273 kJ/mol $\Delta_fH^o$ ($CO_2(g)$) = -393.5 kJ/mol $\Delta_fH^o$ ($H_2O(g)$) = -241.8 kJ/mol $\Delta_fH^o$ ($O_2(g)$) = 0 kJ/mol
Flashcards
Energy
Energy
The capacity to do work or transfer heat.
Work
Work
Energy used to change the position of an object.
Heat
Heat
Energy used to change the temperature of an object.
Kinetic Energy
Kinetic Energy
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Potential Energy
Potential Energy
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Atoms charge
Atoms charge
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Units of energy
Units of energy
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First Law of Thermodynamics
First Law of Thermodynamics
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Thermochemistry
Thermochemistry
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Exothermic Reaction
Exothermic Reaction
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Endothermic Reaction
Endothermic Reaction
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Enthalpy (∆H)
Enthalpy (∆H)
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Hess’s Law
Hess’s Law
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Formation Reaction
Formation Reaction
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Standard States
Standard States
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Study Notes
- CHEM 191, Module 2, covers energetics, rates, and driving forces of chemical reactions
Learning Objectives
- Understand energy and energy change concepts in chemical systems
- Define and understand enthalpy (H)
- Distinguish between ΔH and ΔH°
- Apply Hess’s Law to determine ΔH°
What is Energy?
- Energy is the capacity to do work or transfer heat
- Work is energy used to change an object's position
- Heat is energy used to change an object's temperature
Types of Energy
- Kinetic energy is the energy of motion
- Potential energy is the energy of position relative to other objects or forces
Potential Energy in Chemical Systems
- Atoms consist of charged particles, leading to electrostatic potential energy
- Electrostatic potential energy has has the equation: Ep = k(q1q2/r2)
- Chemical bonds store potential energy
- If charges q1 and q2 are opposite (attractive interaction), Ep is negative
Units of Energy
- Energy is measured in Joules (J)
- Chemistry often uses kilojoules (kJ) because one Joule is a small amount of energy
- 1 kJ = 1000 J
- Entropy (S) and the gas constant (R) are often in J K⁻¹ and J K⁻¹ mol⁻¹, respectively; unit conversion is important when using these
- When reporting energy quantities, provide the number, unit, and sign
First Law of Thermodynamics
- Energy is conserved, meaning it can change forms but not be created or destroyed
- Chemistry focuses on energy changes, especially between potential energy and heat
- ΔE = Efinal - Einitial
- In a chemical reaction, ΔrE = Eproducts - Ereactants
Thermochemistry
- Heat (q) is the most measurable form of energy in chemistry
- If products have lower heat content than reactants, the reaction releases heat (exothermic)
- Δrq = qproducts – qreactants = negative
- If products have higher heat content, the reaction absorbs heat (endothermic)
- Δrq = qproducts – qreactants = positive
Enthalpy (H)
- Many chemical and biochemical reactions occur at constant pressure
- Heat changes under these conditions are enthalpy changes (ΔH)
- Δrq = ΔrH = Hproducts - Hreactants
- For 2H₂(g) + O₂(g) → 2H₂O(g), ΔrH = -483.6 kJ
- ΔrH = H(2 moles of H₂O(g)) – H(2 moles of H₂(g) plus 1 mole of O₂(g)) = -483.6 kJ
- Exothermic reactions have a negative ΔrH, products have less H than reactants
Hess's Law
- Enthalpy is a state function; the enthalpy change in a conversion from A to B is the same regardless of the number of steps
- Hess’s Law: If a reaction occurs in multiple steps, the ΔrH for the overall reaction equals the sum of ΔrH values for each step
- It allows the calculation of ΔrH values that are difficult to measure experimentally
- If the equations add to the overall equation, then the ∆rH values will add to give the ∆rH value that is wanted
- If an equation is reversed, then the sign on the ∆rH value must be changed
- When multiplying an equation by a number, the ∆rH value must also be multiplied by the same number
Hess's Law Example
- 3C(s) + 4H₂(g) → C₃H₈(g)
- C(s) + O₂(g) → CO₂(g) ΔrH = -394 kJ
- H₂(g) + ½ O₂(g) → H₂O(l) ΔrH = -286 kJ
- C₃H₈(g) + 5O₂(g) → 4H₂O(l) + 3CO₂(g) ΔrH = -2220 kJ
- Solution
- 3C(s) + 3O2(g) -> 3CO2(g) ∆H = 3 x (-394) = -1182 kJ
- 4H2(g) + 2O2(g) -> 4H2O(I) ∆H = 4 x (-286) = -1144kJ
- 4H2O(l) + 3CO2(g) -> C3H8(g) + 5O2(g) ∆H = +2220 kJ
- 3C(s) + 4H2(g) -> C3H8(g) ∆H = (-1182 + -1144 + 2220)kJ = -106 kJ
Enthalpies of Formation
- Hess’s Law allows the calculation of enthalpy changes for various sorts of chemical reactions, most importantly, formation reactions
- ΔcomH is for combustion reactions
- ΔfusH is for fusion (melting) reactions
- ΔvapH is for vaporization (boiling) reactions
- A formation reaction describes the formation of 1 mole of a compound from its constituent elements in their standard states
- Standard states are the most stable form of an element under 'normal' conditions
Standard Enthalpy of Formation
- ΔfHo is the standard enthalpy of formation which refers to the standard conditions formation reaction
- Standard conditions include all gases at standard pressure (1 bar, ~1 atm)
- For example, ΔfHo (H₂O(l)) refers to H2(g) + ½O2(g) à H2O(l)
- Tables list ΔfHo values; enable calculation of ΔrHo for almost any reaction
Calculating ΔrHo
- ΔrHo = Σ[ΔfHo (products)] - Σ[ΔfHo (reactants)]
- The standard enthalpy change for a reaction equals the sum of the enthalpies of formation of the products minus the enthalpy of formation of the reactants
- Σ means “sum of”
- Any element in its standard state has ΔfHo value = 0 kJ mol⁻¹
Enthalpies of Formation Example
- Combustion of glucose: C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(g)
- ΔfHo (C6H12O6(s)) = -1273 kJ mol⁻¹
- ΔfHo (O2(g)) = 0 kJ mol⁻¹
- ΔfHo (CO2(g)) = -393.5 kJ mol⁻¹
- ΔfHo (H2O(g)) = -241.8 kJ mol⁻¹
- ∆rHo = ∑(∆fHo (products) - ∑(∆fHo (reactants)
- ∆rHo = ((6 x -393.5) + (6 x -241.8)) – (-1273 + (6 x 0)) = -2538.8 kJ
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