Energy and Percent Yield

I. Enthalpy is the energy stored in chemical bonds

                A. The symbol of enthalpy is H

                                a. units of joules (J)

                B. Change in enthalpy is ΔH

                C. In exothermic reactions enthalpy decreases

                D. In endothermic reactions enthalpy increases

II. Calorimetry

                A. To experimentally determine the heat released we need to know 3 things

                                a. temperature change (ΔT)

                                b.  mass (m)

                                c. specific heat capacity (C)

These are related by the equation:

ΔH = mCΔT

Calculate the heat required to warm a cup of 723 g of water (c=6.98J/g˚C) from 30.0˚C to 50.0˚C
C = 6.98J/g˚C
M = 723g
T = 20.0˚C

▲H = mC▲T
= (723g)(6.98 J/g˚C)(20.0˚C)
= 100930.8
= 1.01×10⁴

Ex. 9500 J of heat are added to a 778 g glass of water initially at 56.0˚C calculate the final temperature of the water (c = 6.21 J/g˚C)
▲H = 9500 J
m = 778 g
C = 6.21 J/g˚C
Ti = 56.0˚C

▲H = mC▲T
= mC (Tf-Ti)

95000J = (778g)(6.21J/g˚C)(Tf-56.0˚C)
95000J = (4831J/˚C)(Tf-56.0˚C)
95000J / 4831J/˚C = (4831J/˚C )(Tf-56.0˚C) / (4831J/˚C )
19.7˚C = Tf-56.0˚C
Tf = 75.7˚C

III. Percent Yield

                A.  The theoretical yield of a reaction is the amount of products that should be formed.

                B. The actual amount depends on the experiment

                c. The percent yield is like a measure of success

                                a. How close is the actual amount to the predicted amount?

Ex. The production of Octane (C₈H₁₈) is given by the equation:
C₈H₁₈ → 8C + 9H₂

If 38.0g of Octane are produced, determine the theoretical (predicted) yield of Carbon.

If 38.0g of Octane are produced, determine the theoretical (predicted) yield of Carbon.

 What is the percent yield of Carbon if 11.0 g is produced?
(11.0 g / 32 g) x 100 = 34%