Calculating+Heat+Reactions

Calculating Heats of Reactions

 * (Working at SATP - Standard Ambient Temperature and Pressure; pressure of 100.00 kPa and a temperature of 25 °C).**

Da Bomb Calorimeter is used to measure the amount of energy gained or lost in a chemical reaction**.**
 * Laboratory Method:**
 * This works since energy lost in an exothermic reaction (where ΔH is negative) is equal to energy gained by water (Q).
 * N.B.** Signs are opposite. If it is an exothermic reaction (-ΔH), then energy is gained by water (+).


 * Graphically:**
 * ∆Hr = ∑ potential energy of products - ∑ potential energy of reactants**



In practice, at STP, we look up the heat of formation (which is potential energy).
 * ∆H reaction = ∑H formation (products) - ∑ H formation (reactants)**

1) There is **no** heat of transformation for elements. 2) Diatomic elements (HOFBrINCl) or polyatomic elements (S) have no heat formation.
 * Rules:**

2H2 + O2 → 2H2O (According to rule 2 above, 2H2 has 0 heat formation and so does O2 since they are diatomic).Therefore, ∆Hr = 2[∆H formation (H2O)] - 0 ∆Hr = 2[∆H formation (H2O)]** In this reaction, heat of reaction in the molecules is the heat of formation of molecule.
 * Example:**
 * ∆Hr = ∑Hf(products) - ∑ Hf (reactants)

We //can// also calculate **∆**Hr (not be standard; however, using the energy of bonds). At STP, we will always get a specific value but with this method, it would be an estimate since same type of bond will have different energy according to where it is attached. Methanol Water The O-H bonds in methanol will haev different energy than O-H bonds in Water (it depends on the environment; can be due to polarity and other factors). N.B. ∆Hr = 2 (H-H) + 1(OO) - 4(H-O)** To break bond, input of energy is required. To form a bond, energy is released.
 * Other method to calculate ∆Hr**
 * Ex.**
 * 2H2 + O2 → 2H2O**
 * ∆Hr = ∑Bonds energy of reactants - ∑Bonds energy of products

Chemical reactions can be added together, and so can their heat of formation. Example: __Summation Reaction for Light Reaction (photosynthesis):__ 12 (H2O + λ + NADP + ADP → ½ O2 + NADPH2 + ATP) __Dark Reactions:__ 6 CO2 + 12 NADPH2 + 12 ATP → glucose (C6H12O6) + 6 H2O + NADP + ADP __Final Reaction:__ 6CO2 + 6H2O → glucose + 6O2
 * Hess’ Law**

What Hess’ Law says is that if we add two reactions together ∆Hr = ∆Hf(A) + ∆Hf (B)