February 21st -- Ahmad Kilani

Enthalpy Calculations

Key Concepts

• The heat content of a chemical system is called the enthalpy (symbol: H)
• The enthalpy change (H) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.
• H = H_(products) - H_(reactants)
• H is specified per mole of substance as in the balanced chemical equation for the reaction

The enthalpy change, or H,  for each reaction is unique. So what relationship exists between the heat produced or absorbed by a reaction and the amount of reacting substance?

The energy change in these reactions varies directly as the number of moles of substance reacted or formed. The factor that determines the amount of heat absorbed or released is the H of the reaction: 

Moles of compound reacted or formed	Varies directly with	Heat absorbed or released in the reaction
	<--------------------->
	H reaction (kJ)

The energy is a variable because it differs from one reaction to another. However, it is a given that it is going to be positive when it is absorbed (i.e LHS, or reactants side); and negative when it is produced (i.e RHS, or products side). 

Examine the following reaction:

2 C₄H₁₀ (l) + 13 O₂ (g) ---> 8 CO₂ (g) + 10 H₂O (g) + 5315 kJ

The equation tells us that 8 moles of CO2 (g) releases 5317 kJ , since the relationship is a direct one then 2 moles would release one quarter as much heat or 2 moles would release 1,329 kJ.

To calculate heat changes using equations, we will perform the following steps

1) calculate the number of moles of substance reacted or formed.
2) create a proportion using the balances and heat in the chemical equation.
3) solve for missing quantity.

Question:

Calculate the amount of heat released when 25.0 grams of C₄H₁₀ (l) is burned in oxygen using the equation above.

Solution:

.

Calculate the number of moles of C₄H₁₀ (l), and times it by the energy per one mole.

MM of  C₄ is  12x4= 48.0 g/mol

MM of   H₁₀  is  1x10=10.0 g/mol

Total MM is 58.0 g/mol

Therefore, 

the conversion is from grams to moles to energy (heat).

25.0 g  x  1 mole   x  2657.5  KJ     =  1145.47 KJ   ---> ( 3 sig figs) = 1150 KJ 

                 58.0 g           1 mole

So 1150 KJ  of heat would be released when 25 grams is burned.