Why is the term "mean" used when referring to bond enthalpies?

Because the same type of bond has slightly different bond enthalpies in different molecular environments, so an average value across many compounds is used.

Is breaking bonds an endothermic or exothermic process?

Endothermic (energy is required to break bonds).

Is forming bonds an endothermic or exothermic process?

Exothermic (energy is released when bonds form).

AQA A-Level Chemistry 7405

Atomic Structure

Topic overview Fundamental Particles Mass Number and Isotopes Time of Flight Mass Spectrometry: Ionisation and Acceleration Time of Flight Mass Spectrometry: Drift, Detection and Calculating m/z Electron Configuration: Sub-shells and Orbitals Electron Configuration: Writing Configurations for Atoms and Ions Ionisation Energies and Periodic Trends

Amount of Substance

Topic overview Relative Atomic and Molecular Mass The Mole and the Avogadro Constant The Ideal Gas Equation Empirical and Molecular Formula Balanced Equations and Reaction Calculations Atom Economy and Percentage Yield Acid-Base Titrations: Principles, Indicators and Apparatus Acid-Base Titrations: Standard Solutions and Calculations

Bonding

Topic overview Ionic Bonding Covalent, Dative Covalent and Metallic Bonding Crystal Structures: Giant Ionic Lattices and Diamond Crystal Structures: Graphite and Simple Molecular Solids Shapes of Molecules and Ions: VSEPR and Electron Pair Repulsion Shapes of Molecules and Ions: Predicting Shapes and Bond Angles Electronegativity and Bond Polarity Intermolecular Forces

Energetics and Thermodynamics

Topic overview Enthalpy Change and Standard Conditions Calorimetry and Heat Change Calculations Hess's Law and Enthalpy Cycles Mean Bond Enthalpies Lattice Enthalpy and Born-Haber Cycles Enthalpies of Solution and Hydration Entropy Change Gibbs Free Energy and Reaction Feasibility

Kinetics

Topic overview Collision Theory and Activation Energy Maxwell-Boltzmann Distribution and the Effect of Temperature Effect of Concentration and Pressure, and Catalysts Rate Equations: The Rate Equation and Orders of Reaction Rate Equations: The Effect of Concentration on Rate The Rate Constant and the Arrhenius Equation Determining Rate Equations: The Initial Rates Method Determining Rate Equations: Continuous Monitoring Rate Equations and Reaction Mechanisms

Chemical Equilibria

Topic overview Dynamic Equilibrium and Le Chatelier's Principle Industrial Compromise Conditions The Equilibrium Constant Kc Partial Pressures and Mole Fractions The Equilibrium Constant Kp

Redox and Electrochemistry

Topic overview Redox: Definitions and the Rules for Oxidation States Redox: Identifying Oxidation and Reduction in a Reaction Half-Equations and Combined Redox Equations Standard Electrode Potentials and the Hydrogen Electrode Electrochemical Cells and EMF Calculations Predicting Redox Reactions from Electrode Potentials Commercial Cells and Fuel Cells

Acids and Bases

Topic overview Brønsted-Lowry Acid-Base Equilibria The pH Scale and Strong Acids The Ionic Product of Water and Strong Bases Weak Acids and Ka pH Curves and Acid-Base Titrations Indicators Buffer Solutions

Periodicity and Group Chemistry

Topic overview s, p, d and f Block Classification Trends in Period 3 Group 2 Trends in Atomic Radius, Ionisation Energy and Melting Point Reactions of Group 2 with Water Solubility Trends of Group 2 Hydroxides Solubility Trends of Group 2 Sulfates Uses of Group 2 Compounds Test for Sulfate Ions Trends in Electronegativity and Boiling Point of Halogens Halogens as Oxidising Agents and Displacement Reactions Halide Ions as Reducing Agents: Chloride and Bromide Halide Ions as Reducing Agents: Iodide, and the Equations Test for Halide Ions Using Silver Nitrate Tests for Ammonium, Group 2, Hydroxide and Carbonate Ions Reactions of Chlorine with Water Reactions of Chlorine with Cold Dilute NaOH and Water Treatment

Period 3 Elements and Their Oxides

Topic overview Reactions of Na and Mg with Water Period 3 Elements with Oxygen: The Oxides Formed and Their Bonding Period 3 Elements with Oxygen: Structures and Oxidation States Melting Points of Period 3 Oxides Period 3 Oxides with Water: Sodium and Magnesium Oxides Period 3 Oxides with Water: The Acidic Oxides and the pH Trend Period 3 Oxides with Acids and Bases: The Reactions Period 3 Oxides with Acids and Bases: Acidic, Basic and Amphoteric

Transition Metals

Topic overview General Properties of Transition Metals Complex Ions: Ligands, Coordinate Bonds and Denticity Complex Ions: Why a Species Can Act as a Ligand, and Coordination Number Substitution Reactions with Monodentate Ligands Substitution Reactions with Bidentate and Multidentate Ligands Haemoglobin and the Chelate Effect Shapes of Complex Ions and Stereoisomerism Formation of Coloured Ions and the d-d Transition Colorimetry and Concentration Determination Variable Oxidation States and Vanadium Chemistry Tollens' Reagent and Redox Titrations Heterogeneous Catalysis Vanadium(V) Oxide and the Contact Process Homogeneous Catalysis and Autocatalysis Metal-Aqua Ions and Acidity Metal Aqua Ions with Bases: Reactions with Hydroxide and Ammonia Metal Aqua Ions with Bases: Why They React, and Their Equations Amphoteric Hydroxides Identifying Transition Metal Ions by Test-Tube Reactions

