Chemical Energetics

The One Thing

Chemical reactions involve energy transfer because breaking bonds requires energy whereas forming bonds releases energy. The balance between these processes determines whether a reaction is exothermic or endothermic, while combustion and electrochemical cells provide important applications of chemical energy transfer.

Reactions, Processes and Experiments

Calorimetry results may differ from the true energy change because heat can escape to the surroundings, combustion may be incomplete, or the fuel may not burn completely. If the reacting solution has density approximately 1 g/cm³, its mass in grams is often estimated as equal to its volume in cm³.

Key Terms

• Chemical energetics: The study of energy changes that occur during chemical reactions.
• Exothermic reaction: A reaction that releases energy to the surroundings, usually causing the temperature of the surroundings to increase.
• Endothermic reaction: A reaction that absorbs energy from the surroundings, usually causing the temperature of the surroundings to decrease.
• Surroundings: Everything outside the reacting chemicals, including the container, air and measuring equipment.
• Activation energy: The minimum energy that reacting particles must have for a reaction to occur.
• Energy-level diagram: A diagram showing the energy of reactants and products and the overall energy change during a reaction.
• Enthalpy change: The heat energy change of a reaction under constant pressure, commonly represented by ΔH.
• Bond breaking: An endothermic process because energy is needed to separate bonded atoms.
• Bond making: An exothermic process because energy is released when new bonds form.
• Combustion: A reaction in which a substance burns in oxygen and releases energy.
• Fuel: A substance that releases useful energy when it reacts, commonly by combustion.
• Complete combustion: Burning in a plentiful supply of oxygen to form carbon dioxide and water from a hydrocarbon.
• Incomplete combustion: Burning with insufficient oxygen, producing carbon monoxide and/or carbon alongside water.
• Renewable fuel: A fuel made from resources that can be replaced naturally in a relatively short time, such as bioethanol or biogas.
• Non-renewable fuel: A fuel formed over millions of years and available in limited amounts, such as coal, petroleum and natural gas.
• Electrochemical cell: A device in which a chemical reaction produces an electrical current.
• Electrode: A conducting material through which electrons enter or leave an electrochemical cell.
• Electrolyte: An ionic substance or solution that conducts electricity because it contains mobile ions.
• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Redox reaction: A reaction in which oxidation and reduction happen at the same time.
• Battery: One or more electrochemical cells connected together to provide electrical energy.
• Fuel cell: An electrochemical cell supplied continuously with fuel and an oxidant, producing electricity while reactants are available.

Easily Confused

• Exothermic and endothermic reactions: Exothermic reactions release energy and usually increase the surroundings’ temperature; endothermic reactions absorb energy and usually decrease it.
• Reactants and products on an energy-level diagram: Reactants are the starting substances; products are the substances formed, with products lower than reactants in an exothermic reaction and higher in an endothermic reaction.
• Bond breaking and bond making: Breaking bonds requires energy; forming bonds releases energy.
• Complete and incomplete combustion: Complete combustion occurs in plentiful oxygen and produces carbon dioxide and water; incomplete combustion occurs with insufficient oxygen and may produce carbon monoxide and/or carbon alongside water.
• Renewable and non-renewable fuels: Renewable fuels derive from resources replaced naturally in a relatively short time; non-renewable fuels form over millions of years and are limited.
• Primary and rechargeable cells: A primary cell is designed for single use; a rechargeable cell can be restored by passing an electric current through it.
• Oxidation and reduction: Oxidation is loss of electrons; reduction is gain of electrons.
• An electrochemical cell and a battery: An electrochemical cell is a device in which a chemical reaction produces an electrical current; a battery consists of one or more electrochemical cells connected together.
• A battery and a fuel cell: A battery contains one or more electrochemical cells, whereas a fuel cell is supplied continuously with fuel and an oxidant and produces electricity while reactants are available.

What Gets Asked

• Classifying a reaction as exothermic or endothermic: Use the temperature change of the surroundings, the relative energy of reactants and products, and the sign of ΔH. A temperature increase, lower-energy products and negative ΔH indicate an exothermic reaction; the reverse indicates an endothermic reaction.
• Interpreting or drawing an energy-level diagram: Include the relative energies of reactants and products, the activation energy, and the overall energy change. The activation energy is the difference between the reactants and the highest point of the pathway.
• Calculating energy changes from calorimetry: Apply q = mcΔT, use the stated or estimated mass, and calculate molar enthalpy change using q ÷ amount of reacting substance in moles. A common error is confusing solution volume with mass when the density assumption has not been applied.
• Calculating enthalpy change from bond energies: Use ΔH = total energy needed to break bonds − total energy released when bonds form. The distinction between energy absorbed for bond breaking and energy released for bond making determines the sign.
• Comparing fuels: Consider energy output, efficiency, cost, safety, availability, renewability and environmental effects. Fossil fuels are energy-rich but non-renewable and polluting; renewable fuels may reduce some environmental impacts but can require land, water or energy to produce.
• Explaining electrochemical cells and fuel cells: State that coupled oxidation and reduction reactions generate electrical energy, that electrons move through the external circuit from the more reactive electrode to the less reactive electrode, and that the electrolyte permits ion movement. For hydrogen fuel cells, include 2H₂ + O₂ → 2H₂O and distinguish water produced at the point of use from the environmental effects of manufacturing hydrogen.