Unit 7 Thermochemistry

thermochemistry the study of heat changes in chemical reactions and physical changes

SURROUNDINGS SYSTEM system—specific part being analyzed surroundings—everything outside the system (usually the immediate area) universe = system + surroundings SURROUNDINGS SYSTEM open closed isolated **Exchange: mass & energy energy nothing

Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances Nuclear energy is the energy stored within the collection of neutrons and protons in the atom Electrical energy is the energy associated with the flow of electrons Potential energy is the energy available by virtue of an object’s position

Energy Changes in Chemical Reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. Temperature = Thermal Energy 400C 900C greater thermal energy

Units for Measuring Heat Flow calorie - the amount of heat needed to raise the temperature of 1 g of H2O by 1 oC. -Used except when referring to food Calorie (capital C) refers to the energy in food

Units for Measuring Heat Flow 1 Calorie = 1 kilocalorie = 1000 cal SI unit of heat and energy = Joule 4.184 J = 1 cal 1 J = 0.2390 cal   

Energy Transformations Heat – (variable is q) – energy that transfers from one object to another only changes can be detected! flows from warmer to cooler object

2H2 (g) + O2 (g) 2H2O (l) + energy Exothermic - process that gives off heat- energy goes from system to surroundings. 2H2 (g) + O2 (g) 2H2O (l) + energy Energy is written on the product (right) side of the equation q and ΔH are negative (-) Δ is a Greek letter – means change in

exothermic The products are lower in energy than the reactants Thus, energy is released. ΔH = -395 kJ • The negative sign does not mean negative energy, but instead that energy is lost.

q and ΔH are positive (+) Endothermic - process in which heat has to be supplied – energy goes from surroundings to system energy + 2HgO (s) 2Hg (l) + O2 (g) energy is written on the reactant (left) side of the equation q and ΔH are positive (+)

endothermic The products are higher in energy than the reactants Thus, energy is absorbed. ΔH = +176 kJ • The positive sign means energy is absorbed

Changes of state When a substance changes state the temperature remains constant during the actual phase change.- this is because the heat energy is being used up by the phase change process.

The specific heat (C) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. Specific heat formula q = m X Dt X C q = heat m = mass ( in grams) Dt = tfinal – tinitial C = specific heat q = DH

Dt = tfinal – tinitial = 50C – 940C = - 890C How much heat is given off when an 869 g iron bar cools from 940C to 50C? C of Fe = 0.444 J/g • 0C m = 869 g Dt = tfinal – tinitial = 50C – 940C = - 890C q = mCDt = 869 g x 0.444 J/g • 0C x –890C = -34,000 J 6.4

The specific heat of water is high due to hydrogen bonding

enthalpy changes are measured with a calorimeter Calorimetry the measurement of the heat into or out of a system heat released = the heat absorbed enthalpy changes are measured with a calorimeter No heat enters or leaves!

The Law of Conservation of Energy in any chemical or physical process, energy is neither created nor destroyed. - All the energy is accounted for as work, stored energy, or heat.

thermo chemical equations thermo chemical equations— equations that show heat changes enthalpy (H)—heat content of a substance Cannot measure directly but we CAN measure change in enthalpy = ΔH; heat change for a process; usually measured in kJ (kilojoules)

Heat of reaction The physical states of all reactants and products must be specified H2O (s) H2O (l) DH = 6.01 kJ H2O (l) H2O (g) DH = 44.0 kJ Equation – = given mol x DH 1 # mol

Thermo chemical Equations How much heat is evolved when 266 g of white phosphorus (P4) burn in air? P4 (s) + 5O2 (g) P4O10 (s) DH = -3013 kJ 1 mol P4 123.9 g P4 x 266 g P4 3013 kJ 1 mol P4 x = 6470 kJ

ΔHf = HEAT OF FORMATION = heat absorbed or released to make 1 mol of a compound from its elements ΔHsoln = Heat of Solution = heat change caused by dissolving of one mole of substance

DHf for the formation of rust (Fe2O3) is –826 kJ/mol DHf for the formation of rust (Fe2O3) is –826 kJ/mol. How much energy is involved in the formation of 5 grams of rust 1 mol Fe2O3 160 g Fe2O3 x 5.0 g Fe2O3 1 826 kJ 1 mol Fe2O3 x = 25.9 kJ

When 1. 0 g of solid NaOH (DHsoln = – 445 When 1.0 g of solid NaOH (DHsoln = – 445.1 kJ/mol) dissolves in 10 L of water, how much heat is released? 1 mol NaOH 40.0 g NaOH x 445.1 kJ 1 mol NaOH x 1.0 g NaOH 1 = 11.1 kJ

Molar Heat of Combustion = ΔHcomb = heat released in combustion of 1 mol of substance Molar Heat of Fusion (DHfus.) = the heat absorbed by 1 mol of a substance in melting from a solid to a liquid Molar Heat of Solidification (DHsolid.) = the heat lost when 1 mol of liquid solidifies (or freezes) to a solid

Molar Heat of Vaporization (DHvap Molar Heat of Vaporization (DHvap.) = the amount of heat necessary to vaporize 1 mol of a given liquid. Molar Heat of Condensation (DHcond.) = amount of heat released when 1 mol of vapor condenses to a liquid

Standard heat of reaction Hess’s law of heat summation states that if you add two or more thermochemical equations you can add the heats of reaction to give the final heat of reaction

Standard Heats of Formation The standard heat of formation (ΔHf0) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound Standard heat of formation for elements = 0

Standard Heat of Reaction DH = H (products) – H (reactants) Multiply the standard heat of formation by the number of moles for each reactant and product. Add the reactants together Add the products together Subtract the sum of the reactants from the sum of the products

Calculate DH for the following reaction. C2H4(g) + H2(g) C2H6(g) DH for C2H4(g) = 52.5 kJ/mol; DH for H2(g) = 0 kJ/mol; (free element) H for C2H6(g) = –84.7 kJ/mol - 84.7 - 52.5 = - 137.2 kJ