Exploring Heat Content (Molar Enthalpy) in Thermodynamics and Chemical Reactions
Heat Content (molar enthalpy)
Introduction:
Heat content, also known as molar enthalpy, is a fundamental concept in thermodynamics and chemistry. It represents the total energy stored within a substance, accounting for the internal energy of its atoms, molecules, and intermolecular forces. Understanding heat content is crucial for analyzing chemical reactions, determining their spontaneity, and predicting their outcomes.
Heat content:
-Each chemical substance has a different number and type of atoms and different type of bonds between its atoms so it has a specific amount of energy called internal energy.
-The internal energy of a chemical substance is the summation of energies stored in it.
1-Stored chemical energy in the atom
It is represented in the energy of electrons in the energy level Energy of electron = kinetic energy + potential energy.
2-Stored chemical energy in the molecule
It is the energy of chemical bonds between its atoms ionic or covalent.
3- Intermolecular forces
The attraction force between molecules is called Van der Waals force
4-Hydrogen bond
If the compound is polar and has hydrogen in its structure.
The summation of these energies are called Heat content
Heat content of a substance (molar enthalpy) H:
The sum of the stored energy in one mole of a substance.
• Heat content for the element = zero
Heat content change (∆H):
The difference between the sum of the heat content of the products and the sum of the heat content of the reacting substances.
Heat content = Heat content of products – Heat content of reactants
ᅀ H = ∑H Products – ∑H reactants
Standard heat content ΔH0= q/n
Comparison of values of different reactions under standard conditions
The standard conditions are:
-Pressure = 1 atm
-Temperature = 250C
-Solution concentration 1 M
Thermo chemical equation
It is a symbolic chemical equation that includes the heat change accompanying the chemical reaction and this heat change is represented in the equation as one of the reactants or products.
Example: Calculate the change in heat content Δ H resulted from the decomposition of 136 g of ammonia gas under constant pressure to give hydrogen and nitrogen gases.
2NH3 >>> N2 + 3H2 ΔH = 92.2 kJ
Answer:
Molar mass of NH3 = 14 + (3×1) = 17 g/mol
No. of moles of NH3 =136/17 = 8 mol
From the equation:
2mol of ammonia need 92.2 KJ
so 8mol neeed ??
ΔH=368.8 KJ
Types of chemical reactions in thermochemistry:
Exothermic reactions :
- Release energy.
Heat transfer from the system to the surrounding. - Heat content of product less than reactants H.
- Heat content has negative sign.
Heat content of the products > Heat content of the reactants
Endothermic reactions:
- -Absorb energy.
- -Heat transfer from the surrounding to the System.
- -Heat content of reactants are less than the heat conent of products
- -H has positive sign.
- -Heat content of the products < Heat content of reactants
Bond energy:
-It is the amount of energy absorbed to break the bonds or released during formation of bonds in one mole of the substance
- -Breaking of bonds is endothermic reaction( absorb energy from the surrounding) .
- -Formation of bonds is exothermic reaction (energy of the surrounding increases).
- • Energy must be absorbed to break the bond or energy released when the bond is formed in one mole of the substances
Example:
Calculate the heat of the following reaction and determine if the reaction is exothermic or endothermic.
CH4(g) + 2O2(g) >>> CO2(g)+ 2H2O(g)
Knowing that the bond energy is estimated by the unit (KJ/mol) as follows:
{C 〓 O)745, (O – H) 467, (C 〓 H) 413 , (O=O)498}
The energy required to break reactant bonds = [ 4 × (C – H) ] + [2 × (O O)]
=[4 × 413] + [2 × 498]=2648 KJ
The energy released from formation of bonds in the products = [2 × (C〓O)]
+[2 × 2(O H) ] =[2 × 745] + [2×2×467]=3358KJ
ᅀH = (PRODUCT + REACTION)
= (-3358) +2648 = -710 KJ
The reaction is exothermic because H is negative
Quiz: Understanding Heat Content (Molar Enthalpy)
1-What contributes to the internal energy of a chemical substance?
a) Stored chemical energy in the atom
b) Stored chemical energy in the molecule
c) Intermolecular forces
d) All of the above
Explanation: Internal energy includes energies from the atom, molecule, and intermolecular forces.
2-What is the definition of heat content (molar enthalpy)?
a) The energy stored in one mole of a substance
b) The energy required to break bonds in a substance
c) The energy released during bond formation
d) The energy transferred between a system and its surroundings
Explanation: Heat content is the sum of the stored energy in one mole of a substance.
3-Which condition yields a standard heat content?
a) Pressure = 1 atm, Temperature = 25°C, Solution concentration = 1 M
b) Pressure = 1 atm, Temperature = 100°C, Solution concentration = 1 M
c) Pressure = 0.5 atm, Temperature = 25°C, Solution concentration = 1 M
d) Pressure = 1 atm, Temperature = 25°C, Solution concentration = 0.5 M
Explanation: Standard conditions include pressure of 1 atm, temperature of 25°C, and solution concentration of 1 M.
4-Which type of reaction releases energy to the surroundings?
a) Endothermic
b) Exothermic
c) Isobaric
d) Isochoric
Explanation: Exothermic reactions release energy to the surroundings.
5-What is bond energy?
a) The energy required to break bonds
b) The energy released during bond formation
c) The energy stored in intermolecular forces
d) The energy stored in the nucleus
Explanation: Bond energy is the amount of energy absorbed to break bonds or released during formation.
6-Which reaction is exothermic?
a) CH4(g) + 2O2(g) >>> CO2(g) + 2H2O(g)
b) 2NH3 >>> N2 + 3H2
c) H2(g) + Cl2(g) >>> 2HCl(g)
d) 2H2O(l) >>> 2H2(g) + O2(g)
Explanation: The reaction CH4(g) + 2O2(g) >>> CO2(g) + 2H2O(g) is exothermic because it releases energy, indicated by a negative change in enthalpy.
