Podcast
Questions and Answers
What is the primary reason why ΔH and ΔE can differ in a chemical reaction?
What is the primary reason why ΔH and ΔE can differ in a chemical reaction?
What is the expression for calculating the change in enthalpy (ΔH) for a chemical reaction?
What is the expression for calculating the change in enthalpy (ΔH) for a chemical reaction?
Under which condition is the change in internal energy (ΔE) equivalent to the change in enthalpy (ΔH)?
Under which condition is the change in internal energy (ΔE) equivalent to the change in enthalpy (ΔH)?
Which of the following defines the term 'Enthalpy' in a system?
Which of the following defines the term 'Enthalpy' in a system?
Signup and view all the answers
What happens to the term PΔV for reactions studying gases at constant temperature and pressure?
What happens to the term PΔV for reactions studying gases at constant temperature and pressure?
Signup and view all the answers
What is the heat of formation (ΔHF)?
What is the heat of formation (ΔHF)?
Signup and view all the answers
Which statement is true regarding the heat of combustion (ΔHc)?
Which statement is true regarding the heat of combustion (ΔHc)?
Signup and view all the answers
What does the heat of solution (ΔHsolution) represent?
What does the heat of solution (ΔHsolution) represent?
Signup and view all the answers
What is the correct interpretation of the sign of ΔH in a heat of combustion reaction?
What is the correct interpretation of the sign of ΔH in a heat of combustion reaction?
Signup and view all the answers
Which of the following is NOT a type of enthalpy change?
Which of the following is NOT a type of enthalpy change?
Signup and view all the answers
What is the heat of formation of methane at constant volume?
What is the heat of formation of methane at constant volume?
Signup and view all the answers
Which of the following represents an exothermic reaction?
Which of the following represents an exothermic reaction?
Signup and view all the answers
What occurs when a thermochemical equation is reversed?
What occurs when a thermochemical equation is reversed?
Signup and view all the answers
What is the physical state symbol for a liquid in thermochemical equations?
What is the physical state symbol for a liquid in thermochemical equations?
Signup and view all the answers
In a balanced thermochemical equation, what must be mentioned?
In a balanced thermochemical equation, what must be mentioned?
Signup and view all the answers
How is the enthalpy change (ΔH) affected if both sides of the reaction are multiplied by a factor n?
How is the enthalpy change (ΔH) affected if both sides of the reaction are multiplied by a factor n?
Signup and view all the answers
What is the value of ΔH for the combustion of 1 mole of methane at 25°C?
What is the value of ΔH for the combustion of 1 mole of methane at 25°C?
Signup and view all the answers
Which of the following is NOT a requirement for a thermochemical equation?
Which of the following is NOT a requirement for a thermochemical equation?
Signup and view all the answers
What does a negative value of ΔH indicate about a reaction?
What does a negative value of ΔH indicate about a reaction?
Signup and view all the answers
In the equation ΔH = ΔE + PΔV, what does PΔV account for?
In the equation ΔH = ΔE + PΔV, what does PΔV account for?
Signup and view all the answers
Which of the following describes an endothermic reaction?
Which of the following describes an endothermic reaction?
Signup and view all the answers
What happens to the enthalpy change (ΔH) when the heat of combustion reaction is conducted at constant volume?
What happens to the enthalpy change (ΔH) when the heat of combustion reaction is conducted at constant volume?
Signup and view all the answers
Which process is an example of an exothermic reaction?
Which process is an example of an exothermic reaction?
Signup and view all the answers
Under what condition is ΔH equal to zero?
Under what condition is ΔH equal to zero?
Signup and view all the answers
How can you calculate the change in volume (ΔV) from a general reaction?
How can you calculate the change in volume (ΔV) from a general reaction?
Signup and view all the answers
What does it mean when ΔH is greater than zero?
What does it mean when ΔH is greater than zero?
Signup and view all the answers
Study Notes
Physical Chemistry Lecture 3 Notes (F/R-010)
- Professor: Dr. Hossieny Ibrahim
- Course: Physical Chemistry
- Institution: Badr University in Assiut, School of Biotechnology
- Contact: [email protected], Bio-326
Unit 1: Enthalpy of a Reaction
- Thermochemical measurements are made at constant volume or constant pressure.
