Name: how bond energy predicts reaction heat
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 49 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Bond Energies and Bond Lengths

Ogni reazione chimica è associata ad una variazione dell'entalpia, che aiuta a determinare se l'energia viene rilasciata o richiesta durante la reazione. Questo cambiamento di entalpia può essere stimato utilizzando energie di legame medie. L'energia richiesta per rompere un legame specifico in 1 mole di un composto chimico gassoso è detta energia di legame ed è espressa in kilojoule per mole.

Questa energia dipende dal tipo di atomi legati e dal numero di coppie di elettroni condivise. L'energia di legame è solitamente espressa come media delle energie di legame dello stesso legame in composti diversi. L'energia di legame media può essere utilizzata per determinare se una reazione chimica è esotermica o endotermica.

Considerate che l'etene e il bromo reagiscano per formare 1, 2-dibromoetano. Inizialmente, il doppio legame del carbonio e il legame singolo del bromo si rompono, un processo che richiede un input di energia, che aumenta l'energia potenziale degli atomi. Pertanto, la rottura del legame è un processo endotermico con un cambiamento positivo dell'entalpia.

Successivamente, si formano nuovi legami fra atomi di carbonio e bromo che danno origine al prodotto. La formazione di legami aumenta la stabilità della molecola, riducendo l'energia potenziale. Pertanto, la formazione del legame è un processo esotermico che causa una variazione negativa dell'entalpia.

Secondo la legge di Hess, la somma delle variazioni di entalpia di reagenti e prodotti è uguale alla variazione di entalpia complessiva della reazione. La variazione di entalpia dei reagenti è la somma dell'entalpia dei legami rotti, mentre la variazione di entalpia dei prodotti è la somma dell'entalpia dei nuovi legami formati. Pertanto, la formazione di 1, 2-dibromoetano, con un'entalpia di 255 kilojoule per mole, è una reazione endotermica.

Oltre alle entalpie di legame, il tipo di legami e gli atomi legati influenzano anche la lunghezza del legame, che è la distanza media fra i nuclei di due atomi legati. Considerate i diversi legami fra due atomi di azoto e due atomi di carbonio. Gli atomi con più legami, come i tripli legami, sono tenuti più strettamente insieme, e questo porta a legami più brevi e più forti.

Di conseguenza, la molecola è più stabile e richiede una maggiore energia per dissociarsi. Generalmente, la forza del legame è indirettamente proporzionale alla lunghezza del legame, con alcune dovute eccezioni.

energie di legame e lunghezze di legame

Le molecole stabili esistono perch i legami covalenti tengono insieme gli atomi. La forza di un legame covalente si misura dall'energia necessaria per romperlo, cio dall'energia necessaria per separare gli atomi legati. Separare qualsiasi coppia di atomi legati richiede energia: pi forte un legame, maggiore l’energia necessaria per romperlo.

L'energia richiesta per rompere uno specifico legame covalente in una mole di molecole gassose chiamata energia di legame o energia di dissociazione del legame. L'energia di legame per una molecola biatomica, DX–Y, definita come la variazione di entalpia standard per la reazione endotermica:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-01.png" style="height: 40px;" />

Ad esempio, l'energia di legame del legame H–H covalente puro, DH–H, 436 kJ/mol di legami H–H rotti:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-02.png" style="height: 40px;" />

Le molecole con tre o pi atomi hanno due o pi legami. La somma di tutte le energie di legame in tale molecola uguale alla variazione di entalpia standard per la reazione endotermica che rompe tutti i legami nella molecola. Ad esempio, la somma delle quattro energie del legame C–H in CH_4, 1660 kJ, uguale alla variazione di entalpia standard della reazione:

<img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067Image_1.svg" style="height: 60px;" />

L’energia media del legame C–H, DC–H, 1660/4 = 415 kJ/mol perch ci sono quattro moli di legami C–H rotti per mole di reazione. Sebbene i quattro legami C – H siano equivalenti nella molecola originale, non richiedono ciascuno la stessa energia per rompersi; una volta rotto il primo legame (che richiede 439 kJ/mol), i restanti legami sono pi facili da rompere. Il valore di 415 kJ/mol la media, non il valore esatto richiesto per rompere un qualsiasi legame.

