Enthalpy of Neutralization

Topics: Thermodynamics, Enthalpy, Specific heat capacity Pages: 20 (2260 words) Published: November 8, 2013


Introduction
Enthalpy change, ΔH, is defined as the heat output of a system as it goes through a reaction under constant pressure. It is an important aspect of thermochemistry, which is the study of energy changes during a chemical or physical reaction . When we calculate enthalpy change, we always assume that the pressure is constant. We are able to calculate enthalpy change numerous ways, notably by the increase in heat, Q, given by an exothermic reaction or the heat absorbed by an endothermic reaction. To do this, we use the concept of calorimetry, the measuring of heat of chemical reactions or physical changes. For this concept, we use a device called a calorimeter, which is a device that creates an isolated system that enables the user to accurately measure the change in temperature, ΔT. We use the following formula in order to calculate Q, quantity of heat, given the mass of the substance, m; its specific heat capacity, c; and the change in temperature, ΔT, given in °C: Q = m · c · Δt [1]

Using this equation, we are able to calculate the heat lost or gained by one of the substances in the reaction. Since heat lost = heat gained in a reaction, we are able to therefore say that the heat lost by the first substance, is equal to the heat gained by the second substance, and therefore: Qsubstance 1 = -Qsubstance 2 [2]

Since the law of conservation of energy states that energy cannot be created or destroyed, the change in energy must equal zero and therefore the left side of this equation must be equal in magnitude but opposite in sign of the right side. By substituting equation [1] into equation [2] we are able to derive: m1 · c1 · Δt1 = - (m2 · c2 · Δt2) [3]

and thus we are able to calculate an unknown given the other 5 variables.
The determination of the change in temperature can be done by taking the temperatures in intervals and giving a graphical interpretation of these values. In these experiments, the initial temperature of the substance in question is taken before the second substance is mixed into the calorimeter. Therefore, the determination of the final temperature is slightly more complex. In the perfect case, the final temperature would be able to be calculated immediately after the substances were mixed, and we would be able to form a graph that consists of a straight line with constant slope. In the case of a dissolution, a curve is formed, because the temperature change occurs as the substance is dissolving into the solvent. Therefore, when a plateau has been reached, this is the final temperature.

We can find enthalpy using many different methods. We are able to find the enthalpy of neutralization, a reaction between an acid and a base, by measuring the heat absorbed or produced per mole of a certain substance. Therefore we can use the following equation, where ΔHN is equal to the enthalpy of neutralization; QN is the heat absorbed or produced; and nsub is the number of moles of the substance under consideration: ΔHN = QN / nsub [4]

Furthermore, we can measure the enthalpy of a solution, or of a dissolution, by the of addition the heat absorbed or produced per mole (n) of substance dissolved dissolved by the two substances involved in the reaction. In all cases, a positive enthalpy value results in an endothermic reaction because the solution absorbs energy, and a negative enthalpy value produces an exothermic reaction because the solution loses energy (Waterloo University). Procedure:

The experiment was performed following the procedure outlined in Lab # 3: The Enthalpy of Various Reactions. No modifications were made (CHM 1311 Laboratory Manual , 2013). Observations:
Table 1: Enthalpy of Copper
Measurement
Trial 1
Trial 2
Mass of metal (g)
10.68
10.51
Mass of calorimeter (g)
10.87
10.87
Mass of Lids (g)
7.088
7.088
Volume of Distilled Water (mL)
20.0
20.5
Mass H2O and Calorimeter (g)
30.97
31.48
----
-----
----
Initial Temp. of H2O (°C)...

Cited: Parker, V . B ., Thermal Properties of Uni-Univalent Electrolytes, Natl . Stand . Ref . Data Series — Natl . Bur . Stand .(U .S .), No .2, 1965
Randall, Merle, Rossini, Frederick D., The Journal of the American Chemical Society, Vol. 51(2), February 1929, Table VII, pp.334-335.
tutorvista. (2010). Enthalpy of Neutralization. Retrieved Oct 7, 2013, from tutorvista.com: http://www.tutorvista.com/bow/enthalpy-of-neutralisation
Waterloo University. (n.d.). Thermochemistry. Retrieved Oct 7, 2013, from www.science.uwaterloo.ca: http://www.science.uwaterloo.ca/~cchieh/cact/c120/thermosum.html
What in the World Isn 't Chemistry?(2013). General Chemistry. CHM 1311. Laboratory Manual: University of Ottawa.
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