- The magnitudes of changes vary between these two conditions.
- Internal energy (ΔΕ) is the heat change accompanying a reaction at constant volume.
- No external work is done at constant volume.
- Most chemical reactions in labs occur at constant pressure.
- Enthalpy (ΔΗ) is a new term introduced by chemists to study heat changes at constant pressure and temperature.
Unit 1: Enthalpy of a Reaction (Cont'd)
- Enthalpy (H) of a system is the sum of internal energy (E) and the product of pressure (P) and volume (V).
- H = E + PV
- Like internal energy, enthalpy is a state function, and its absolute value cannot be determined directly.
- However, the change in enthalpy (ΔH) accompanying a process is measurable.
- The change in enthalpy during a reaction is calculated using the following expression: ΔH = Hproducts - Hreactants
Unit 1: Enthalpy of a Reaction (Cont'd)
- For reactions involving solids and liquids, the change in volume (ΔV) is very small, and the term PΔV becomes negligible.
- ΔH ≈ ΔE
- For gas-phase reactions, the term PΔV is substantial and needs consideration in enthalpy calculations.
- ΔH = ΔE + PΔV
Unit 1: Exothermic and Endothermic Reactions
- A general chemical reaction (A + B → C + D) at constant pressure is considered.
- Enthalpies of reactants (HA, HB) and Products (HC, HD) are defined.
- ΔH = Hproducts - Hreactants = (HC + HD) – (HA +HB)
- ΔH value can be zero, negative, or positive.
- ΔH = 0: Hproducts = Hreactants (no heat absorbed or evolved).
- ΔH < 0 (Negative): Hproducts < Hreactants (heat evolved, exothermic reaction).
- ΔH > 0 (Positive): Hproducts > Hreactants (heat absorbed, endothermic reaction).
Unit 1: Exothermic and Endothermic Reactions (Cont'd)
- Examples of exothermic processes include mixing water and strong acids, mixing water with calcium chloride, rusting iron, and freezing.
- Examples of endothermic processes include melting solid salts, evaporation of water, sublimation, mixing water and ammonium nitrate.
Unit 1: Calculation of ΔH from ΔE
- ΔH = ΔE + PΔV
- Change in number of moles (Δn) is defined for a reaction (aA + bB → cC + dD) as Δn = (c + d) - (a + b).
- PΔV = RTΔn (where R is the ideal gas constant, and T is temperature).
- Substituting for PΔV in the ΔH equation, gives: ΔH = ΔE + ΔnRT
Unit 1: Solved Problem
- Heat of combustion of ethylene is calculated under both constant volume and constant pressure.
- ΔH is determined using the relation ΔH = ΔE + ΔnRT.
Unit 1: Solved Problem 2
- Calculating the heat of combustion of carbon monoxide from the value of heat at constant volume.
- The standard equation for the reaction is also mentioned
Unit 1: Thermochemical Equations
- Thermochemical equations represent the heat changes (evolved or absorbed) in a reaction or process.
- The equations should be balanced.
- The value of ΔH should be specified along with the physical states of reactants and products: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous states.
Unit 1: Thermochemical Equations (cont.)
- The sign of ΔH indicates whether the reaction is exothermic or endothermic – Positive ΔH is endothermic, and negative ΔH is exothermic.
Unit 1: Different Types of Heat (Enthalpy) of Reaction
- Heat of Formation (ΔHf): The change in enthalpy when one mole of a compound forms from its constituent elements.
- Heat of Combustion (ΔHc): The change in enthalpy when one mole of a substance is completely burned in excess oxygen.
- Heat of Solution (ΔHsolution): The change in enthalpy when one mole of a substance is dissolved in a specific amount of solvent.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
Explore the key concepts of enthalpy and internal energy as discussed in Professor Hossieny Ibrahim's Physical Chemistry class. This lecture covers thermochemical measurements and the significance of constant volume versus constant pressure in chemical reactions.
