2thermochemistrythe study of heat changes in chemical reactions and physical changes
3SURROUNDINGS SYSTEM system—specific part being analyzed surroundings—everything outside the system (usually the immediate area)universe = system + surroundingsSURROUNDINGSSYSTEMopenclosedisolated**Exchange:mass & energyenergynothing
4Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and moleculesChemical energy is the energy stored within the bonds of chemical substancesNuclear energy is the energy stored within the collection of neutrons and protons in the atomElectrical energy is the energy associated with the flow of electronsPotential energy is the energy available by virtue of an object’s position
5Energy 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 Energy400C900Cgreater thermal energy
6Units 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 foodCalorie (capital C) refers to the energy in food
7Units for Measuring Heat Flow 1 Calorie = 1 kilocalorie = 1000 calSI unit of heat and energy = Joule4.184 J = 1 cal1 J = cal
8Energy Transformations Heat – (variable is q) – energy that transfers from one object to anotheronly changes can be detected!flows from warmer to cooler object
92H2 (g) + O2 (g) 2H2O (l) + energy Exothermic - process that gives off heat- energy goes from system to surroundings.2H2 (g) + O2 (g) H2O (l) + energyEnergy is written on the product (right) side of the equationq and ΔH are negative (-)Δ is a Greek letter – means change in
10exothermicThe products are lower in energy than the reactantsThus, energy is released.ΔH = -395 kJ• The negative sign does not mean negative energy, but instead that energy is lost.
11q and ΔH are positive (+) Endothermic - process in which heat has to be supplied – energy goes from surroundings to systemenergy + 2HgO (s) Hg (l) + O2 (g)energy is written on the reactant (left) side of the equationq and ΔHare positive (+)
12endothermicThe products are higher in energy than the reactants Thus, energy is absorbed.ΔH = +176 kJ• The positive sign means energy is absorbed
13Changes of stateWhen 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.
14The 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 formulaq = m X Dt X Cq = heatm = mass ( in grams)Dt = tfinal – tinitialC = specific heatq = DH
16Dt = 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 = J/g • 0C m = 869 gDt = tfinal – tinitial = 50C – 940C = - 890Cq = mCDt= 869 g x J/g • 0C x –890C= -34,000 J6.4
17The specific heat of water is high due to hydrogen bonding
18enthalpy changes are measured with a calorimeter Calorimetry the measurement of the heat into or out of a system heat released = the heat absorbedenthalpy changes are measured with a calorimeterNo heat enters or leaves!
19The 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.
20thermo chemical equations thermo chemical equations— equations that show heat changesenthalpy (H)—heat content of a substanceCannot measure directly but we CAN measurechange in enthalpy = ΔH;heat change for a process; usually measured in kJ (kilojoules)
21Heat of reactionThe physical states of all reactants and products must be specifiedH2O (s) H2O (l)DH = 6.01 kJH2O (l) H2O (g)DH = 44.0 kJEquation –= given mol x DH# mol
23Thermo chemical Equations How much heat is evolved when 266 g of white phosphorus (P4) burn in air?P4 (s) + 5O2 (g) P4O10 (s) DH = kJ1 mol P4123.9 g P4x266 g P43013 kJ1 mol P4x= 6470 kJ
24ΔHf = 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
25DHf 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 rust1 mol Fe2O3160 g Fe2O3x5.0 g Fe2O31826 kJ1 mol Fe2O3x= kJ
26When 1. 0 g of solid NaOH (DHsoln = – 445 When 1.0 g of solid NaOH (DHsoln = – kJ/mol) dissolves in 10 L of water, how much heat is released?1 mol NaOH40.0 g NaOHx445.1 kJ1 mol NaOHx1.0 g NaOH1= kJ
27Molar 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 liquidMolar Heat of Solidification (DHsolid.) = the heat lost when 1 mol of liquid solidifies (or freezes) to a solid
28Molar 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
29Standard 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
30Standard 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 compoundStandard heat of formation for elements = 0
31Standard 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 togetherAdd the products togetherSubtract the sum of the reactants from the sum of the products
32Calculate 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= kJ