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Thermochemistry Chapter 10 thermo #3.ppt
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Today, you will learn… How to solve problems that include stoichiometry and thermochemistry The definition of specific heat How to solve problems using Q=mC T How heat is transferred with and without a phase change
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Reaction Enthalpies All chemical reactions either release or absorb heat Exothermic reactions: heat is a product Reactants products + energy as heat ( H = negative) Endothermic reactions: heat is a reactant. Reactants + energy as heat products ( H = positive)
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Reaction Enthalpies Example: __ SO 2 (g) + __ O 2 (g) __ SO 3 (g) H = -198.2 kJ How much heat is released when 454 g of SO 2 are burned in air? First, balance the equation: 2 SO 2 (g) + 1 O 2 (g) 2 SO 3 (g) H = -198.2 kJ 454 g ? kJ Use stoichiometry (treat enthalpy like a coefficient, ignore sign):
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Specific Heat (C), (called S in your textbook, but usually C. S stands for entropy… a coming attraction!) Things heat up or cool down (change temperature) at different rates. Land heats up and cools down faster than water, and aren’t we lucky for that!?
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Specific heat (also called specific heat capacity) is the amount of heat required to raise the temperature of 1 g a material by one degree (C or K, they’re the same size). C water = 4.184 J / g C (“holds” its heat) C sand = 0.664 J / g C (less E to change) This is why land heats up quickly during the day and cools quickly at night and why water takes longer to heat/cool.
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Why does water have such a high specific heat? Water molecules form strong attractions with each other water molecule (including H-bonds!); so it takes more heat energy to break the attractive forces. Metals have weak bonds (remember the “sea of e-) and do not need as much energy to break them. water metal
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Q = m x C x T Q = change in thermal energy m = mass of substance T = change in temperature (T f – T i ) C = specific heat of substance Memorize this!
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Specific Heat Capacity If 25.0 g of Al cool from 310 o C to 37 o C, how many joules of heat energy are involved? (C Al ) = 0.897 J/gK heat gain / loss = Q = (m)(C)(∆T) where ∆T = T final - T initial Q = (25.0 g)(0.897 J/gK)(37 - 310)K Q = - 6120 J Notice that the negative sign on Q signals heat “lost by” or transferred OUT of Al.
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Transferred heat can cause a change in temperature q transferred = (c)(mass)(∆T)
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Or… Heat Transfer can cause a Change of State Changes of state involve energy (at constant T) Ice + 333 J/g (heat of fusion) -----> Liquid water Use the units to help you solve!
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Heat Transfer and Changes of State Liquid (l) Vapor (g) Requires energy (heat). Why do you… cool down after swimming ? use water to put out a fire?
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Remember this – it’s the Heating/Cooling Curve for Water! Note that T is constant as phase changes (Evaporate water)
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WHY DO I NEED H vap or H fus WHEN I HAVE Q = (m)(C)(∆T)? Well, when a phase changes THERE IS NO change in temperature ( T = 0)… but there is definitely a change in energy!
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A separate step is required for EACH SEGMENT on the graph. Slanted segments = temperature changes, use Slanted segments = temperature changes, use Q = (m)(C)(∆T) Flat segments = phase changes Use Q = (mass)( H fus ) or Q = (mass)( H vap ) WATCH YOUR UNITS!!!! J and kJ are not the same thing.
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For ice at 0˚C to steam at 100˚C, there are 3 steps. Q = (mass)( H fus ) Q = (mass)( H vap ) Q = (mass)(c)( T)
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Example: What quantity of heat is required to melt 500.0 g of ice and heat the water to steam at 100 o C? Heat of fusion of ice = 333 J/g Specific heat of water = 4.184 J/gK Heat of vaporization = 2,260 J/g +333 J/g +2260 J/g
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How much heat is required to melt 500. g of ice and heat the water to steam at 100 o C? 1. To melt ice: Q 1 = (500.0 g)(333 J/g) = 1.665 x 10 5 J 2.To raise water from 0 C to 100 C: Q 2 = (500.0 g)(4.184 J/gK)(100 - 0)K = 2.092 x 10 5 J 3.To evaporate water at 100 C: Q 3 = (500.0 g)(2,260 J/g) = 1.130 x 10 6 J 4. Total heat energy = Q 1 + Q 2 + Q 3 Q total = 1.665 x 10 5 J + 2.092 x 10 5 J + 1.130 x 10 6 J Q total = 1.506 x 10 6 J = 1,506 kJ
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Molar Heat of Combustion The amount of energy released in completely burning one mole of substance. Molar heat of solidification (freezing) and molar heat of condensation – related to molar heat of fusion and molar heat of vaporization. Same value, opposite sign! H fus = - H solid H vap = - H cond Some more terms!
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