ENERGETICS ENERGETICS CAPE Chemistry Module 1 Objectives Objectives 1. State that chemical reactions take place through energy changes (usually in the form of heat) associated with the breaking and making of bonds 2. State that energy changes occur in chemical reactions associated with the making and breaking of bonds Explain the differences between exothermic and endothermic reactions using energy profile diagrams Explain the term ‘bond energy’ Explain how bond energy data may be used to show the relationship between strength of covalent bonds and reactivity of covalent molecules 3. 4. 5. 6. 7. 8. 9. Apply concepts associated with enthalpy changes Explain the effect of ionic charge and radius on the magnitude of lattice energy Calculate enthalpy changes from appropriate experimental data State Hess’ law of constant heat summation Energetics& &Thermodynamics Thermodynamics Energetics Energetics is the branch of physical science that deals primarily with energy and its transformations Thermodynamics is the particular branch of energetics which deals with heat energy and its transformations Enthalpy is the total energy associated with any system http://www.sophia.org/define-enthalpy/define-enthalpy-tutorial Energy& &Chemical ChemicalReactions Reactions Energy Chemical reactions take place through energy changes (usually in the form of heat) − Enthalpy (H) is the energy possessed by a system − Enthalpy change or ΔH is the energy change in a reaction http://www.sophia.org/define-enthalpy/define-enthalpy-tutorial EnthalpyChange Changein inaaSystem System Enthalpy Enthalpy Change (ΔH) = Eproducts – Ereactants If Eproducts > Ereactants then ΔH is positive reaction is endothermic reaction absorbs heat from the surroundings reaction vessel gets colder If Eproducts< Ereactants then ΔH is negative The reaction is exothermic reaction releases heat to the surroundings reaction vessel gets hotter EnthalpyChange Changein inaaSystem System Enthalpy ChemicalReactions Reactions Chemical Two steps necessary in a chemical reaction: •FIRST - bonds are broken - energy is needed or absorbed during bond breaking (endothermic) •AFTER - bonds are formed - energy is released during bond formation (exothermic) BOTH processes ALWAYS occur in a chemical reaction http://employees.csbsju.edu/cschaller/Reactivity/thermo/TDbondmakebreak.png ChemicalReactions Reactions Chemical Chemical reactions may be classified as endothermic or exothermic • Exothermic - more energy is released than is absorbed in a chemical reaction (ΔH is negative) • Endothermic - less energy is released than is absorbed in a chemical reaction (ΔH is positive) http://www.gcsescience.com/rc24-energy-level-diagram.htm EnergyProfile ProfileDiagrams: Diagrams:Exothermic ExothermicReactions Reactions Energy • Most reactions are exothermic eg. acid-base reactions, combustion reactions and respiration • Temperature of the system increases • ΔH is negative (Energy of reactants is higher than energy of products) EnergyProfile ProfileDiagrams: Diagrams:Endothermic EndothermicReactions Reactions Energy • Fewer reactions are endothermic eg. Photosynthesis, thermal decomposition, dissolving some salts in water • Temperature of the system decreases • ΔH is positive (Energy of reactants is lower than energy of products) http://www.gcsescience.com/rc25-exothermic-endothermic-reaction.htm BondEnergy Energy Bond Bond energy is : the amount of energy required to dissociate a molecule into its respective atoms directly related to the strength of the covalent bond and is indirectly related to its reactivity Factorsaffecting affectingBond BondEnergy Energy Factors 1. Strength of covalent bond 2. Size of the atoms in the molecule 3. Degree of orbital overlap in the covalent bond 4. Bond Order (single, double or triple) BondEnergy Energyand andReactivity Reactivity Bond 1. If bonds can be broken easily, the reaction would occur quickly 2. If the bonds cannot be broken easily, the reaction would occur slowly Which is more reactive? N2 B.E = 945 kJ mol-1 ; bond length = 110 pm OR O2 B.E = 498 kJ mol-1 ; bond length = 121 pm BondEnergy Energy& &Chemical ChemicalStability Stability Bond • The energy