Introduction to Organic Chemistry

Topic overview Nomenclature and Formula Representations Homologous Series and General Formulae IUPAC Naming: Stems, Prefixes and Suffixes Free Radical and Curly Arrow Mechanisms Structural Isomerism E-Z Stereoisomerism and CIP Priority Rules Chirality and Enantiomers Racemic Mixtures and Optical Activity

Hydrocarbons

Topic overview Alkanes, Fractional Distillation and Cracking Combustion of Alkanes and Pollutants Free Radical Chlorination of Alkanes Alkenes: Structure, Bonding and Electrophilic Addition Markovnikov's Rule and Carbocation Stability Addition Polymers and Repeating Units Properties and Uses of Polyalkenes

Halogenoalkanes, Alcohols and Organic Analysis

Topic overview Nucleophilic Substitution of Halogenoalkanes Effect of Bond Enthalpy on Reaction Rate Elimination Reactions and Competing Mechanisms CFCs and Ozone Depletion Industrial Production of Alcohols Biofuels and Carbon Neutrality Classification and Oxidation of Alcohols Distinguishing Aldehydes and Ketones Elimination of Water from Alcohols Test-Tube Reactions for Functional Groups Mass Spectrometry and Molecular Formula Infrared Spectroscopy, Functional Groups and Global Warming

Carbonyls, Carboxylic Acids and Derivatives

Topic overview Oxidation of Aldehydes and Distinguishing Tests Reduction of Carbonyls with NaBH4 Nucleophilic Addition with KCN and Hydroxynitrile Formation Hazards of KCN and Formation of Enantiomers Structure and Acidity of Carboxylic Acids Esterification and the Uses of Esters Vegetable Oils, Fats and the Hydrolysis of Esters Soap and Biodiesel Production from Fats Structure of Acid Anhydrides, Acyl Chlorides and Amides Nucleophilic Addition-Elimination Reactions of Acyl Chlorides Acid Anhydrides and the Industrial Synthesis of Aspirin Purifying organic solids and liquids

Aromatic Chemistry and Amines

Topic overview Bonding and Stability of Benzene Electrophilic Substitution: Nitration Electrophilic Substitution: Friedel-Crafts Acylation Preparation of Aliphatic and Aromatic Amines Base Strength of Amines Amines as Nucleophiles, and Quaternary Ammonium Salts Amines with Acyl Chlorides and Acid Anhydrides

Polymers, Amino Acids, Proteins and DNA

Topic overview Condensation Polymers: Polyesters Condensation Polymers: Polyamides Intermolecular Forces in Condensation Polymers Biodegradability and Disposal of Polymers Amino Acids and Zwitterions Peptide Bonds and Protein Structure Hydrogen Bonding and Disulfide Bridges in Proteins Hydrolysis of Proteins and TLC Identification Enzymes, Active Sites and Inhibitors in Drug Design DNA: Nucleotides, the Double Helix and Base Pairing Cisplatin and Anticancer Drug Action

Organic Synthesis, NMR and Chromatography

Topic overview Multi-Step Synthesis Routes Green Chemistry Principles in Synthesis Design Principles of NMR and the Delta Scale Carbon-13 NMR Interpretation Proton NMR Chemical Shift and Integration Spin-Spin Splitting and the n+1 Rule TMS and Deuterated Solvents Thin-Layer Chromatography and Rf Values Column and Gas Chromatography, and GC-MS

Energetics and Thermodynamics

Mean Bond Enthalpies

3 things to know
Papers 1 and 3

Overview

Why bond enthalpies are averages, and estimating enthalpy change from bonds broken and made.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is the definition of mean bond enthalpy?

Key term

What physical state must reactants and products be in when using mean bond enthalpies to calculate enthalpy changes?

Key term

In a reaction, what happens to bonds in the reactants and what happens to form products?

Questions from this subtopic

Each one with the model answer underneath.

1
What is the definition of mean bond enthalpy?
Model answer
The enthalpy change when one mole of bonds in a gaseous molecule is broken, averaged over a range of different compounds containing that bond.
2
Why is the term "mean" used when referring to bond enthalpies?
Model answer
Because the same type of bond has slightly different bond enthalpies in different molecular environments, so an average value across many compounds is used.
3
What physical state must reactants and products be in when using mean bond enthalpies to calculate enthalpy changes?
Model answer
The gaseous phase.
4
In a reaction, what happens to bonds in the reactants and what happens to form products?
Model answer
Bonds in reactants are broken and new bonds are formed in products.
5
Is breaking bonds an endothermic or exothermic process?
Model answer
Endothermic (energy is required to break bonds).
6
Is forming bonds an endothermic or exothermic process?
Model answer
Exothermic (energy is released when bonds form).

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Atomic Structure

Amount of Substance

Bonding

Energetics and Thermodynamics

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AOxidation is the loss of electrons.

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