Title: Heat in Chemical Reactions
1 Heat in Chemical
Reactions
210.1 Chemical Reactions that Involve Heat
- Heat Energy (symbol - q) that is transferred
from one object to another due to a difference in
temperature. Measured in Joules (symbol - J) - 2. Thermochemistry The study of heat changes in
a chemical reaction.
33. Types of Chemical Reactions
- a. Exothermic Reactions release heat into their
surroundings. Heat is a product of the reaction
and temperature increases. This occurs during
bond formation. - Combustion reactions are exothermic burning
propane - C3H8 5O2 ? 3CO2 4H2O 2043 kJ
surroundings
Exothermic Reaction
?T
?H -
4Exothermic Reaction
5- b. Endothermic Reactions Heat is absorbed by the
reactants and stored in the chemical bonds of the
products. Heat acts as a reactant and
temperature decreases. This occurs during bond
breaking. - Electrolysis of water requires electrical energy.
- 2H2O 572kJ ? 2H2 O2
surroundings
Endothermic Reaction
?T -
?H
6Exothermic/Endothermic Reactions
710.2 Heat and Enthalpy Changes
- 1. Enthalpy The heat content of a system at
constant pressure (symbol is H ). - 2. Enthalpy Change The heat absorbed or released
during a reaction (symbol is ?H ). - SKIP TO TOP of PAGE 3
8- 4. When reactions take place at standard
temperature and pressure, q H. - 5. Standard Enthalpy Change (?H?) Enthalpy
change that occurs when reactants in their
standard states (most stable form) change to
products in their standard states. STP Standard
Temperature and Pressure are 0 ?C and 1 atm. - The ?H? is listed after the equation. If the
?H? is positive the reaction is endothermic and
heat was absorbed. If the ?H? is negative, the
reaction is exothermic and heat was released. - 6. The amount of heat absorbed or released in a
reaction depends upon the number of moles of
reactants.
97. Enthalpy Changes in Stoichiometry Problems
- Ex) How much heat will be released if 5.0 g of
H2O2 decomposes? - 2H2O2 ? 2H2O O2 ?H? -190 kJ
10- Ex) How much heat is transferred when you eat a
10. g Jolly rancher which is made of glucose
(C6H12O6)? It reacts in your body with oxygen
according to the following equation. If 4.184kJ
1 Cal, how many Calories are in the Jolly
Rancher? - C6H12O6 6O2 ? 6CO2 6H2O ?H? -2803 kJ
C6H12O6 6O2 ? 6CO2 6H2O 2803 kJ
11 3. Enthalpy Diagrams 1 2
121 2
a. Which has a higher enthalpy? Products or Reactants R P
b. Was heat absorbed or released? R A
c. Is this an endothermic or exothermic reaction? Exo Endo
d. Is ?H for this reaction positive or negative? -
e. Would the ?H be on the left or right side of the yield sign? R L
f. Is the reverse reaction exothermic or endothermic? Endo Exo
13g. Rewrite each equation with the heat term in
the reaction as a reactant or product
- 1) C3H8 5O2 ? 3CO2 4H2O 2043 kJ
- 2) C H2O 113kJ ? CO H2
14 ? change in ?H Hproducts - Hreactants
?H Hproducts Hreactants
Exothermic Reaction - low high
Endothermic Reaction high low
1510.3 Hesss Law - (1802-1850)
- The enthalpy change for a reaction is the sum of
the enthalpy changes for a series of reactions
that adds up to the overall reaction. - This is also called the Law of Heat of Summation
(S) - 3. This allows you to determine the enthalpy
change for a reaction by indirect means when a
direct method cannot be done.
164. Steps for using Hesss Law
- 1. Identify the compounds
- 2. Locate the compounds on the Heat of Reaction
Table. - 3. Write the reaction from the table so the
compound is a reactant or product. - 4. Write appropriate ?H for each sub equation.
- If needed, multiply equation and enthalpy change
value. (coefficients) - If you reverse the equation, change the sign of
the enthalpy change. - 5. Add the equations to arrive at the desired net
(original) equation. - 6. Add ?H (enthalpy changes) of each sub
equation.
1710.3 Calorimetry
- 1. The Kinetic Theory states that heat results
from the motion vibration of particles. - 2. Heat The transfer of kinetic energy from a
hotter object to a colder object. Heat is
dependent on composition and amount. - 3. Temperature is a measure of how hot or cold
something is specifically it is the measure of
the average kinetic energy (speed) of the
particles in an object. It is independent of
amount.
18Heat
- Temperature is not the same as heat.
- Temperature is a measure of the average kinetic
energy of the particles in an object. - A temperature change is a result of a energy
transfer. - Julius Sumner Miller - Physics - Heat
Temperature - Heat vs. Temperature Animation
19- 4. Calorimetry is the study of heat flow and
measurement. - 5. Calorimetry experiments determine the heats of
reactions (enthalpy changes) by making accurate
measurements of temperature changes produced in a
calorimeter. - 6. A Calorimeter is an insulated device used to
measure heat absorbed or released in a chemical
or physical change.
20- 7. Specific Heat (Cp) The amount of heat needed
to raise 1 g of a substance by 1?C. - Formula for specific heat
- mmass (substance)
- ?Tchange in temperature of the substance (Tf-Ti)
- Specific Heat of Water 4.184 J/g ºC
- 1 calorie or .001food Calorie
21- 8. Measuring Specific Heat of a Metal
- Ex 1) What is the specific heat of a nickel if
the temperature of a 32.2 g sample of nickel is
increased by 3.5ºC when 50. J of heat is added.
22- Ex 2) How much heat is absorbed to be able to
increase the temperature of a 26.2 g sample of
aluminum (Cp 0.897 J/gºC) from 25.3 ºC to 65.9
ºC?
23- 9. Measuring Heat (q) of a Substance Dissolved in
Water - You can rearrange this formula to determine
the heat released or absorbed by the surroundings
(solution) as the substance dissolves based on
this assumption - q reaction -q surroundings
-
- ?q
- 1) Calculate q for the surroundings (solution)
and determine qrxn. - 2) Calculate the moles of solute dissolved in the
water. - 3) Calculate ?H
?T Tf - Ti
24- Ex) When a 4.25 g sample of solid NH4NO3
dissolves in 60.0 g of H2O in a calorimeter,the
temperature drops from 21.0 ºC to 16.9 ºC.
Calculate ?H. Rewrite the thermochemical
equation with the heat term as a reactant or
product. - NH4NO3(s) ? NH4(aq) NO3-(aq) ?H
? - qsurmCp?T (64.3g)(4.184J/gC)(-4.1C)
- qsur-1100J
- qrxn -qsur 1100J
- 2.
- 3.
21 kJ
2510. Foods as Fuels
- A. Carbohydrates typically have high enthalpies
however, the products of their combustion, CO2
and H2O, have low enthalpies. - B. Therefore, the combustion of carbohydrates and
fats, is exothermic. - C. Sugars and Starches break down to glucose,
which reacts with O2 in a combustion reaction. - D. Nutritional information on food labels can be
gathered using a calorimeter. - Carbs 4 Cal/g Protein 4 Cal/g Fat 9 Cal/g