required to break: O-H in H2O(g) is 463 kJ mol-1 O-O bond in H2O2 (g) is 139 kJ mol-1 • H2O2 is less stable than water because it contains the O-O bond which is easier to break than the O-H bond in H2O • Hydrogen peroxide will be more chemically reactive than water Homework Homewor State whether the following processes are exothermic or endothermic: • Reaction between sodium hydroxide and hydrochloric acid N2 + 3H2 → 2NH3 ΔHrxn= - 92 kJ mol-1 • The burning of gasoline • The dissolving of potassium nitrate in water • Standard Enthalpy Changes and definitions StandardEnthalpy EnthalpyChanges Changes Standard • StandardEnthalpy EnthalpyChange ChangeofofReaction Reaction()() Standard The standard enthalpy change when the mole quantities in the balanced equation react under standard conditions (1 atm and 298 K) At 1 atm and 25°C (298 K) reactants and products are in their usual or standard states StandardEnthalpy EnthalpyChange ChangeofofFormation Formation()() Standard • StandardEnthalpy EnthalpyChange ChangeofofFormation Formation()() Standard • Standard Enthalpy Change of combustion () Standard Enthalpy Change of combustion () StandardEnthalpy EnthalpyChange ChangeofofNeutralization Neutralization()() Standard StandardEnthalpy EnthalpyChange ChangeofofAtomization Atomization()() Standard • StandardEnthalpy EnthalpyChange Changeofofhydration hydration()() Standard StandardEnthalpy EnthalpyChange Changeofofsolution solution()() Standard StandardEnthalpy EnthalpyChanges Changes Standard • Factorsaffecting affecting Factors 1. Size of the ions 2. Charge on the ions http://chemistry.tutorvista.com/inorganic-chemistry/lattice-energy.html http://www.chemhume.co.uk/A2CHEM/Unit%202b/9%20Lattice%20enthalpy/Ch9Lattice.htm Determining Enthalpy Determining Enthalpy Changes Changes Determiningenthalpies enthalpiesofofreaction reaction) ) Determining • Measuring Experimentally Experimentally 1.1.Measuring − − − − − Chemical reaction occurs in an insulated container called a calorimeter Styrofoam coffee cup with a lid serves as calorimeter Thermometer measures heat change of the reaction The change in temperature is proportional to the energy released by the reaction Since the pressure is constant, the heat evolved or absorbed is equal to ΔHr https://www.learner.org/courses/chemistry/text/text.html?dis=U&num=Ym5WdElUQS9PQ289&sec=YzJWaklUQS9OeW89 Assumptionsin inmeasuring measuring experimentally experimentally Assumptions − calorimeter perfectly prevents the gain or loss of heat from the solution to its surroundings − heat gained by the solution is produced by the chemical reaction or heat lost by the solution is absorbed by the chemical reaction Measuringthe the for foracid-base acid-basereactions reactions Measuring Measuring ofofaasalt salt Measuring Question 5g of ammonium nitrate (NH4NO3) was dissolved in 50 cm3 of water in a Styrofoam cup. The temperature fell from 22 oC to 14 oC. Calculate the molar enthalpy change of solution for the above reaction. Measuringthe the of offuels fuels Measuring heat from combustion of the fuel is used to heat up the water assume energy released by combustion of fuel is absorbed by the water in the calorimeter heat capacity of the water and the temperature rise are needed to calculate how much heat was released by a specific mass of fuel very inaccurate method because of huge losses of heat e.g. radiation from the flame Measuringthe the of offuels fuels Measuring Question 100 cm3 of water was measured into a calorimeter. A spirit burner containing ethanol fuel, C2H5OH, weighed 18.62 g at the start. The initial temperature of the water was measured. After burning some time, the flame was extinguished and the water stirred gently. The final temperature of the water was measured. The burner and fuel are then reweighed to see how much fuel had been burned. After burning, 17.14g of ethanol remained and the temperature of the water rose from 18 to 89 oC. Calculate the enthalpy of combustion for ethanol. http://www.docbrown.info/page03/3_51energyD.htm#Calculations Calculating from fromBond BondEnergies Energies 2.2.Calculating Calculating from Bond Energies Calculating from Bond Energies Calculating