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BACK Heat in Chemical Reactions
I Chemical Reactions That Involve Heat
A. most chemical reactions involve energy
1. bond breaking releases energy
2. bond formation requires energy
3. results in an energy flow
B. thermochemistry
1. study of heat in a chemical reaction
a. reactions that release energy called exothermic
b. reactions that absorb energy called endothermic
2. part of Thermodynamic - examines energy and work transformations in systems
C. exothermic reactions
1. releases more from the formation of new bonds than energy used to break old bonds
2. heat value appears on the right side of the equation
3. unit of measurement - joule (j)
D. endothermic reactions
1. heat value appears on the left side of the equation
2. energy released during bond formation is less than energy used to break old bonds
3. additional energy supplied by surroundings
4. additional energy stored in bonds of the products
II Heat and Enthalpy Changes
A. enthalpy
1. determines the heat absorbed or released in a reaction
2. symbol - H
3. involves the energy plus pressure and volume of the substance
4. at a constant pressure, heat absorbed or released is equal to the enthalpy change
B. enthalpy change, ^H
1. ^H (enthalpy change) = H products - H reactants
a. if H products is greater than H reactants --> ^ H is positive, therefore endothermic
b. if H reactants is greater than H products --> ^ H is negative, therefore exothermic
c. the amount absorbed or released depends on pressure, temperature, and physical state
d. enthalpy changes report at standard conditions - 1 atmosphere and 25oC
e. reactants and products must be in standard state
f. standard state - pure form at 1 atmosphere
g. elements must be present in its most stable form
h. enthalpy changes of standard state substances is called standard enthalpy change --> ^Ho
C. using enthalpy changes
1. amount of heat absorbed or released depends on the number of moles of reactants involved
2. problem solution
a. convert grams of given to moles
b. multiple moles of given times the conversion factor: ^Ho / moles of given
2 H2O2 --->2 H2O + O2 ^H0 = -190 Kj
5.0g H2O2 x 1 mol H2O2 = .145 mol H2O2
34.0g H2O2
.145 mol H2O2 x -190 Kj = -14.0 Kj
2 mol H2O2
III Hess's Law
A. the net enthalpy change for a series of reactions is equal to the sum of the enthalpy changes for the individual reactions
B. useful when the net reaction is given and the ^H's are known
C. must know 2 rules to manipulate the reactions
1. if coefficients are multiplied by a factor, the enthalpy changes must multiplied by the same
factor
A + B --> AB ^H0 = - 100Kj
2AB --> 2A +2B ^H0 = - 200Kj
2. if the equation is reversed, the sign of ^H is reversed
A + B --> AB ^H0 = - 100Kj
AB -->A + B ^H0 = + 100Kj
D. heat is transferred during physical changes as well
1. melting - heat of fusion
2. vaporization - heat of vaporization
IV Calorimetry
A. the study of heat flow and heat measurement
B. calorimeter - device used to measure temperature changes (enthalpy changes) of reactions
C. heat and temperature
1. exothermic reaction increases the surrounding's temperatures
2. how much depends on:
a. amount of heat released
b. heat capacity of the surroundings
3. heat capacity - the amount of heat needed to raise the temperature of the object by 1 Celsius degree
4. heat capacity depends on:
a. mass
b. its composition
5. heat capacity of 1 gram of a substance is called specific heat
6. specific energy is a physical property
7. metals - low specific heat
8. water high specific heat - 4.184 j / g 0C
9. transfer of heat is detected by temperature changes
10. measuring heat transfer
a. uses calorimeter - insulated container
b. filled known mass of water
c. reaction takes place in water
d. initial temperature recorded
e. final temperature recorded
f. temperature changes
1. increase - reaction is exothermic
2. decrease - reaction is endothermic
g. heat transferred in a reaction (qrxn) = but opposite in sign to the heat transferred of the surroundings (qsur)
h. qrxn = - qsur
i. qsur = m x C x (Tf - Ti)
D. foods as fuels
1. carbohydrates and fats - high enthalpies
2. products CO2 and H2O low enthalpies
3. reaction is exothermic
4. carbohydrates = 17Kj / g
5. fats = 38Kj / g
V. What is Heat?
A. the caloric theory
1. Lavoisier - heat was an invisible fluid that flowed from hot to cold
2. fluid called caloric
3. fluid filled spaces around atoms
4. Thompson's and Joule's experiments cast doubt on the caloric theory
B. the kinetic theory
1. heat - the result of the motion and vibration of particles of matter
2. kinetic energy is transferred object to object
3. hotter objects to colder objects
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