Heat in Chemical Reactions
Name ___________________________
Period ______ Date________________
10-1 Chemical Reactions that Involve Heat
1. Heat: _____________ (symbol - ____) that is transferred from one object to another
due to a difference in ____________________. Measured in Joules (symbol - _____)
2. Thermochemistry: The study of ___________________________ in a chemical reaction.
3. Types of Chemical Reactions
a. Exothermic Reactions: ________________ heat into their surroundings. Heat is a
________________ of the reaction and temperature ______________. This occurs
during bond _______________________________.
surroundings
Exothermic
Reaction
sur
sur
sur
Combustion reactions are exothermic: burning propane
C3H8 + 5O2
→
3CO2 + 4H2O + 2043 kJ
b. Endothermic Reactions: Heat is ________________ by the reactants and stored in the
chemical _______________ of the products. Heat acts as a __________________ and
temperature _________________. This occurs during bond _____________________.
Electrolysis of water requires __________________ energy.
2H2O + 572kJ → 2H2 + O2
surroundings
sur
Endothermic
Reaction
sur
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10.2 Heat and Enthalpy Changes
1. Enthalpy: The ______________ content of a system at constant pressure (symbol is ______ ).
2. Enthalpy Change: The heat absorbed or released during a reaction (symbol is ________ ).
3. Enthalpy Diagrams:
#1
#2
#1
#2
a. Which has a higher enthalpy? Products or Reactants
_______
_______
b. Was heat absorbed or released?
_______
_______
c. Is this an endothermic or exothermic reaction?
_______
_______
d. Is ΔH for this reaction positive or negative?
_______
_______
e. Would the ΔH be on the left or right side of the yield sign? _______
_______
f. Is the reverse reaction exothermic or endothermic?
_______
_______
g. Rewrite each equation with the heat term in the reaction as a reactant or product:
#1)
#2)
2
 = “change in”
H = Hproducts ─ Hreactants
ΔH
H products
H reactants
Exothermic
Reaction
Endothermic
Reaction
4. When reactions take place at standard temperature and pressure, ______ = ______.
5. Standard Enthalpy Change (H): Enthalpy change that occurs when reactants in their
standard states (_____________ _____________ _____________) change to products in their
standard states. STP Standard Temperature and Pressure are: _______C and _______atm.
The H is listed after the equation. If the H is positive the reaction is
____________________ and heat was ____________________. If the H is negative, the
reaction is _________________ and heat was ____________________.
6. The amount of heat absorbed or released in a reaction depends upon the number of
_____________ of reactants.
7. Enthalpy Changes in Stoichiometry Problems:
Ex) How much heat will be released if 5.0 g of H2O2 decomposes?
2H2O2 → 2H2O + O2
ΔH = -190 kJ
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
ΔH = -2803 kJ
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10.3 Hess’s Law - (1802-1850)
1. The enthalpy change for a reaction is the _________ of the enthalpy changes for a series of
reactions that __________ _______ to the overall reaction.
2. This is also called the ____________________________________________________
3. This allows you to determine the enthalpy change for a reaction by _________________ means
when a direct method cannot be done.
4. Steps for using Hess’s 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 ΔH for each ‘sub equation.”
a. If needed, multiply equation and enthalpy change value. (coefficients)
b. 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 ΔH (enthalpy changes) of each “sub equation.”
5. Calculate ΔH for the following examples:
C(s)
2CO(g)
+
→
H2O(g)
+
O2(g)
CO(g)
→
+
2CO2(g)
ΔH=?
H2(g)
ΔH=?
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10-3 Calorimetry:
1. The Kinetic Theory states that heat results from the _______________ &________________
of particles.
2. Heat: The transfer of _________________ energy from a __________ object to a
______________ object. Heat is __________________ on composition and ______________.
3. Temperature is a measure of how hot or cold something is; specifically it is the measure of the
_____________________________of the particles in an object. It is __________________ of
amount.
4. Calorimetry is the study of ___________ ____________ and ________________________.
5. Calorimetry experiments determine the heats of reactions (enthalpy changes) by making
accurate measurements of temperature changes produced in a _________________________.
6. A Calorimeter is an ___________________ device used to measure heat absorbed or released
in a chemical or physical change.
7. Specific Heat (Cp): The amount of heat needed to raise _______ of a substance by 1C.
Formula for specific heat:
m=mass (substance) T=change in temperature of the substance (Tf─Ti)
Specific Heat of Water = 4.184 J/g ºC = 1 calorie or .001Calorie (food)
8. Measuring Specific Heat of a Metal:
Ex #1) What is the specific heat of nickel if the temperature of a 32.2 g sample is increased
by 3.5 º C when 50. J of heat is added.
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?
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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.
heat (q)
3) Calculate H =
mol
Ex) When a 4.25 g sample of solid NH4NO3 dissolves in 60.0g 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 = ?
10. Foods as Fuels:
A. Carbohydrates typically have ____________ enthalpies; however, the products of
their combustion, CO2 and H2O, have ____________ enthalpies.
B. Therefore, the combustion of carbohydrates and fats, is ______________.
C. Sugars and Starches break down to _____________, which reacts with O2 in a
combustion reaction.
D. Nutritional information on food labels can be gathered using a calorimeter.
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