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Questions and Answers
Which of the following statements accurately describes the relationship between work and heat in thermodynamics?
Which of the following statements accurately describes the relationship between work and heat in thermodynamics?
- Work and heat are interchangeable terms, both referring to any form of energy transfer.
- Work is defined as the energy used to change the temperature of an object, while heat changes the position.
- Work is the energy used to change the position of an object, while heat is the energy used to change the temperature. (correct)
- Heat is a subset of work, specifically referring to the energy involved in mechanical movements.
Kinetic energy is best described as energy of position relative to other objects or a force.
Kinetic energy is best described as energy of position relative to other objects or a force.
False (B)
What three components must be included when reporting energy quantities?
What three components must be included when reporting energy quantities?
number, unit, and sign
The first law of thermodynamics states that '______' is conserved.
The first law of thermodynamics states that '______' is conserved.
Match the type of reaction with the correct sign of ∆q.
Match the type of reaction with the correct sign of ∆q.
What condition must be met for enthalpy changes to accurately describe heat changes in a chemical reaction?
What condition must be met for enthalpy changes to accurately describe heat changes in a chemical reaction?
According to Hess's Law, the enthalpy change of a reaction depends on the path taken from reactants to products.
According to Hess's Law, the enthalpy change of a reaction depends on the path taken from reactants to products.
According to Hess's Law, if a reaction is reversed, what happens to the sign of the ∆H value?
According to Hess's Law, if a reaction is reversed, what happens to the sign of the ∆H value?
A ______ reaction describes the formation of 1 mole of a compound from its constituent elements in their standard states.
A ______ reaction describes the formation of 1 mole of a compound from its constituent elements in their standard states.
Match the term with its meaning.
Match the term with its meaning.
What is the value of ∆H° for an element in its standard state?
What is the value of ∆H° for an element in its standard state?
Energy is measured in Newtons.
Energy is measured in Newtons.
What is the formula for electrostatic potential energy?
What is the formula for electrostatic potential energy?
According to the slides on the first page, Brown et al., 15th edition, Chapter ______ discusses chemical energetics.
According to the slides on the first page, Brown et al., 15th edition, Chapter ______ discusses chemical energetics.
Match the definitions with their respective terms in thermodynamics.
Match the definitions with their respective terms in thermodynamics.
According to the First Law of Thermodynamics, which of the following statements is most accurate?
According to the First Law of Thermodynamics, which of the following statements is most accurate?
An exothermic reaction is characterized by a positive value of ∆q because the reaction absorbs heat from its surroundings.
An exothermic reaction is characterized by a positive value of ∆q because the reaction absorbs heat from its surroundings.
If the ΔH for the reaction $2H_2(g) + O_2(g) → 2H_2O(g)$ is -483.6 kJ, what would the ΔH be for the reaction $4H_2(g) + 2O_2(g) → 4H_2O(g)$?
If the ΔH for the reaction $2H_2(g) + O_2(g) → 2H_2O(g)$ is -483.6 kJ, what would the ΔH be for the reaction $4H_2(g) + 2O_2(g) → 4H_2O(g)$?
Hess’s Law states that if a reaction is conducted in a series of steps, ΔH for the ______ reaction will equal the sum of the enthalpy changes for the individual steps.
Hess’s Law states that if a reaction is conducted in a series of steps, ΔH for the ______ reaction will equal the sum of the enthalpy changes for the individual steps.
Match each symbol with its definition, as they relate to Hess's Law.
Match each symbol with its definition, as they relate to Hess's Law.
Which of the following mathematical relationships correctly applies Hess's Law to calculate the enthalpy change (ΔH) for a reaction, given the enthalpies of formation (ΔfH°) of products and reactants?
Which of the following mathematical relationships correctly applies Hess's Law to calculate the enthalpy change (ΔH) for a reaction, given the enthalpies of formation (ΔfH°) of products and reactants?
Entropy, S, and the gas constant, R, are typically measured in units of kJ K-1 and kJ K-1 mol-1, respectively, eliminating the need to convert units in thermodynamic calculations.
Entropy, S, and the gas constant, R, are typically measured in units of kJ K-1 and kJ K-1 mol-1, respectively, eliminating the need to convert units in thermodynamic calculations.
A reaction yields products with higher heat content than the reactants. Is this process endothermic or exothermic?
