1. Energy and Chemical Equations

2. Endothermic and Exothermic
When chemical reactions occur, there are also energy changes.
In an exothermic reaction, like combustion, energy is released:
Reactants  Products + energy
The chemical bonds of the reactants have more energy than the products.
By convention, exothermic processes are given a (-) sign. (products-reactants = negative number)

3. Endothermic Reactions
In an endothermic reaction, energy is absorbed from the surroundings:
Reactants + Energy  Products
The chemical bonds of the products have more energy than the reactants do.
By convention, endothermic reactions are (+)
(products – reactants) = (+) number

4. Thermochemical Equations
Energy changes are often represented in chemical equations.
The energy change is called the Heat of Reaction (∆H) and is measured in kJ/mol
Table I gives some examples of thermochemical equations.

5. This equation can also be represented as follows:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) kJ
Note that the energy term is a product – heat is produced in the reaction
The minus sign goes away…

6. For an endothermic reaction:
This equation can be represented as:
N2(g) + O2(g) kJ 2NO(g)
Energy is a reactant in this case.
This reaction is an unintended consequence of combustion!!
It is responsible for 1/3 of anthropomorphic acid rain.

7. Heat of Solution
Note that thermochemical equations can be used to represent physical changes as well.
The Heat of Solution is the energy change that occurs when solutes dissolve in solvents, usually at 25⁰C.
From Table I:

8. Thermochemistry Problems Using Table I
How many kilojoules of heat are released when 2 moles of carbon dioxide are formed from its elements?

9. Which of the salts have endothermic heats of solution?

10. The combustion of 0.5 moles of methane (CH4) produces how many kilojoules of heat?

11. How much heat is absorbed by hydrogen and iodine to produce 1 mole of HI?

12. How many moles of glucose (C6H12O6) are needed to produce 1402 kilojoules of heat when burned in air?

13. Base your answer to the question on the information below.
Several steps are involved in the industrial production of sulfuric acid. One step involves the oxidation of sulfur dioxide gas to form sulfur trioxide gas. A catalyst is used to increase the rate of production of sulfur trioxide gas. In a rigid cylinder with a movable piston, this reaction reaches equilibrium, as represented by the equation below.
2SO2(g) + O2(g)  2SO3(g) kJ
Determine the amount of heat released by the production of 1.0 mole of SO3(g).

14. Given the balanced equation representing a reaction:
H+(aq) + OH-(aq) → H2O(l) kJ
In this reaction there is conservation of
mass, only
mass and charge, only
mass and energy, only
mass, charge, and energy

15. According to Reference Table I, Heats of Reaction, the dissolving of NH4Cl(s) in water is
exothermic and the heat of reaction is negative
exothermic and the heat of reaction is positive
endothermic and the heat of reaction is negative
endothermic and the heat of reaction is positive

16. Heat of Solution Lab
Objective: To determine energy changes which occur when various solids dissolve in water.
Safety Warning: All solutions in this lab are irritating to the skin. Do not touch any chemical or solution with your hands! Clean up all spills with plenty of cold water.

17. Procedure
Determine the mass of a clean, dry Styrofoam cup (calorimeter).
Measure about 100 ml of water in a graduated cylinder and place it in the Styrofoam cup. Determine the mass of the water and the Styrofoam cup. Calculate the mass of the water in your cup.
Record the initial temperature of the water in your cup. Do not remove the thermometer from your cup.
Measure out 8 to 12 grams of ammonium chloride. Record the mass of this compound in your data table.

18. Add the NH4Cl to the water in the calorimeter and stuff gently until the solid is completely dissolved.
Determine the highest or lowest temperature reached by the solution.
Carefully pour the solution down the drain, then rinse and dry the cup.
Repeat steps 2-7 with 8-12 grams of CaCl2.
Clean up your lab table and wash your hands before starting your calculations.

19. Calculations
Calculate the change in temperature for each reaction and enter the values in the data table.
Calculate the energy in joules absorbed or released by the water in each reaction. These calculations must be shown on a separate sheet of paper. Remember,
q=mCΔT
where q=heat (in Joules)
C= specific heat of water(J/ºC) = 4.18J//ºC
m=mass of water, and ΔT= change of temperature

20. Calculate the energy released or absorbed in Joules per gram of solid used in each solution. Be sure to include your calculations.
Using the given accepted values, calculate your % error and enter your results in your data table.

21. Initial Temp. of water (ºC) Final Temp. of water (ºC)
Compound
NH4Cl
CaCl2
Mass used (g)
Mass water used (g)
Initial Temp. of water (ºC)
Final Temp. of water (ºC)
Temperature Change(ΔT) (ºC)
Total Joules released or absorbed
Joules per gram of compound dissolved
ΔHsolution Accepted Value (J/g)
271
746
% Error

22. Conclusions (on a separate sheet of paper)
Classify each reaction as endothermic or exothermic. Remember that the “system” is the compound and the “surroundings” is the water.
Give one possible source of experimental error in this lab, and suggest a way to reduce the error.
What kind(s) of bonding do these substances have? Justify your answer in terms of the types of elements (metal/non-metal) in each compound.

23. Write equations for the dissolving of ammonium chloride and calcium chloride in water. Include the states of the materials in parenthesis next to each product and reactant.
If you dissolve a mole of each compound in a kg of water, which solution would have the lowest freezing point? Explain your answer in terms of the number of solute particles.