Chem +: Chapter 16: Thermochemistry

Energy

Capacity to do work

Kinetic Energy

Motion

Potential Energy

Stored energy (chemical bonds)

Number of cal in a Cal

1000

Number of joules in a cal

4\.184

Temperature

Measure of average kinetic energy of matter. Average kinetic energy directly proportional to absolute temperature (Kelvin). 0K = no kinetic energy. °C + 273 = K. ΔT = Change in temp. = T(final) - T(initial)

Heat (q)

Form of energy. Can’t measure directly, so changes in temperature are used to determine whether heat is gained or lost.

Enthalpy (ΔH, kj/mol)

Change in potential energy (stored) during a reaction due to breaking (requires energy) and making (releases energy) bonds. ΔH = H(products - H(reactants). ΔH = q(system)/# of moles = -q(surroundings)/# of moles.

Exothermic

Reaction that releases energy. Heat is released, so ΔH is negative. An increase in temperature will be observed.

Activated complex

Time in a reaction when there are neither reactants nor products, but temporary unstable particles.

Endothermic

Reaction that absorbs energy. Heat is absorbed, so ΔH is positive. A decrease in temperature will be observed.

Heat curve

Shows the relationship of heat and temperature. When solids, liquids, or gases are being heated, a slanted line is shown. This shows that both temperature and heat are increasing. When substances are freezing or melting (heat of fusion) or condensing or boiling (heat of vaporization,) a flat line is shown. This indicates that only heat is increasing. They also indicate a phase change.

ΔH=mΔTC

Formula used to determine the amount of heat needed to change the temperature of a sample from one temperature to another. Can only be used when the temperature is changing. m is the mass, ΔT is the change in temperature, and C is the specific heat capacity.

ΔH=mH(fus) or ΔH=mH(vap)

Formula used to determine the amount of heat needed to boil, condense, freeze, or melt a substance. The heats of fusion and vaporization will be given with the problem. This can only be used during phase changes.

Calorimeter

Insulated container where a reaction takes place and the resultant ΔT can be measured accurately. Provides the data necessary to determine the heat of the reaction (ΔH) or the specific heat capacity.

Calorimetry Calculations

Assume heat released or absorbed by a reaction is absorbed or released by the surroundings. ΔH(reaction) = -ΔH(surroundings). q=mCΔT, where m is the mass of everything in the calorimeter.

Predicting Enthalpy Changes with Stoichiometry

Energy can be used as a conversion factor and can be added to any balanced reaction.

Predicting Enthalpy Changes with Enthalpy of Formation (ΔH°(f))

Heat of formation is the energy change to form one mole of a compound from elements in their natural state. They can be endothermic or exothermic. It can be used to determine ΔH. ΔH(reaction) = ΣΔH°(f)(products) - ΣΔH°(f)(reactants). You can use fractions when balancing equations to make sure there is 1 mol of each product.

Predicting Enthalpy Changes with Hess’s Law

If a series of reactions are added together, the enthalpy change for the net reaction will be the sum of the enthalpy changes for the individual steps. Reactions should be treated as manipulatable equations

Rules of Hess’s Law

If the coefficients of a reaction are multiplied by a factor, the ΔH is multiplied by the same factor. If the reaction is reversed, the sign of the ΔH is reversed.

Entropy (ΔS)

Disorder/chaos/randomness. Reactions tend to proceed in the direction that produces disorder, or more of this. Gases are more disorderly than liquids, and liquids are more disorderly than solids. The more particles there are on a certain side of a reaction, the more disorderly that side of the reaction is.

Gibb’s Free Energy (ΔG)

Describes energy available to do work. Used to determine if reactions are spontaneous. This quantity must be negaitve for a reaction to be spontaneous. The spontaneity, or the sign of this number, is dependent on ΔH and ΔS. Exothermic (-ΔH) reactions tend to be spontaneous. High entropy (+ΔS) tend to be spontaneous.

Gibbs-Helmholtz Equation

ΔG = ΔH - TΔS. ΔH and ΔS must be in kilojoules. T must be in Kelvin. This is used to determine the sponaneity of a reaction when one condition is favorable and the other is unfavorable. If ΔH and ΔS are positive, the reaction will be spontaneous at higher temperatures. If ΔH and ΔS are negative, the reaction will be spontaneous.