A reaction yields products with higher heat content than the reactants. Is this process endothermic or exothermic?
If one multiplies an equation by a constant factor in Hess's Law, one must also multiply the ______ value by the same factor.
If one multiplies an equation by a constant factor in Hess's Law, one must also multiply the ______ value by the same factor.
Match the following types of energy with the correct description or example:
Match the following types of energy with the correct description or example:
Which statement best describes a 'state function' in the context of thermodynamics?
Which statement best describes a 'state function' in the context of thermodynamics?
The standard conditions for gases when determining standard enthalpy changes are defined as 298 K and 1 atm pressure.
The standard conditions for gases when determining standard enthalpy changes are defined as 298 K and 1 atm pressure.
What is the standard state of oxygen when determining standard enthalpies of formation?
What is the standard state of oxygen when determining standard enthalpies of formation?
Chemical bonds store ______ energy.
Chemical bonds store ______ energy.
Match the change in conditions with the appropriate type of energy transfer:
Match the change in conditions with the appropriate type of energy transfer:
Given the reaction $C(s) + O_2(g) → CO_2(g)$ with ΔH = -393.5 kJ and $CO(g) + 1/2O_2(g) → CO_2(g)$ with ΔH = -283.0 kJ, which of the following steps is necessary to calculate the ΔH for $C(s) + 1/2O_2(g) → CO(g)$ using Hess's Law?
Given the reaction $C(s) + O_2(g) → CO_2(g)$ with ΔH = -393.5 kJ and $CO(g) + 1/2O_2(g) → CO_2(g)$ with ΔH = -283.0 kJ, which of the following steps is necessary to calculate the ΔH for $C(s) + 1/2O_2(g) → CO(g)$ using Hess's Law?
If the products of a reaction have a higher heat content than the reactants, the reaction is classified as exothermic.
If the products of a reaction have a higher heat content than the reactants, the reaction is classified as exothermic.
In calculating enthalpy changes using Hess's Law, what must be considered if an equation is multiplied by a coefficient?
In calculating enthalpy changes using Hess's Law, what must be considered if an equation is multiplied by a coefficient?
Enthalpy is a ______ function, meaning that the change in enthalpy upon conversion from one state to another is the same regardless of the number of steps.
Enthalpy is a ______ function, meaning that the change in enthalpy upon conversion from one state to another is the same regardless of the number of steps.
Match these terms with their definitions:
Match these terms with their definitions:
Using the information provided, what additional information is needed to calculate the exact electrostatic potential energy between two oppositely charged ions?
Using the information provided, what additional information is needed to calculate the exact electrostatic potential energy between two oppositely charged ions?
The textbook for the course is "Chemistry – the central science 16th Ed" by Brown et al..
The textbook for the course is "Chemistry – the central science 16th Ed" by Brown et al..
What is the sign convention for ΔH in an exothermic reaction, signifying that the system releases heat?
What is the sign convention for ΔH in an exothermic reaction, signifying that the system releases heat?
The standard enthalpy of formation for a compound is defined for the formation of ______ mole(s) of the compound.
The standard enthalpy of formation for a compound is defined for the formation of ______ mole(s) of the compound.
Match the process with its relative change in potential energy:
Match the process with its relative change in potential energy:
Which of the following statements accurately describes the relationship between heat transfer and temperature change?
Which of the following statements accurately describes the relationship between heat transfer and temperature change?
Consider a chemical reaction where the products have a higher heat content than the reactants. Which of the following statements is true regarding this reaction?
Consider a chemical reaction where the products have a higher heat content than the reactants. Which of the following statements is true regarding this reaction?
Given the reaction $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$ with $\Delta H = -483.6 \text{ kJ}$, what would be the value of $\Delta H$ for the reaction $4H_2(g) + 2O_2(g) \rightarrow 4H_2O(g)$?
Given the reaction $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$ with $\Delta H = -483.6 \text{ kJ}$, what would be the value of $\Delta H$ for the reaction $4H_2(g) + 2O_2(g) \rightarrow 4H_2O(g)$?
Explain how Hess's Law can be utilized to determine the enthalpy change ($\Delta H$) for a reaction that is difficult to measure directly.
Explain how Hess's Law can be utilized to determine the enthalpy change ($\Delta H$) for a reaction that is difficult to measure directly.
