temperature changes in a system indicate energy changes
energy changes in a system can be described as endothermic/exothermic
heating or cooling of a substance
phase changes
chemical transformations
when a chemical reaction occurs, the energy of a system:
decreases (exothermic reaction)
energy lost by the reacting species (system) is gained from the surroundings
heat transfer
work done by the system
increases (endothermic reaction)
system gains energy from the surroundings
heat transfer
work done by the system
remains the same
the formation of a solution may be exothermic or endothermic
depends on the relative strengths of intermolecular/interparticle interactions before and after the dissolution
energy diagrams: show the endothermic or exothermic nature of a physical or chemical process
heat transfer and thermal equilibrium
the particles in a warmer body have a greater average kinetic energy than those in a cooler body
transfer/exchange of energy/heat: occurs with collisions between particles in thermal contact
thermal equilibrium: average kinetic energy (temperatures) of both bodies are the same
heat capacity and calorimetry
amount of heat transferred between two bodies
calorimetry: used to measure the transfer of heat
first law of thermodynamics: energy is conserved in chemical and physical processes
the transfer of a given amount of thermal energy will not produce the same temperature change in equal masses of matter with differing specific heat capacities
heating a system increases the energy of the system, and vice versa
the specific heat capacity and the molar heat capacity are used in energy calculations
chemical systems change their energy through:
heating/cooling
phase transitions
chemical reactions
in calorimetry experiments involving dissolution, temperature changes of the mixture within the calorimeter can be used to determine the direction of energy flow
temperature increases: thermal energy is released by the dissolution process (exothermic)
temperature decreases: thermal energy is absorbed by the dissolution process (endothermic)
energy of phase changes
energy must be transferred to a system to cause a substance to melt/boil
energy increases as the system undergoes a solid-to-liquid/solid-to-gas phase transition
system releases energy when it freezes/condenses
energy decreases as the system undergoes a liquid-to-solid/gas-to-liquid phase transition
temperature of a pure substance remains constant during a phase change
the energy absorbed during a phase change is equal to the energy released during a complementary phase change in the opposite direction
e.g. molar enthalpy of condensation of a substance is equal to the negative of its molar enthalpy of its vaporization
e.g. molar enthalpy of fusion can be used to calculate the energy absorbed when melting a substance and the energy released when freezing a substance
enthalpy change: amount of energy released (negative) or absorbed (positive) by a chemical reaction at constant pressure
when products are at a different temperature than their surroundings, they exchange energy with the surroundings to reach thermal equilibrium
exothermic reaction: thermal energy is transferred to the surroundings as the reactants convert to products
endothermic reactions: thermal energy is transferred from the surroundings as the reactants convert to products
the chemical energy of the products of a reaction is different from that of the reactants because of the breaking and forming of bonds
the energy difference results in a change in the kinetic energy of the particles, which manifests as a temperature change
during a chemical reaction, bonds are broken/formed, changing the potential energy of the system
the average energy can be estimated by adding up the average bond energies of the bonds
reactants: required to break bonds
products: released to form bonds
exothermic: energy released (forming) is greater than energy required (breaking)
endothermic: energy required (breaking) is greater than energy released (forming)
tables of standard enthalpies of formation can be used to calculate the standard enthalpies of reactions
Hess’s law
many processes can be broken down into a series of steps
each step in a series has its own energy change
the enthalpy change (net thermal energy transferred) in the sequence will be equal to the sum of the enthalpy changes in each of the steps
total energy is conserved (first law of thermodynamics) and each individual reaction in a sequence transfers thermal energy to or from the surroundings
thermal energy transfers are the result of potential energy changes among the species in the reaction sequence
changes to the reaction changes the enthalpy value
reaction is reversed: mathematical sign of enthalpy is reversed
reaction is multiplied: enthalpy change is multiplied by the same factor
reactions are added: the individual enthalpy changes of each reaction are added to obtain the net enthalpy change
