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Thermochemistry Chapter 10 thermo #3.ppt. Today, you will learn… How to solve problems that include stoichiometry and thermochemistry The definition of.

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Presentation on theme: "Thermochemistry Chapter 10 thermo #3.ppt. Today, you will learn… How to solve problems that include stoichiometry and thermochemistry The definition of."— Presentation transcript:

1 Thermochemistry Chapter 10 thermo #3.ppt

2 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

3 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)

4 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):

5 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!?

6 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.

7 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

8 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!

9 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.

10 Transferred heat can cause a change in temperature q transferred = (c)(mass)(∆T)

11 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!

12 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?

13 Remember this – it’s the Heating/Cooling Curve for Water! Note that T is constant as phase changes (Evaporate water)

14 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!

15 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.

16 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)

17 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

18 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

19 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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