from fromBond BondEnergies Energies Calculating Use Bond enthalpies (found in data booklet) to estimate the enthalpy change in the following reaction: CalculatingΔH ΔHr rfrom fromΔH ΔHf f 3.3.Calculating ΔHr = ΣΔHf, products– ΣΔHf, reactants ΔHr is the enthalpy change of reaction ΔHf, products is the enthalpy change of formation of products ΔHf, reactants is the enthalpy change of formation of reactants CalculatingΔH ΔHr rfrom fromΔH ΔHf f values values Calculating Question Calculate the enthalpy of the following reaction: C2H5OH(l) + 3 O2(g) → 2 CO2 (g) + 3 H2O (l) Standard enthalpies of formation are: C2H5OH = -228 kJ/mol CO2 = -394 kJ/mol H O = -286 kJ/mol CalculatingΔH ΔHrfrom fromΔH ΔHf values Calculating r f values Answer: C2H5OH + 3 O2 → 2 CO2 + 3 H2O ΔHr= ΣΔHf, products– ΣΔHf, reactants ΔHr = [(3 x -286) + (2 x -394)] – [(-228) + (3 x 0)] = -1418 kJ mol-1 CalculatingΔH ΔHrusing usingHess' Hess'Law Law 4.4.Calculating r Hess’ Law states that the enthalpy in converting reactants into products is the same regardless of the route taken. http://commons.wikimedia.org/wiki/File:Hess_Law.png According to Hess' Law: ΔH = ΔH1 + ΔH2 + ΔH3 Hess’ Law Hess’Law Hess’ Law may be used to calculate enthalpy changes which cannot be determined experimentally e.g. lattice enthalpies sum of enthalpies via route 1 = sum of enthalpies via route 2 Sum of clockwise enthalpies = sum of anticlockwise enthalpies http://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htm Hess'Law: Law:Applications Applications Hess' • Hess'Law LawCalculations: Calculations:Energy EnergyCycle CycleDiagrams Diagrams Hess' You are given the following: ΔH°c (kJ mol-1) C6H6(l) -3267 C(s) -394 H2(g) -286 Hess’sLaw LawCalculations: Calculations:Chemical ChemicalEquations Equations Hess’s • Hess’s law can be applied when chemical equations are given instead of a diagram • The chemical equations will be treated like algebraic equations Hess'Law LawCalculations: Calculations:Chemical ChemicalEquations Equations Hess' • The reaction for enthalpy of combustion for ethane is: C2H6(g) + O2(g) 2CO2(g) + 3H2O(l) ∆HC = ? Arrange the three equations in such a way, that when they are added together their sum will be the overall equation above. C2H6(g) 2C(s) + 3H2(g) 2 x [C(s) + O2(g) CO2 (g)] 3 x [H2(g) + O2(g) H2O(l)] ∆H1 = +84.68 kJ/mol ∆H2 = -394 kJ/mol x2 ∆H3 = -286 kJ/mol x3 C2H6(g) + O2(g) + 2C(s) + 3H2(g) 2CO2(g) +3H2O(l) + 2C(s) + 3H2(g) therefore: ∆HC = -∆H1 + 2∆H2 + 3∆H3 ∆HC = -1561.32 kJ/mol http://intro.chem.okstate.edu/1314F00/Lecture/Chapter6/Lec102300.html LatticeEnthalpy Enthalpy)) Lattice Lattice enthalpy is a measure of the strength of the forces between the ions in an ionic solid The greater the lattice enthalpy, the stronger the forces Remember that energy is given out when bonds are made, and is needed to break bonds LatticeEnthalpy Enthalpy)) Lattice Lattice Enthalpy (∆Hlatt) The change when one mole of ionic solid is formed from its gaseous ions (downward arrow) ΔH0latt is negative Reverse Lattice Enthalpy (-∆Hlatt) The change when one mole of ionic solid is broken up to form its scattered gaseous ions (upward arrow) Reverse lattice enthalpy is positive value eg. NaCl, the solid is more stable than the gaseous ions by 787 kJ mol-1 0 0 0 Relationship between ΔH ΔH and ΔH 0 0 0 , , Relationship between ΔH soln ΔH hyd sol,nΔH lat,tand hyd latt 0 0 , and ΔH0 Relationship between ΔH ΔH , soln hyd 0 0 Relationship between ΔH soln, ΔH lattlatt, and ΔH0 hyd Determine the enthalpy of solution of potassium fluoride, KF. Given the following information: ΔH0 of KF = -819 kJ mol-1, ΔH0 = -821 kJ mol-1 hyd latt Enthalpy of solution of KF,ΔHsolution = -ΔHlattice + ΔHhyd = 821 + (-819) kJ mol -1 = +2 kJmol -1 Born-HaberCycle Cycle Born-Haber Born-Haber cycle relates the lattice energy of an ionic compound to other enthalpies that can be measured special application of Hess’Law indirectly measures the lattice enthalpies (ΔHlatt) lattice enthalpies are difficult to determine experimentally What steps are involved in the formation of NaCl? Born-HaberCycle Cyclefor forCaCl CaCl2 Born-Haber 2 http://chemistry.tutorvista.com/inorganic-chemistry/born-haber-cycle.html