La forza di un legame tra due atomi aumenta all'aumentare del numero di coppie di elettroni nel legame. Generalmente, all’aumentare della forza del legame, la lunghezza del legame diminuisce. Pertanto, i tripli legami sono pi forti e pi corti dei doppi legami tra gli stessi due atomi; allo stesso modo, i doppi legami sono pi forti e pi corti dei singoli legami tra gli stessi due atomi. Quando un atomo si lega a vari atomi di un gruppo, la forza del legame tipicamente diminuisce man mano che si scende nel gruppo. Ad esempio, C–F 439 kJ/mol, C–Cl 330 kJ/mol e C–Br 275 kJ/mol.

L'energie di legame possono essere utilizzate per calcolare le variazioni approssimative di entalpia per le reazioni, chiamate anche entalpie di legame, dove le entalpie di formazione non sono disponibili. Calcoli di questo tipo diranno anche se una reazione esotermica o endotermica. Una reazione esotermica (ΔH negativo, calore prodotto) si verifica quando i legami nei prodotti sono pi forti dei legami nei reagenti. Una reazione endotermica (ΔH positivo, calore assorbito) si verifica quando i legami nei prodotti sono pi deboli di quelli nei reagenti.

La variazione di entalpia, ΔH, per una reazione chimica approssimativamente uguale alla somma dell'energia richiesta per rompere tutti i legami nei reagenti (energia “in?, segno positivo) pi l'energia rilasciata quando tutti i legami si sono composti formati nei prodotti (energia “fuori?, segno negativo). Ci pu essere espresso matematicamente nel modo seguente:

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-03.png" style="height: 40px;" />

In questa espressione, il simbolo ? significa “la somma di? e D rappresenta l'energia di legame in kJ/mol, che sempre un numero positivo. L’energia del legame dipender dal fatto che il particolare legame sia singolo, doppio o triplo. Pertanto, nel calcolare le entalpie in questo modo, importante considerare i legami in tutti i reagenti e prodotti. Poich i valori D sono tipicamente medie per un tipo di legame in molte molecole diverse, questo calcolo fornisce una stima approssimativa, non un valore esatto, dell'entalpia di reazione.

Consideriamo la seguente reazione:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-04.png" style="height: 40px;" />

O

<img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-05.png" style="height: 40px;" />

Per formare due moli di HCl, necessario rompere una mole di legami H–H e una mole di legami Cl–Cl. L'energia richiesta per rompere questi legami la somma dell’energia del legame H–H (436 kJ/mol) e del legame Cl–Cl (243 kJ/mol). Durante la reazione si formano due moli di legami H–Cl (energia di legame = 432 kJ/mol), liberando 2 432 kJ; o 864 kJ. Poich i legami nei prodotti sono pi forti di quelli nei reagenti, la reazione rilascia pi energia di quanta ne consuma:

<img alt="Eq6" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-06.png" style="height: 40px;" />

<img alt="Eq7" src="https://www.jove.com/files/core/CDNSource/lessons/1067/1067_Equations-07.png" style="height: 40px;" />

Questa energia in eccesso viene rilasciata sotto forma di calore, quindi la reazione esotermica.

Energie di legame e lunghezze di legame

Le molecole stabili esistono perché i legami covalenti tengono insieme gli atomi. La forza di un legame covalente è misurata dall'energia necessaria per romperlo, ...

Education

Didattica

JoVE Core

Chimica

Chemical Bonding: Basic Concepts

Legami chimici: concetti fondamentali

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

how bond energy predicts reaction heat

Bond energy helps explain why molecules stay together and how much heat a reaction may release or absorb. A covalent bond is stronger when more energy is needed to separate the atoms. The energy needed to break one specific covalent bond in one mole of gaseous molecules is called bond energy, or bond dissociation energy.

For a diatomic molecule, bond energy is the standard enthalpy change for an endothermic reaction that breaks that bond. The H-H bond is a common example. Its bond energy is 436 kJ/mol. Molecules with three or more atoms have several bonds, so the total bond energy is the sum of all the bonds that must be broken.

