Chemistry Chapter on Enthalpy

Defining Enthalpy

• Enthalpy is the heat flow under constant pressure conditions
• Enthalpy change (ΔH) is represented by: ΔH = qp
• Exothermic reactions release heat, ΔH is negative.
• Endothermic reactions absorb heat, ΔH is positive.

ΔH of Phase Changes

• The heat required to convert a liquid to a gas is the heat of vaporization (ΔHvap), which is endothermic.
• The heat released to convert a gas to a liquid is the heat of condensation (ΔHcond), which is exothermic.
• ΔHcond = –ΔHvap
• The magnitude of enthalpy change depends on the substance involved.

Heat of Reaction

• The enthalpy change for a reaction is called the heat of reaction.
• It can be estimated using bond energies.
• Breaking bonds requires energy, making bonds releases energy.
• Bond energy method is used to estimate enthalpy changes for reactions where thermochemical data is unavailable.

Thermochemical Equations

• A thermochemical equation summarizes the overall energetics for a chemical reaction.
• The sign on the ΔH indicates whether the reaction is endothermic or exothermic.
• If stoichiometric coefficients are multiplied by a factor, the heat of reaction must also be multiplied by the same factor.

Heats of Reaction for Specific Reactions

• Heat of combustion (ΔHcomb) is the heat released during a combustion reaction.
• Heat of neutralization (ΔHneut) is the heat released during a neutralization reaction.
• Heat of formation (ΔHf) is the heat change for the formation of one mole of a compound from its elements in their standard states.
• The standard state is the most stable form of an element at 25°C and 1 atm, indicated with a superscript °.
• ΔHf° = 0 for an element in its standard state.

Hess’s Law

• Hess's law states that the enthalpy change for any process is independent of the particular way the process is carried out.
• Enthalpy is a state function, its value relies solely on the state of the system, independent of its history.
• Hess's Law utilizes known enthalpy values for individual reaction steps to calculate the enthalpy change for an overall reaction.

Hess’s Law Steps

• To calculate enthalpy changes using Hess's Law, manipulate given chemical reactions so their overall effect matches the desired reaction.
• Rules for manipulating reactions:
  • Reactions can be reversed, changing the sign of ΔHf.
  • Reactions can be multiplied by a constant, which multiplies the value of ΔHf by the same constant.

Formation Reactions and Hess’s Law

• The enthalpy change for a reaction can be calculated using Hess's law and heats of formation.

Energy and Stoichiometry

• A thermochemical equation allows for the stoichiometric treatment of energy.
• Energy is treated as a product in exothermic reactions and as a reactant in endothermic reactions.
• The thermochemical equation converts between the number of moles of a reactant/product and the amount of energy released/absorbed.

Energy Density and Fuels

• Factors considered when evaluating a fuel's economic merits include:

  • Technology for extracting the fuel
  • Pollution released during combustion
  • Relative safety
  • Ease of transport
  • Energy density

• Energy density is the amount of energy that can be released per gram of fuel burned.

• Fuels with higher energy densities require less fuel transport.

Power Distribution and the Electrical Grid

• The electrical grid balances electricity demand with production.

• Demand varies based on factors like season, time of day, and power plant operating conditions.

• Key components of the electrical grid include:

  • Generating station: produces electricity (e.g., burning fossil fuels).
  • Transmission substation: increases voltage to reduce loss over long distances.
  • Distribution substation: reduces voltage and distributes electricity to consumers.