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Thermochemistry Energy and Chemical Change
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Energy Energy can change for and flow, but it is always conserved.
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The Nature of Energy Energy – the ability to do work or produce heat – Potential energy – Kinetic energy
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Chemical systems contain both kinetic and potential energy – Kinetic energy – Potential energy
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Chemical potential energy Energy that is stored in a substance because of its composition
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Heat Heat = energy Symbol = q Always flows from warmer object to cooler object
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Measuring Heat calorie – the amount of energy needed to raise the temperature of one gram of pure water by one degree Celsius – Calorie vs. calorie
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Joule – the SI unit of energy 1 calorie = 4.184 J A chemical reaction releases 213 J of energy. How many calories has this reaction released?
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A big mac contains 550 Calories, express this energy in Joules.
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Specific Heat Amount of heat needed to raise the temperature of 1 g of a substance by 1 o C – Symbol = c – Unit = J/g o C High specific heat = absorbs a lot of energy w/small changes in temperature – Water = 4.184 J/g o C – Concrete = 0.84 J/g o C
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Calculating changes in heat q = (m)(c)( T)
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If the temperature of 34.4 g of ethanol increases from 25.0 o C to 78.8 o C, how much heat has been absorbed by the ethanol? (specific heat of ethanol = 2.44 J/g o C)
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The temperature of a 10.0 g piece of iron changed from 50.4 oC to 25.0 oC, how much energy was release by the iron? (specific heat of iron is 0.449 J/g o C)
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Heat Calorimeter – insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.
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Chemical Energy and the Universe Thermochemistry – the study of heat changes that accompany chemical reactions and phase changes – System – specific part of the universe that contains the reaction or process you want to study – Surroundings – everything else in the universe other than the system
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Law of conservation of energy 1 st law of thermodynamics – energy cannot be created or destroyed, only transferred
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Energy being absorbed/released is indicated from the system’s point of view -q = energy released by system(exothermic) + q = energy absorbed by system(endothermic) q system = -q surroundings (m 1 )(c 1 )( Δ T 1 ) = - (m 2 )(c 2 )(Δ T 2 )
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A 125 g sample of iron at 93.5 o C is dropped into an unknown mass of water at 25.0 o C. The final temperature of the mixture is 32.0 o C. The C of iron is 0.451 J/g o C, the C of water is 4.18 J/g o C. What is the mass of the water?
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A 118 g piece of tin at 85 o C is dropped into 100 g of water at 35 o C. The final temperature of the mixture is 38 o C. C of water is 4.18 J/g o C. What amount of heat is absorbed by the water? What amount is released by the tin? What is the specific heat of tin?
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Enthalpy and enthalpy change Enthalpy (H) – heat content of a system at a constant pressure Cannot measure enthalpy directly but can measure change in enthalpy (heat absorbed or released in a chemical reaction) Enthalpy of reaction ( H rxn )– change in enthalpy for a reaction
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ΔH rxn = H products – H reactants Exothermic reaction Endothermic reaction
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Thermochemical Equations Thermochemical equations express the amount of heat released or absorbed by chemical reactions
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Writing Thermochemical Equations Thermochemical equation – balanced chemical equation that includes the states of all reactants and products and the energy change CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) ΔH comb = -891 kJ
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Enthalpy (heat) of combustion (ΔH comb ) – the enthalpy change for the complete burning of one mole of the substance Enthalpy (heat) of vaporization (ΔH vap ) – heat required to vaporize one mole of a liquid Enthalpy (heat) of fusion (ΔH fus ) – heat required to melt one mole of a solid
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How much heat is released when 54.0 g of glucose is burned? ΔH comb = -2808 kJ/mol
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H 2 O(l) H 2 O(g) ΔH vap = 40.7 kJ H 2 O(s) H 2 O(l) ΔH fus = 6.01 kJ
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Phase Change Diagram C steam = 1.70 J/g o C Liquid steam: 40.7 kJ/mol C ice = 2.10 J/g o C C Water = 4.18 J/g o C solid liquid: 6.01 kJ/mol
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How much heat must be absorbed to melt 150.0 g of water?
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How much heat is released when 50.0 g of steam cools to 40 o C?
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Challenge!!! A 39.0g sample of ice at -125 o C changes into steam at 125 o C. How much energy is absorbed during this process?
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Calculating Enthalpy Change Hess’s Law – if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction
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What is the energy change for the following reaction: 2S(s) + 3O 2 (g) 2SO 3 (g) a.S(s) + O 2 (g) SO 2 (g) ΔH = -594 kJ b.2SO 3 (g) 2SO 2 (g) + O 2 (g) ΔH = 198 kJ
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What is the energy change for the following? H 2 O 2 (l) 2H 2 O(l) + O 2 (g) a. 2H 2 (g) + O 2 (g) 2H 2 O(l) ΔH = -572kJ b. H 2 (g) + O 2 (g) H 2 O 2 (l) ΔH = -188kJ
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Standard heat of formation The change in enthalpy that accompanies the formation of one mole of the compound in its standard state ΔH f ΔH f of an element = 0
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ΔH rxn = sum of ΔH f products – sum of ΔH f reactants
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What is ΔH rxn for the following equation: CH 4 (g) +2O 2 (g) CO 2 (g) + 2H 2 O(l)
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What is the ΔH rxn for the following: 4NH 3 (g) + 7O 2 (g) 4NO 2 (g) + 6H 2 O(g)
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Reaction Spontaneity Changes in enthalpy and entropy determine whether a process is spontaneous
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Spontaneous processes Any physical or chemical change that once begun, occurs with no outside intervention – Iron rusting – Paper burning Often some energy from the surroundings must be supplied to get the process started
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Entropy Entropy(S) – a measure of the number of possible was that the energy of a system can be distributed – Determined by the freedom of the systems particles to move and the number of ways they can be arranged – Disorder or randomness of a system
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Second law of thermodynamics – spontaneous processes always proceed in such a way that the entropy of the universe increases
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Predicting changes in entropy +ΔS = entropy increases - ΔS = entropy decreases
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Changes resulting in +ΔS Changes in state that allow more molecule movement – (s) (l) – (l) (g) Number of particles increases in a reaction CaCO 3 CaO + CO 2 Solid or liquid dissolves in solvent Increase in temperature
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Changes resulting in -ΔS Phase changes that decrease molecule movement – (g) (l) – (l) (s) Number of particles decreases in a reaction Dissolving of gas in a solvent Decrease in temperature
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Predict the sign of ΔS for the following: – ClF(g) + F 2 (g) ClF 3 (g) – NH 3 (g) NH 3 (aq) – CH 3 OH(l) CH 3 OH(aq) – C 10 H 8 (l) C 10 H 8 (s)
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Gibbs Free Energy ΔG = ΔH – TΔS -ΔG = spontaneous reaction + ΔG = nonspontaneous reaction
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For a process, ΔH = 145 kJ and ΔS = 322 J/K. is the process spontaneous at 382K?
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ΔHΔHΔSΔSΔGΔG Reaction Spontaneity NegativePositiveNegativeAlways spontaneous Negative Negative or positive Spontaneous at low temperatures Positive Negative or positive Spontaneous at high temperatures PositiveNegativePositiveNever spontaneous
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