Ch. 15 Honors Chemistry Test
formation equation
shows how the compound is formed from its component elements
what changes of state produce a positive ΔH?
solid to liquid to gas (endothermic)
Hess' law
states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction
law of conservation of energy (first law of thermodynamics)
states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neith created nor destroyed
second law of thermodynamics
states that spontaneous processes always proceed in such a way that the entropy of the universe increases; ΔSuniverse > 0 for all spontaneous processes
universe
system and the surroundings
energy
the ability to do work or produce heat; chemicals participating in a chemical reaction contain both potential and kinetic energy
calorie (cal)
the amount of energy required to raise the temperature of one gram of pure water by one degree Celsius
specific heat
the amount of heat required to raise one gram of that substance one degree Celsius; because different substances have different compositions, each susbtance has its own specific heat; unit- J/g °C
enthalpy (heat) of reaction (ΔHrxn)
the change in enthalpy for a reaction
standard enthalpy (heat) of formation (ΔH°f)
the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states; expressed in kJ/mol; can be used to calculate the enthalpies of many reactions under standard conditions using Hess' law
free energy (ΔGsystem)
the energy available to do work
why is energy related to entropy useless?
the energy is dispersed and cannot be harnessed to do work
enthalpy (heat) of combustion (ΔHcomb)
the enthalpy change for the complete burning of one mole of a substance
molar enthalpy (heat) of fusion (ΔHfus)
the heat required to melt one mole of a solid substance; ΔHfus = -ΔHsolid
molar enthalpy (heat) of vaporization (ΔHvap)
the heat required to vaporize one mole of a liquid; ΔHvap = -ΔHcond
endothermic reaction
the reactants absorb heat; Hproducts > Hreactants; when Hreactants is substracted from the larger Hproducts, a positive value results; cold-pack process
exothermic reaction
the reactants lose heat; Hproducts < Hreactants; when Hreactants is subtracted from the smaller Hproducts, a negative value for ΔHrxn results; heat-pack process
system
the specific part of the universe that contains the reaction or process you wish to study
thermochemistry
the study of heat changes that accompany chemical reactions and phase changes
in what ways does the number of possible arrangemens available to a system increase?
when volume increases, when energy increases, when the number of particles increases, when the particles' freedom of movement increases
gibbs free energy equation
ΔGsystem = ΔHsystem - TΔSsystem; must convert ΔH because ΔS is expressed as J/K; ΔGsystem is expressed in J; if ΔGsystem is negative, the reaction is spontaneous
the summation equation
ΔH°rxn = ΔH°f(products) - ΔH°f(reactants); find the ΔHf from the chart for the reactants and add them up, do the same for the products, and then put the values into the equation
how do you find entropy?
ΔSsystem = Sproducts - Sreactants; find the S from the chart for the reactants and add them up, do the same for the products, and then put the values into the equation; if entropy increases, Sproducts > Sreactants and ΔSsystem is positive; if entropy decreases, Sproducts < Sreactants, and ΔSsystem is negative
what is the standard enthalpy of formation for elements in their standard states?
0.0 kJ; with zero as the starting point, the experimentally determined enthalpies of formation of compounds can be placed on a scale above and below the elements in their standard states
important conversion factors
1 cal = 4.184 J, 1 Calorie = 1 kcal = 1000 cal
joule (J)
SI unit of heat and energy
ΔT
Tfinal - Tinitial
spontaneous process
a physical or chemical change that occurs with no outside intervention; however, for many spontaneous processes, some energy from the surroundings must be applied to get the process started; a process can be spontaneous if it is exotermic and there is an increase in entropy; a process cannot be spontaneous if it is endothermic and there is a decrease in entropy
calorimeter
an insulated device used for measuring the amount of heat absorbed or released in a chemical reaction or physical process; a known mass of water is placed in an insulated chamber to absorb the energy released from the reacting system or to provide the energy absorbed by the system
thermochemical equation
balanced chemical equation that includes the physical states of all reactants and products and the energy change usually expressed as the change in enthalpy ΔH; usually balanced for one mole of product, so fractional coefficients are used
why is it impossible to know the total energy content of a substance?
because the total amount of energy of a substance depends on many factors; however, chemists are usually more interested in changes in energy during reactions
potential energy
energy due to the composition or position of an object; depends on its composition: type of atoms, number and type of chemical bonds, and particular way the atoms are arranged
kinetic energy
energy of motion; kinetic energy of a susbtance directly related to the constant random motion of its representative particles and is directly proportional to temperature
chemical potential energy
energy stored in a substance due to its composition; plays an important role in chemical reactions (chemical potential energy is stored in the bonds of a substance)
how can entropy be predicted?
entropy changes associated with changes in state can be predicted (entropy increases as a substance changes from a solid to a liquid to a gas); the dissolving of a gas in a solvent always results in a decreases in entropy; assuming no change in physical state occurs, the entropy of a system usually increases when the number of gaseous product particles is greater than the number of gaseous reactant particles; entropy incrases when a solid or a liquid dissolves in a solvent; the entropy of a substance increases as its temperature increases
surroundings
everything else other than the system in the universe
heat
form of energy that flows from a warmer object to a cooler object
what changes of state produce a negative ΔH?
gas to liquid to solid (exothermic)
enthalpy (H)
heat content of a system at constant pressure; although you cannot measure the actual enthalpy of a substance, you can measure the change in enthalpy, which is the heat absorbed or released in a chemical equation; q = H
what does the zero superscript tell you in ΔH°f?
it tells you that enthalpy changes were determined at standard conditions that are one atm and 298K
entropy (S)
measure of the number of possible ways that the energy of a system can be disturbed; related to the freedom of the system's particles to move and number of ways they can be arranged; measure of disorder and randomness of particles
Calorie
nutritional Calorie
equation for calculating heat
q = m × c × ΔT; units: q- J, m- g, ΔT- °C