Calculate the change in heat content (∆H) for the reaction:
CH4(g) + 2O2(g) >>> CO2(g) + 2H2O(g)
Given bond energies: {C 〓 O)745, (O – H) 467, (C 〓 H) 413, (O=O)498}
Explanation: Calculate the energy required to break bonds in reactants and released during bond formation in products.
7-What is the relation between heat content of products and reactants in an exothermic reaction?
a) Heat content of products is greater
b) Heat content of reactants is greater
c) Heat content of products and reactants are equal
d) Heat content is not related to the type of reaction
Explanation: In an exothermic reaction, the heat content of products is less than that of reactants.
8-Which type of bond contributes to the stored chemical energy in a molecule?
a) Ionic bond
b) Covalent bond
c) Metallic bond
d) Hydrogen bond
Explanation: Covalent and ionic bonds contribute to the stored chemical energy in a molecule.
9-What is the significance of standard conditions in determining standard heat content?
a) They ensure consistency in experimental measurements
b) They represent ideal gas behavior
c) They account for varying environmental factors
d) They minimize the effects of temperature changes
Explanation: Standard conditions provide a consistent basis for comparing heat content values.
10-Which factor determines whether a reaction is exothermic or endothermic?
a) Change in temperature
b) Change in pressure
c) Change in volume
d) Change in heat content (∆H)
Explanation: The sign of the change in heat content (∆H) determines whether a reaction is exothermic or endothermic.
11-How does the magnitude of bond energy relate to bond strength?
a) Higher bond energy indicates stronger bonds
b) Higher bond energy indicates weaker bonds
c) Bond energy is not related to bond strength
d) Bond energy is inversely proportional to bond strength
Explanation: Higher bond energy indicates stronger bonds, as more energy is required to break them.
12-What is the enthalpy change for the reverse of an exothermic reaction?
a) It remains the same
b) It becomes positive
c) It becomes negative
d) It depends on the specific reaction
Explanation: The enthalpy change for the reverse of an exothermic reaction is positive.
13-How does the molar enthalpy of an element compare to its compounds?
a) Molar enthalpy of an element is always higher
b) Molar enthalpy of an element is always lower
c) Molar enthalpy of an element is zero
d) Molar enthalpy of an element is negative
Explanation: Molar enthalpy of an element is zero because it does not form compounds in its standard state.
14-What role does the type of intermolecular force play in determining a substance’s heat content?
a) It affects the boiling point of the substance
b) It influences the rate of reaction
c) It contributes to the internal energy of the substance
d) It determines the color of the substance
Explanation: Intermolecular forces contribute to the internal energy of a substance and, consequently, its heat content.
15-What is the relationship between the heat content change (∆H) and the direction of a chemical reaction?
a) ∆H determines the direction of the reaction
b) The direction of the reaction determines ∆H
c) ∆H is independent of the reaction direction
d) ∆H is only relevant in reversible reactions
Explanation: The direction of the reaction determines the sign (positive or negative) of ∆H.
16-Which type of bond is typically stronger: covalent or hydrogen bonds?
a) Covalent bonds
b) Hydrogen bonds
c) Both are equally strong
d) It depends on the specific molecules involved
Explanation: Covalent bonds are typically stronger than hydrogen bonds.
17-In an endothermic reaction, what happens to the heat content of the system?
a) It decreases
b) It remains constant
c) It increases
d) It fluctuates
Explanation: In an endothermic reaction, heat is absorbed by the system, leading to an increase in its heat content.
18-How does the stoichiometry of a reaction affect the calculation of heat content change (∆H)?
a) It does not affect the calculation of ∆H
b) It affects the sign of ∆H
c) It affects the magnitude of ∆H
d) It only matters in non-stoichiometric reactions
Explanation: The stoichiometry of a reaction determines the coefficients used in the calculation of ∆H.
19-What does a positive standard heat content (∆H°) indicate about a reaction?
a) The reaction is spontaneous
b) The reaction is exothermic
c) The reaction is endothermic
d) The reaction has reached equilibrium
Explanation: A positive ∆H° indicates that the reaction is endothermic under standard conditions.
20-How does the magnitude of ∆H relate to the extent of a chemical reaction?
a) There is no relationship between ∆H and the extent of the reaction
b) Larger ∆H values correspond to slower reactions
c) Larger ∆H values correspond to more complete reactions
d) Smaller ∆H values correspond to more complete reactions
Explanation: Larger ∆H values generally correspond to more complete reactions.
Therefore, the change in heat content (∆H) for the reaction is -572 kJ/mol.
Problem:
Given the following thermochemical equation:
2C(s) + 2H2(g) -> C2H4(g)
Calculate the standard heat content change (∆H°) if the bond energies are as follows: C-C = 348 kJ/mol, H-H = 436 kJ/mol, C-H = 413 kJ/mol, C=C = 614 kJ/mol.
Solution:
Energy required to break bonds in reactants = [2(C-C) + 2(H-H)]
= [2(348 kJ/mol) + 2(436 kJ/mol)]
= 696 kJ/mol + 872 kJ/mol
= 1568 kJ/mol
Energy released from formation of bonds in products = [1(C-C) + 4(C-H)]
= [1(348 kJ/mol) + 4(413 kJ/mol)]
= 348 kJ/mol + 1652 kJ/mol
= 2000 kJ/mol
∆H° = (Energy released from formation of bonds) – (Energy required to break bonds)
= 2000 kJ/mol – 1568 kJ/mol
= 432 kJ/mol
Therefore, the standard heat content change (∆H°) for the reaction is 432 kJ/mol.