Methane shows why average bond energy is useful. Breaking all four C-H bonds in CH4 requires 1660 kJ, so the average C-H bond energy is 415 kJ/mol. That is an average value, not the exact energy for every single C-H bond. After the first bond breaks, which requires 439 kJ/mol, the remaining bonds are easier to break.

Bond strength usually increases as the number of electron pairs in the bond increases. As bond strength increases, bond length usually decreases. Triple bonds are therefore stronger and shorter than double bonds between the same atoms, and double bonds are stronger and shorter than single bonds. Bond strength can also decrease down a group. For example, C-F is 439 kJ/mol, C-Cl is 330 kJ/mol, and C-Br is 275 kJ/mol.

Bond energies can also estimate reaction enthalpy, especially when enthalpies of formation are not available. These calculations can show whether a reaction is exothermic or endothermic. An exothermic reaction has a negative ΔH and releases heat. An endothermic reaction has a positive ΔH and absorbs heat.

To estimate ΔH, add the energy needed to break all bonds in the reactants and subtract the energy released when bonds form in the products. In equation form, this is the sum of bond energies broken plus the sum of bond energies formed, with formed bonds counted as negative. Because bond energy values are usually averages for a bond type, the result is an estimate, not an exact value.

The formation of hydrogen chloride is a useful example. To make two moles of HCl, one mole of H-H bonds and one mole of Cl-Cl bonds must be broken. That requires 436 kJ/mol plus 243 kJ/mol. At the same time, two moles of H-Cl bonds form and release 864 kJ because each H-Cl bond has a bond energy of 432 kJ/mol. Since the product bonds are stronger than the reactant bonds, the reaction releases more energy than it uses and is exothermic.

How Bond Energy Predicts Reaction Heat

types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

the born-haber cycle

Predicting Ionic Stability with Lattice Energy

Lattice energy helps explain why ionic compounds are stable. It measures the strength of attraction between positive and negative ions in a crystal. In one common definition, lattice energy is the energy required to separate one mole of an ionic solid into gaseous ions, so it has a positive value. Another definition uses the opposite sign and describes the energy released when gaseous ions form a lattice. When checking data, it is important to confirm which convention is being used. In either...

lewis symbols and the octet rule

Valence Electrons, Lewis Symbols, and Bonding

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration. The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share...

electronegativity

Electronegativity and Polar Bonds

Electronegativity helps explain why some covalent bonds are nonpolar and others are polar covalent. It is a measure of how strongly an atom attracts shared electrons toward itself. That pull controls how electron density is distributed in a bond. Linus Pauling proposed the first electronegativity values by studying bond energies in heteronuclear molecules such as hydrogen fluoride. He expected the bond energy of HF to be about the average of the bond energies of H2 and F2, or 296 kJ/mol. The...

ionic bonding and electron transfer

Ionic Compounds and Crystal Lattices

Ionic compounds form when positive and negative ions attract each other. A cation is a positive ion, and it forms when an atom loses one or more valence electrons. An anion is a negative ion, and it forms when an atom gains one or more valence electrons. Compounds made of these ions are called ionic compounds, or salts, and the force between them is an ionic bond. The properties of ionic solids help show how strong this attraction is. They usually form rigid, brittle crystals and have high...

trends in lattice energy: ion size and charge

trends-in-lattice-energy-ion-size-and-charge

Lattice Energy Patterns in Ionic Solids

Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound. For ionic crystals, lattice energy follows a pattern based on charge and ion size. The...

covalent bonding and lewis structures

Shared Electrons in Covalent Molecules

Covalent molecules form when atoms share electrons instead of transferring them. This bonding usually happens between nonmetal atoms with similar tendencies to attract electrons. Because the atoms pull on the electrons in a shared way, the bond is held together by mutual attraction. Covalent compounds often have physical properties that differ from ionic compounds. The attraction between neutral molecules is weaker than the attraction between charged ions. As a result, many covalent compounds...