According to Hess's Law, if a reaction is ______, the sign on the $\Delta H$ value is changed.
According to Hess's Law, if a reaction is ______, the sign on the $\Delta H$ value is changed.
Flashcards
What is energy?
What is energy?
The capacity to do work or transfer heat.
Work (energy)
Work (energy)
Energy used to change the position of an object.
Heat (energy)
Heat (energy)
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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Potential energy in chemical systems
Potential energy in chemical systems
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Electrostatic Potential Energy
Electrostatic Potential Energy
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Joules (J)
Joules (J)
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First Law of Thermodynamics
First Law of Thermodynamics
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Heat (q)
Heat (q)
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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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ΔH (change in enthalpy)
ΔH (change in enthalpy)
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Hess's Law
Hess's Law
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State function
State function
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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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Standard enthalpy of formation
Standard enthalpy of formation
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∆H° value = 0 kJ mol-1
∆H° value = 0 kJ mol-1
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Standard enthalpy change for a reaction
Standard enthalpy change for a reaction
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Study Notes
- CHEM 191 is module 2
- The module focuses on energetics, rates, and the driving forces behind chemical reactions
- Relevant reading material can be found in Brown et al., 15th edition, Chapter 5, 14, 19, 20, and 23
Module 2, Lecture 1 Objectives
- Understand the concepts of energy and energy change in chemical systems
- Grasp the concept of enthalpy (H)
- Discern the difference between ΔH and ΔHo
- Apply Hess's Law to determine ΔHo
- The textbook for the course is Chapter 5
Energy Defined
- Energy is the capacity to do work or transfer heat
- Work is the energy used when an object's position changes
- Heat is energy used to change an object's temperature
- Energy can enable work, such as making a fan turn
- Energy can transfer heat, like heating a coil
Types of Energy
- Kinetic energy is the energy of motion
- Potential energy is the energy of position relative to other objects or forces
- Kinetic and gravitational potential energy are examples of energy types
Potential Energy in Chemical Systems
- Atoms consist of charged particles
- Electrostatic Potential Energy is defined as Ep = k(q1q2/r2)
- 'k' is a constant, 'q' represents the charges, and 'r' signifies the distance between the charges
- For charges q1 and q2, if the interaction is attractive (ie. opposite charges), Ep is negative
- Chemical bonds store potential energy
Units of Energy
- Energy is measured in Joules (J)
- In chemistry, energy amounts are commonly expressed in kilojoules (kJ) due to the small size of a Joule
- When reporting quantities it is important to include a number, a unit, and a sign
- There are 1000 J in 1 kJ
First Law of Thermodynamics
- Energy is conserved, meaning it can change form but not be created or destroyed
- Chemistry is interested in the changes in energy, especially potential energy changing to heat
- ΔE = Efinal - Einitial describes the change in energy
- In a chemical reaction, ΔrE = Eproducts - Ereactants, where ΔrE is sometimes noted as ΔrxnE in books
Thermochemistry
- Heat (q) is the most readily measurable form of energy in chemistry
- Exothermic reactions release heat when the products have lower heat content than the reactants, making Δrq negative
- Endothermic reactions absorb heat when products have higher heat content than reactants, making Δrq positive
Enthalpy (H)
- Enthalpy (H) is heat energy at constant pressure
- Many chemical and biochemical reactions occur without changes in the pressure of reactants or products, staying at atmospheric pressure
- Under these conditions, heat changes are described as enthalpy changes (ΔH)
- Δrq = ΔrH = Hproducts - Hreactants
Enthalpy Calculations
