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Energy, work, heat and chemical reactions What is work (in chemical terms)? work -- occurs when an object is moved against an applied force Work = Force x Distance d Fgravity Ffriction Moving an object horizontally against the force of friction d d Lifting an object vertically against the force of gravity What is work (in chemical terms)? Ffriction = 0 Moving an object horizontally across a frictionless surface Work = 0 Work and energy work -- occurs when an object is moved against an applied force work -- occurs when an object is moved against an applied force Work = Force x Distance energy -- the capacity of matter to perform work Felectrostatic d Fspring Compressing a spring + d + The greater the amount of energy that a substance or object has, the greater the amount of work that it can perform + The SI unit for energy is the joule (J) Moving a charged particle against an electric field Another commonly used unit for energy is the calorie (cal) 1 cal = 4.184 J Kinetic energy and potential energy Work and energy work -- occurs when an object is moved against an applied force kinetic energy -- energy possessed by matter due to its motion energy -- the capacity of matter to perform work KE = ( 1 / 2 ) m v2 Types of energy more mass higher speed greater kinetic energy kinetic energy -- energy possessed by matter due to its motion baseball at rest potential energy -- stored energy that is possessed by matter (i.e., stored energy that has the potential to be released later) KE = 0 baseball in flight KE > 0 -- gravitational -- elastic (e.g., compressed springs) -- chemical (energy stored in the chemical bonds between atoms in a compound) heat -- internal energy of a substance due to the motion of its atoms (or molecules) Kinetic energy and potential energy Other examples of potential energy potential energy -- stored energy that is possessed by matter (i.e., stored energy that has the potential to be released later) Example: gravitational potential energy -- the energy contained by an object due to its elevation Elastic potential energy -- for example, the energy stored in a compressed spring 100 ft PE = mgh more mass greater height greater potential energy 20 ft PE = 0 10 ft Ouch! #@$#*!! x x PE > 0 Heat Other examples of potential energy heat -- internal energy of a substance due to the motion of its atoms (or molecules) Chemical potential energy -- the energy stored in chemical bonds H • heat is nothing more than atoms (and molecules) in motion H H propane H • the faster an atom/molecule moves around, the more heat it carries C C H C H H H adenosine triphosphate (ATP) water Energy and chemical changes (reactions) Energy and chemical changes (reactions) In all chemical changes, matter either releases or absorbs energy In all chemical changes, matter either releases or absorbs energy Example of energy release: Combustion of methane Example of energy absorption: Photosynthesis CH4 + methane 2 O2 CO2 + 2 H2O + oxygen carbon dioxide + energy water 2 H2 hydrogen electricity CO2 + 2 H2O + carbon dioxide water heat, light Example of energy absorption: Electrolysis of water 2 H 2O energy + O2 oxygen water energy CH2O sunlight organic compounds + O2 oxygen Example of energy release: Respiration CH2O organic compounds + O2 oxygen CO2 + 2 H2O + energy carbon dioxide metabolic energy water Heat Energy and chemical changes (reactions) heat -- internal energy of a substance due to the motion of its atoms (or molecules) • heat is nothing more than atoms (and molecules) in motion • the faster an atom/molecule moves around, the more heat it carries In all chemical changes, matter either releases or absorbs energy Example of energy release: Combustion of methane CH4 + 2 O2 methane CO2 + 2 H2O + carbon dioxide oxygen water heat, light Example of energy absorption: Electrolysis of water 2 H 2O + energy 2 H2 water hydrogen Energy and chemical changes (reactions) In all chemical changes, matter either releases or absorbs energy water O2 oxygen energy CH2O sunlight organic compounds Bond energy (chemical potential energy) Energy is released when a bond is formed 58 kcal / mol Example: Formation of Cl2 molecule Example of energy absorption: Photosynthesis carbon dioxide + electricity water CO2 + 2 H2O + energy + O2 oxygen Cl Cl Cl Cl Energy must be supplied to break a bond Example of energy release: Respiration Example: Breaking Cl-Cl bond CH2O organic compounds + O2 oxygen CO2 + 2 H2O + energy carbon dioxide metabolic energy water 58 kcal / mol Cl Cl This is called the bond dissociation energy Cl Cl Bond dissociation energy is a measure of bond strength Conservation of energy Law of conservation of energy Stronger bonds require more energy to break Energy can be neither created nor destroyed -- but it can be changed from one form to another 58 kcal / mol Cl Cl Cl Cl For chemical changes: total energy before reaction = total energy after reaction 226 kcal / mol N Reactions that release energy: N N N Total bond energy of reactants = Total bond energy of products + released energy Chemical potential energy Conservation of energy Heat Heat in chemical reactions Reactions are classified as either exothermic or endothermic Law of conservation of energy Energy can be neither created nor destroyed exothermic reaction -- heat (energy) is released by the reaction -- but it can be changed from one form to another CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) + 890 kJ For chemical changes: total energy before reaction = total energy after reaction Reactions that absorb energy: Total bond energy of reactants + Bond energy of reactants > Bond energy of products endothermic reaction -- heat (energy) is absorbed by the reaction absorbed energy = Total bond energy of products 2 H2O(l) + 286 kJ 2 H2(g) + O2(g) Bond energy of reactants < Bond energy of products Chemical potential energy Heat Chemical potential energy Heat of reaction Energy and chemical changes (reactions) heat of reaction ( enthalpy change: !H ) -- the quantity of heat (energy) produced or absorbed during a reaction Example of exothermic reaction: Combustion of methane CH4 !Hreaction = Hproducts – – Energy of bonds broken in reactants Hreactants Exothermic reactions: Heat of reaction (!H) is negative 2 O2 CO2 + 2 H2O + 890 kJ CH4 + 2 O2 Chemical potential energy Energy of bonds formed in products heat of = reaction + reactants Energy (released) !H = – 890 kJ CO2 + 2 H2O products Endothermic reactions: Heat of reaction (!H) is positive Bond energy of reactants > Bond energy of products Energy and chemical changes (reactions) + 286 kJ 2 H2 + exothermic reaction: heat (energy) is released by the reaction O2 -- heat of reaction is negative Chemical potential energy 2 H2 + O2 products Energy (absorbed) Heat in chemical reactions Reactions are classified as either exothermic or endothermic Example of endothermic reaction: Electrolysis of water 2 H 2O CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) + 890 kJ CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) !H = – 890 kJ !H = 286 kJ endothermic reaction: heat (energy) is absorbed by the reaction -- heat of reaction is positive 2 H 2O reactants Bond energy of reactants < Bond energy of products !H is negative !H is positive 2 H2O(l) + 286 kJ 2 H2(g) + O2(g) 2 H2O(l) 2 H2(g) + O2(g) !H = 286 kJ Homework assignment Chapter 6 Problems: 7.25, 7.26, 7.27, 7.28, 7.29