bond polarity, dipole moment, and percent ionic character

bond-polarity-dipole-moment-and-percent-ionic-character

How Bond Polarity Shapes Molecular Dipoles

Bond polarity depends on electronegativity, the ability of an atom to attract shared electrons. The absolute value of the electronegativity difference, or ΔEN, gives a rough guide to bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is larger, the bond may be polar covalent or ionic. Examples show how this works in real compounds. H–H has a ΔEN of 0, so it is nonpolar. H–Cl has a ΔEN of 0.9, so it is polar covalent. Na–Cl has a ΔEN of 2.1, so it...

lewis structures of molecular compounds and polyatomic ions

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Drawing Lewis Structures with Valence Electron Counts

Drawing Lewis structures starts with counting valence electrons and building the atom arrangement step by step. Valence electrons are the outer-shell electrons that help show how atoms bond. For ions, a positive charge means one electron is removed for each plus sign, and a negative charge means one electron is added for each minus sign. The next step is to sketch the skeleton structure. The least electronegative atom usually goes in the center, while hydrogen is almost never central. Fluorine...

resonance

Resonance Forms in Lewis Structures

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement. If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that...

formal charges

Lewis Structures with Formal Charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry. Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then...

exceptions to the octet rule

Octet Rule Exceptions in Lewis Structures

Some covalent molecules do not follow the octet rule in their Lewis structures. Their central atoms can fall into three groups: odd-electron molecules, electron-deficient molecules, and hypervalent molecules. These cases show why a Lewis structure may not always give every atom eight valence electrons. Odd-electron molecules have an odd number of valence electrons, so one electron remains unpaired. Molecules with an odd number of electrons are called radicals. Nitric oxide, NO, is one example,...

bond energies and bond lengths

Bond energy helps explain why molecules stay together and how much heat a reaction may release or absorb. A covalent bond is stronger when more energy is needed to separate the atoms. The energy needed to break one specific covalent bond in one mole of gaseous molecules is called bond energy, or bond dissociation energy. For a diatomic molecule, bond energy is the standard enthalpy change for an endothermic reaction that breaks that bond. The H-H bond is a common example. Its bond energy is...

bonding in metals

Electron Sea Model in Metals

Electron sea model explains how metallic bonding holds metals together. In this model, most metal atoms do not have enough valence electrons to form ionic or covalent bonds in the usual way. Their valence electrons are held loosely because metals have low electronegativity, and their ionization energy is low, so electrons are removed easily. When a metal atom loses these outer electrons, it becomes a positively charged metal ion. The freed electrons form a delocalized electron cloud around...

key terms and concepts

Learn how bond energy, bond length, and bond dissociation energy help predict whether reactions absorb or release heat, with examples like H-H, CH4, and HCl.

How Bond Energy Predicts Reaction Heat | JoVE K12

chemistry

JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-4

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Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Bonding and compounds

Structure And Properties Of Matter

Chemical bonding

Tennessee Academic Standards

Energy

CHEM1.PS3.3

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.d

Physics

PH.9

PH.9.e

Minnesota Academic Standards in Science

3.1.1 9C.2.2.1.1

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New Jersey Student Learning Standards for Science

Matter and Its Interactions

Alabama Course of Study Science

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Motion and Stability: Forces and Interactions

9-12.C.2.2.b

Next Generation Science Standards

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Chemistry and Physics

IPC.7

IPC.7.A

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South Carolina College and Career Ready Science Standards

C-PS1-4

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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-2.

HS-PS1-4.

HS-PS1-12.

Georgia Standards of Excellence

SC2.

Energy Changes in Chemical Reactions

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Subatomic Particles

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Elements: Chemical Symbols and Isotopes

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Use models to explain how the scientific understanding of atomic structure has evolved.

PS.Chm.1.1

Radioactivity and Nuclear Equations

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Types of Radioactivity

The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
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Nuclear Binding Energy

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Radioactive Decay and Radiometric Dating

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Nuclear Fission

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Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

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Nuclear Transmutation

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Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

PS.Chm.1.2

Atomic Orbitals

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The Pauli Exclusion Principle

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The Energies of Atomic Orbitals

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The Aufbau Principle and Hund's Rule

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Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

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Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

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Ionization Energy

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

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Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

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Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

Analyze the structure of atoms and isotopes.

How Bond Energy Predicts Reaction Heat

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