1. 12-1 Chemical Reactions That Involve Heat
Law of conservation of energy - energy is neither created or destroyed. 
Most chemical reactions involve changes in energy. 
The making and breaking of chemical bonds either absorb or release energy.  
Heat is the transfer of energy from one object to another.  
Temperature is a measure of kinetic energy  
Thermochemistry- the study of changes in heat in chemical reactions.

2. Exothermic Reactions Release heat (exo=outside, thermic=heat)
Combustion is an exothermic reaction. 
When you burn things heat is put out, you feel it!  
When reactions are exothermic, release heat, ENERGY is released.  
Therefore, the products have less energy than the reactants.

3. Endothermic Reactions
Reactions that absorb heat (endo=inside, thermic=heat).
Chemical bonds formed store heat as energy.
Energy must be supplied to make these reactions occur. 
Products have more energy than reactants because of energy stored in bonds.

4. Is this an example of an endothermic or exothermic reactions?
GUMMY BEAR SACRIFICE!
Is this an example of an endothermic or exothermic reactions?
How do you know???

5. 12-1 Section Review Questions…
1.  In an exothermic reaction, which side of the chemical equation would you write the term energy?
2.  In an endothermic reaction, which side of the chemical equation would you write the term energy?
3. Is cloud formation exothermic or endothermic?     Why do you feel cooler on a hot summer day when you mist yourself with water and let it evaporate off you?  Hint: Who has more kinetic energy- solids, liquids or gases? So is energy being lost or gained in each phase change?
4. Why do you think your body produces sweat?  Why are you in trouble when you stop sweating (2 parts)?

6. 12-2 Heat and Enthalpy Changes
Enthalpy Change (delta ΔH) the heat gained or lost during a chemical reaction.
The formula is:
ΔH = enthalpy of products – enthalpy of reactants
Energy is measured in kilojoules (kJ)!
Enthalpy-depends on temperature, phase (state of matter), and composition.  
When pressure remains constant, the heat released or absorbed in a chemical reaction is equal to the enthalpy change.

7. Enthalpy Change for Endothermic Reactions…
ΔH for endothermic = positive
Who has the higher energy the reactants or the products?
ΔH for exothermic = negative

8. Stoichiometry & Enthalpy!
Solving enthalpy problems requires stoichiometry
Observe the following reaction:
2H2O2(l)  2H2O(l) + O2(g)      = -190kJ
Is this rxn endo or exo thermic?

9. Sample Enthalpy Problem
How much heat is transferred when 9.22g of glucose in your body reacts with oxygen in respiration, which produces carbon dioxide and water.  
1 mole of glucose delta H  = -2803kJ
9.22g glucose 1 mol glucose -2803kJ
180g glucose 1mol glucose
= kJ

10. Sample #2
147g of Nitrogen dioxide is dissolved in water to produce nitric acid and nitrogen monoxide.
3mol nitrogen dioxide delta H = –138kJ
3NO2 + H2O  2HNO3 + NO
147g NO2 1mol NO kJ
46g NO2 3mol NO2
= kJ

11. 12-3 Hess’s Law
Hess’s Law –The enthalpy change for the net rxn (rxn=REACTION) is the sum of the enthalpy’s of the individual rxns.
An indirect method of measuring enthalpy given a series of rxns.
Rules for manipulating the equations:
If coefficients are multiplied or divided, ΔH must be also
If an equation is reversed, so is the sign on ΔH

12. Hess’s Law Problems ΔH = +118 + -113 = +68kJ
The brown haze of smog over large cities is often due to the production of NO2 from N2 and O2. Given the information below, what is the net enthalpy change for this reaction?
N2 + O2  2NO ΔH= +181 kJ
2NO + O2  2NO2 ΔH= - 113kJ
If reactants and products are on proper sides of arrows, add equations together.
N2 + O2 + 2NO + O2  2NO + 2NO2
N2 + 2O2  2NO2
ΔH = = +68kJ

13. More Practice 2. Calculate ΔH of S + O2  SO2
From the following enthalpy changes:
2SO2 + O2  2SO3 ΔH = –196kJ
2S + 3O2  2SO3 ΔH = -760kJ

14. 3. Calculate the ΔH of PCl3 + Cl2  PCl5
From the following enthalpy changes
2P + 3Cl2  2PCl delta H = -640kJ
2P + 5Cl2  2PCl delta H = -760kJ

15. One More  4. Calculate the delta H prime for 2S + 2OF2  SO2 + SF4
From the following rxns:
OF2 + H2O  O2 + 2HF delta H = -277kJ
SF4 + 2H2O  SO2 + 4HF delta H = -828kJ
S + O2  SO delta H = -297kJ

16. 12-4 Calorimetry
Calorimetry- the study of heat flow and heat measurement. 
These experiments determine the heats (enthalpy changes) of rxns by measuring the changes of temperature in a device called a calorimeter (we’ll make one in lab…)
Heat Capacity-the amount of heat needed to raise the temperature of the object by 1 degree Celsius. 
This is dependent on the mass and the composition of the substance.
Heating lots of water vs. a drop…
Heating water vs. metal…

17. SPECIFIC HEAT Specific heat-
The amount of heat needed to raise the temperature of 1g of water by 1 Co. 
Water’s specific heat is J

18. TRUE OR FALSE?
FALSE!
A small change in temperature may be produced by a large quantity of heat…
…in an object that has a very large heat capacity!
HEAT & TEMP ARE RELATED, BUT NOT THE SAME!
Specific heat is the amount of heat needed to raise the temperature
A small temperature change means that a small quantity of heat was transferred….

19. The Equation qsur = m x C x (Tf-Ti) qrxn = - qsur &
q = heat transfer measurement made in a calorimeter.
Sur = surrounding solution
Rxn = overall reaction
m = mass of water
C = specific heat
(Tf-Ti) is final temperature minus initial temperature

20. Sample Problem
g NH4NO3 dissolves in 60.0 g of water and the temp drops from 21.0 oC to 16.9oC. Calculate  .
NH4NO3  NH4  NO3
qsur = m x C x (Tf-Ti)
qsur = (60.0g)(4.184J/goC)( oC) = x 103 J (heat transferred to the surroundings)
qrxn = 1.03 x 103 J (heat transferred in the reaction)
But this is not  yet…

21. Everything Goes Through Moles!
4.25g  NH4NO3 1mol NH4NO x103 J
80.0gNH4NO3 1 mol NH4NO3
Is this reaction endo or exo thermic????

22. Again!
13.7g of solid lead II nitrate dissolves in 85.0g of water in a calorimeter, the temp drops from 23.4C to 19.7 C. Calculate the delta H for this rxn
Fill in the correct numbers and solve
Convert the lead II nitrate grams to mols
Convert the mols from above to J for 1 mol (or the given balanced eq.)

23. Do problem #6 on page 397 text
It’s a good one…

24. 12-5 What is Heat?
The Caloric Theory- scientists like Lavoisier of the 1700's believed heat was fluid.  That it flows…
Count Rumford and James Joule did experiments indicating that heat was a form of energy.
The Kinetic Theory -In the 1900 century scientists began to understand that heat was the result of motion and vibration of particles of matter.
Heat-the transfer of kinetic energy from a hotter object to a cooler one. 
Now, hopefully you understand why temperature is a measure of ___________ ____________!

25. Entropy
Disorder – randomness in a reaction with regard to energy that cannot be reharnessed (heat!) ΔS
A reaction is ALWAYS spontaneous if
ΔS + and ΔH –
A reaction is NEVER spontaneous if
ΔS - and ΔH +
If both signs are the same, spontaneity
depends on temperature