Unit+12+-+Thermochemistry

The energy is heat measured in Joules or Kilojoules. There are 1000 joules in one kilojoule. The following are the equations and when to use them. This is when one substance is changing temperature or you are trying to find the heat that one substance used. q=heat(energy) in joules m=mass in grams c=specific heat (delta)T=is the change in temperature found by (Tfinal-Tstarting).
 * q=mc(delta)T**

When two substances are mixed together then the heat is evenly distributed until and equilbrium is reached. q1=-q2 Which is really: mc(delta)T=-mc(delta)T The variables are the same as as the previous equation.

It takes more energy to cause a phase change. q=moles X (delta)H

The (delta)H value depends on what phase change is occuring. (delta)Hfus=6.01 KJ/moles
 * When changing a substance from ice to liquid or back the formula is**:

(delta)Hvap=44.01 KJ/moles
 * When changing from vapor to liquid this formula**

If the phase change is going backwards(ex. from water to ice) then the value of energy is Negative!

1) Calculate q when 28.6 g of water is heated from 22.0°C to 78.3°C. A. 0.385 kJ B. 1.61 kJ D. 9.37 kJ E. 1.61 × 103 kJ The equation is q=28.6(4.184)(56.3) 2) If, as a pioneer, you wished to warm your room by taking an object heated on top of a pot-bellied stove to it, which of the following 15-pound objects, each heated to 100°C, would be the best choice? The specific heat capacity (in J/(g·K)) for each substance is given in parentheses. Iron (0.450), copper (0.387), granite (0.79), gold (0.129), water (4.18). A. iron B. copper C. granite D. gold The answer is granite because it has the highest specific heat which means it will hold its heat for longer. 3) Ethylene glycol, used as a coolant in automotive engines, has a specific heat capacity of 2.42 J/(g·K). Calculate q when 3.65 kg of ethylene glycol is cooled from 132°C to 85°C. A. -1900 kJ C. -99 kJ D. -0.42 kJ E. -4.2 × 10-6 kJ The equation is q=3650(2.42)(-47). 4) A 275-g sample of nickel at l00.0°C is placed in 100.0 mL of water at 22.0°C. What is the final temperature of the water? Assume that no heat is lost to or gained from the surroundings. Specific heat capacity of nickel = 0.444 J/(g·K) B. 40.8°C C. 61.0°C D. 79.2°C E. 82.4°C The equation is 275(.444)(X-100)=-100(4.184)(X-22). 5) Benzene is a starting material in the synthesis of nylon fibers and polystyrene (styrofoam). Its specific heat capacity is 1.74 J/(g·K). If 16.7 kJ of energy is absorbed by a 225-g sample of benzene at 20.0°C, what is its final temperature? A. -22.7°C B. 36.7°C C. 42.7°C E. none of these choices is correct 16700=225(1.74)(DeltaT) 6) When Karl Kaveman adds chilled grog to his new granite mug, he removes 10.9 kJ of energy from the mug. If it has a mass of 625 g and was at 25°C, what is its new temperature? Specific heat capacity of granite = 0.79 J/(g·K) A. 3°C B. 14°C C. 22°C The equation is 10900=625(.79)(DeltaT) E. none of these choices is correct 7) The Starship Enterprise is caught in a time warp and Spock is forced to use the primitive techniques of the 20th century to determine the specific heat capacity of an unknown mineral. The 307-g sample was heated to 98.7°C and placed into a calorimeter containing 72.4 g of water at 23.6°C. The heat capacity of the calorimeter was 15.7 J/K. The final temperature in the calorimeter was 32.4°C. What is the specific heat capacity of the mineral? A. 0.124 J/(g·K) C. 0.138 J/(g·K) D. 0.145 J/(g·K) E. none of these choices is correct 307(X)(32.4-98.7)=-72.4(4.18)(8.8) 8) A piece of copper metal is initially at 100.0°C. It is dropped into a coffee cup calorimeter containing 50.0 g of water at a temperature of 20.0°C. After stirring, the final temperature of both copper and water is 25.0°C. Assuming no heat losses, and that the specific heat (capacity) of water is 4.18 J/(g·K), what is the heat capacity of the copper in J/K? A. 2.79 J/K B. 3.33 J/K D. 209 J/K E. none of these choices is correct 50(c)(-75)=-50(4.184)(-25)
 * C. 6.74 kJ**
 * E. water**
 * B. -420 kJ**
 * A. 39.6°C**
 * D. 62.7°C**
 * D. 47°C**
 * B. 0.131 J/(g·K)**
 * C. 13.9 J/K**