Energy and Chemical Change Chemistry – Chapter 16

Energy Energy – ability to do work or produce heat Potential energy – energy due to composition or position of an object Ex: Water stored behind a dam Kinetic energy – energy of motion Law of conservation of energy – in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed Chemical potential energy – energy stored in a substance b/c of its composition Ex: Octane in gasoline Heat (q) – energy that is in the process of flowing from a warmer object to a cooler object

Measuring heat Calorie (cal) – amount of heat required to raise the temp. of 1 g of pure water by 1 0C (metric system) Nutritional Calorie is 1000 calories (1 kcal) Joule (J) – SI unit of heat and energy 1 cal = 4.184 J Specific Heat – amount of heat required to raise the temp. of 1 g of that substance by 1 0C Water has a high specific heat so it can absorb and release large quantities of heat

Heat absorbed or released by a substance during a temp Heat absorbed or released by a substance during a temp. change depends on the specific heat, mass of the substance, and amount by which the temp. changes. q = c x m x ΔT q = heat absorbed/released c = specific heat of substance m = mass of sample in grams ΔT = change in temp. in 0C

Heat in Chemical Reactions and Processes Measuring Heat Calorimeter – insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process Calorimeter can be used to determine the specific heat of an unknown metal Chemical Energy and the Universe Thermochemistry – study of heat changes that accompany chemical reactions and phase changes Heat pack – O reacts w/ Fe in an exothermic reaction Cold pack – ammonium nitrate dissolves in water (endothermic reaction)

Universe = System + Surroundings System – in thermochemistry, this is the specific part of the universe that contains the reaction or process you wish to study Surroundings – everything in the universe other than the system Universe – system plus the surroundings Universe = System + Surroundings Enthalpy (H) – heat content of a system at constant pressure You can’t measure actual energy or enthalpy of a substance but you can measure the change in enthalpy, which is the heat absorbed or released in a chemical reaction Heat of reaction: ΔHrxn ΔHrxn = Hproducts - Hreactants

Sign of enthalpy of reaction: Heat pack reaction is exothermic 4Fe + 3O2 2Fe2O3 + 1625 kJ 4Fe + 3O2 2Fe2O3 ΔHrxn = -1625kJ Cold pack reaction is endothermic 27kJ + NH4NO3 NH4+ +NO3- NH4NO3 NH4+ +NO3- ΔHrxn = 27kJ

Thermochemical Equations Thermochemical equation – balanced equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy (ΔH) Ex: Highly exothermic combustion of glucose occurs in the body as food is metabolized to produce energy ΔHcomb = -2808 kJ Energy released is enthalpy of combustion – enthalpy change for the complete burning of one mole of a substance

Changes of State Molar enthalpy of vaporization (ΔHvap) – heat required to vaporize one mole of a liquid Molar enthalpy of fusion (ΔHfus) – heat required to melt one mole of a solid substance Both are endothermic, so ΔH is positive

Calculating Enthalpy Change Can not always measure heat change, so there is a way to theoretically determine ΔH Hess’s law – states that if you can add 2 or more thermochemical equations to produce a final equation for a reaction, the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction Enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known

2S + 3O2 2SO3 ΔH = ? a. S + O2 SO2 ΔH = -297 kJ First, chemical equations are needed that contain the substances found in the desired equation and have known enthalpy changes a. S + O2 SO2 ΔH = -297 kJ b. 2SO3 2SO2 + O2 ΔH = 198 kJ The desired equation shows 2 moles of sulfur reacting, so Equation a must be rewritten for 2 moles of sulfur by multiplying the coefficients by 2. The enthalpy change (ΔH) must also be doubled b/c twice the energy will be released if 2 moles of sulfur react (Equation a becomes Equation c) c. 2S + 2O2 2SO2 ΔH = -594 kJ

d. 2SO2 + O2 2SO3 ΔH = -198 kJ 2S + 2O2 2SO2 ΔH = -594 kJ B/c you want to determine ΔH for a reaction in which SO3 is a product rather than a reactant, Equation b must be reversed (sign of ΔH changes) and becomes Equation d. d. 2SO2 + O2 2SO3 ΔH = -198 kJ Now add Equations c and d. 2S + 2O2 2SO2 ΔH = -594 kJ 2SO2 + O2 2SO3 ΔH = -198 kJ 2SO2 + 2S + 3O2 2SO2 + 2SO3 ΔH = -792 kJ

Entropy Entropy – measure of the disorder or randomness of the particles that make up a system Law of disorder – states that spontaneous processes always proceed in such a way that the entropy of the universe increases Entropy is always increasing The more time that passes, the messier your room gets until you clean it again