- For the reaction 2H2(g) + O2(g) → 2H2O(g), ΔrH = -483.6 kJ
- ΔrH = Hproducts - Hreactants, taking stoichiometry into account
- Given H(2 moles of H2O(g)) - H(2 moles of H2(g) plus 1 mole of O2(g)) = -483.6 kJ, the reaction releases heat
- Because the ΔrH value is negative, the reaction is exothermic, and the products' heat is less than the reactants'
- For 4H2(g) + 2O2(g) → 4H2O(g), the ΔrH would be double that of the original reaction, since all coefficients are doubled
Hess's Law
- Enthalpy is a state function, meaning the change in enthalpy in the conversion of A to B will be the same regardless of the number of steps in the process
- If a reaction is conducted in a series of steps, the ΔH for the overall reaction equals the sum of the enthalpy changes for each step
- The complete reaction of solid carbon with oxygen gas to make carbon monoxide is difficult, as carbon dioxide also forms
Applying Hess's Law
- To measure the enthalpy change for completely converting solid carbon to carbon dioxide the equation is C(s) + O2(g) → CO2(g) where ΔH = -393.5 kJ
- To measure the enthalpy change for converting carbon monoxide into carbon dioxide, the equation is CO(g) + 1/2O2(g) → CO2(g) where ΔH = -283.0 kJ
Calculating ΔH
- ΔH values from two reactions determine the ΔH value for the conversion of C(s) to CO(g)
- To calculate the conversion is C(s) + 1/2O2(g) → CO(g)
- The following steps are taken:
- C(s) + O2(g) → CO2(g) has a ΔH of -393.5 kJ
- CO2(g) → CO(g) + 1/2O2(g) has a ΔH of +283.0 kJ
- C(s) + 1/2O2(g) → CO(g) has a ΔH of (-393.5) + (+283.0) = -110.5 kJ
- Hess's Law states that if the equations can add to give the overall equation, then the ΔH values will add to give the overall equation's ΔH value
Rules of Hess's Law
- If one of the equations is reversed then the associated sign on the ΔH value must be changed
- If an equation is multiplied by a number, the ΔH value must also be multiplied by that number
Applying Hess’s Law: Example
- To determine the value of ΔH for the reaction 3C(s) + 4H2(g) → C3H8(g), utilize the following information:
- C(s) + O2(g) → CO2(g), ΔH = -394 kJ
- H2(g) + 1/2 O2(g) → H2O(l), ΔH = -286 kJ
- C3H8(g) + 5O2(g) → 4H2O(l) + 3CO2(g), ΔH = -2220 kJ
- Steps to the solution:
- Multiply the first equation by 3: 3C(s) + 3O2(g) → 3CO2(g), ΔH = 3 x (-394) = -1182 kJ
- Multiply the second equation by 4: 4H2(g) + 2O2(g) → 4H2O(l), ΔH = 4 x (-286) = -1144 kJ
- Swap the third equation around: 4H2O(l) + 3CO2(g) → C3H8(g) + 5O2(g), ΔH = +2220 kJ
- Therefore, 3C(s) + 4H2(g) → C3H8(g), ΔH = (-1182 + -1144 +2220) kJ = -106 kJ
Enthalpies of Formation
- Hess's Law helps calculate enthalpy changes for various sorts of chemical reactions
- ∆comH describes combustion reactions
- ∆fusH describes fusion (melting) reactions
- ∆vapH describes vaporisation (boiling) reactions
- Formation reactions describe the formation of 1 mole of a compound from its constituent elements in their standard states
Standard States
- Standard states are the most stable form of an element under 'normal' conditions
- Standard enthalpy of formation, also known as ΔfHo, refers to the formation reaction occurring under standard conditions, including having all gases at standard pressure (1 bar, ~1 atm)
- For example, ΔfHo (H2O(l)) refers to the equation H2(g) + 1/2O2(g) → H2O(l), where both gases are at 1 bar pressure, and 1 mol of H2O is formed
Calculating Enthalpies
- Tables of ΔfHo values are in data books and online and enable the calculation of ΔrHo values for almost any reaction
- An equation for this relationship is: ΔrHo = Σ[ΔfHo (products)] - Σ[ΔfHo (reactants)], while accounting for stoichiometry
- The standard enthalpy change for a reaction equals the sum of formation enthalpies of products minus the sum of formation enthalpies of reactants
Enthalpies of Formation
- Reaction: C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(g) to calculate the value of ΔrHo for the combustion of glucose
- 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
- Any element in its standard state will have ΔfHo value = 0 kJ mol-1
Enthalpies of Formation
- Consider the reaction is C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(g)
- ΔrHo = Σ(ΔfHo (products) - Σ(ΔfHo (reactants)
- ΔrHo = ((6 x ΔfHo (CO2(g)) + (6 x ΔfHo (H2O(g)) - (ΔfHo (C6H12O6(s)) + (6 x ΔfHo (O2(g))
- ΔrHo = ((6 x -393.5) + (6 x -241.8)) – (-1273 + (6 x 0))
- ΔrHo = -2538.8 kJ
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