exam review time

get ready boys...notebook

friday nov 26

ep 5 quiz lab equipment
ep 6/1 lab gas tests for Hydrogen
ep 4/1 molecular formula quiz

Assignments Nov 22, 2010

EP 4--empirical formulae and molecular formulae calculations
EP 5--gas detection and flame tests along with heat of formation
ep 6--flame tests, liquid tests for cations, and mixing HCl and Zinc

experiments this week

ep 4 anions lab day 1 and cations lab day 2
ep 5 thermochemistry
ep 6 identifying unknowns-gases

:D have fun boys

textbooks and notebook checks this week

i'm checking to see you brought your textbook and did your notes and hw this week

Debate Results

AC 2 Teechayut, Tanachai, and Kasidey finished 5th overall at the 5th Thailand High School Debate Championships!

Reminder textbooks next week

can everyone have textbooks

Thermochemistry Lab: EP 5 EP 6

Thermochemistry and Hess's Law

In this experiment students determined the enthalpy change that occurs when sodium hydroxide and hydrochloric acid solutions are mixed. Next, the enthalpy change for the reaction between sodium hydroxide and ammonium chloride was measured. Lastly, they determined the enthalpy change for the reaction between ammonia and hydrochloric acid. An algebraic combination of the first two equations can lead to the third equation. Therefore, according to Hess's law, an algebraic combination of the enthalpy changes of the first two should lead to the enthalpy of the third reaction.

The molecular equations for the reactions are as follows:

(1) NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l)

(2) NH4CI(aq) + NaOH(aq) --> 4NH3 (aq) + NaCl(aq) + H2O(l)

(3) NH3 (aq) + HCl(aq) --> NH4CI(aq)

There is no single instrument that can directly measure heat in the way a balance measures mass or a thermometer measures temperature. However, it is possible to determine the heat change when a chemical reaction occurs. The change in heat is calculated from the mass, temperature change, and specific heat of the substance which gains or loses heat.

The equation that is used to calculate heat gain or loss is:

q = (grams of substance) x (specific heat) x D T

where q = the heat energy gained or lost and DT is the change in temperature. Since DT = (final temperature minus initial temperature), an increase in temperature will result in a positive value for both DT and q, and a loss of heat will give a negative value. A positive value for q means a heat gain, while a negative value means a heat loss.

Acid-base neutralization is an exothermic process. Combining solutions containing an acid and a base results in a rise of solution temperature. The heat given off by the reaction (which will cause the solution temperature to rise) can be calculated from the specific heat of the solution, the mass of solution and the temperature change. This heat quantity can then be converted to the enthalpy change for the reaction in terms of kJ/mole by using the concentrations of the reactants.

According to Hess, if a reaction can be carried out in a series of steps, the sum of the enthalpies for each step should equal the enthalpy change for the total reaction. Another way of stating "Hess's Law" is: If two chemical equations can algebraically be combined to give a third equation, the values of DH for the two equa­tions can be combined in the same manner to give DH for the third equation. An examination of the acid-base equations above shows that if equation (2) is subtracted from equation (1), equation (3) will result. Therefore, if the value of DH for equation (2) is subtracted from that of equation (1), the enthalpy change for equation (3) should result.