Introduction

The study of energy is an important component chemists use to study chemistry. Most of the energy used in society comes from chemical reactions, which include the combustion of fossil fuels. Another example of the practical uses of this theory is when gasoline is combusted in a car engine, the engine block becomes hot and the car’s pistons move (Brain, 2007). This example demonstrates two ways heat can be transferred, which are through heat(temperature change) or work. Since a significant amount of work is not produced in chemical reactions, most of the energy comes in the form of heat (Jones, 2007). This component of chemistry is known as thermochemistry (Jones, 2007). During a chemical reaction, chemical bonds are created and destroyed, which means energy can be either released or absorbed. With constant pressure, the change in energy of a reaction is known as the heat of reaction or enthalpy change (ΔH) (Jones, 2007). In order to determine the amount of energy release or absorbed, chemists carry out the reactions inside a container, known as a calorimeter, that helps insulates the reaction from its surrounding (University of Florida, 2007). Therefore, all of the enthalpy change of a reaction is confined to the container, raising or lowering the temperature of its contents (University of Florida, 2007). This, in short, is the basis of calorimetry. However, for this experiment the heat capacity of the calorimeter is neglected in the calculations. The amount of heat released or absorbed can be determined by measuring the change in temperature (Jones, 2007). If heat or energy is released during the reaction, this reaction is exothermic and ΔH is negative (Jones, 2007). On the other, if heat or energy is absorbed during the reaction, it is an endothermic reaction and ΔH is positive (Jones, 2007).

Reactions between strong acids and bases are also reactions between strong electrolytes, while weak acids and bases are also known as weak electrolytes (Tracey, 2003). Electrolytes are solutions or substances in solution that consists of various chemicals that can carry electric charges (Tracey, 2003). As stated before, reactants can fall into two categories, strong and weak electrolytes. Strong electrolytes are substances that completely, or almost completely, ionize and dissassociate in solution (Tracey, 2003). Some examples include solutions of HCl, KOH, NaOH and KCl (Tracey, 2003). Weak electrolytes, on the other hand, only partially dissociate into ions (Tracey, 2003). These include acetic acid, phenol and ammonia (Tracey, 2003). With the neutralization of weak electrolytes, the heat of reaction can be either smaller or larger than the value of the amount of heat produced with strong electrolytes, which is -55.90 kJ of heat per mol of H+ (Tracey, 2003). In this experiment, the weak electrolyte/acid phenol has a ΔH value of 25.3 kJ/mol, which means this process is endothermic (Tracey, 2003).

In order to determine the ΔH of a reaction, chemists carry out chemical reactions and use certain results. First, the amount of heat (Q) required to raise the temperature of a sample must be calculated (Helmenstine, 2007) . To do this, the total mass of the sample must be multiplied by the temperature change, ΔT, and the specific heat capacity (s) (Helmenstine, 2007).  The specific heat capacity, which is the amount of heat needed to raise the temperature of one gram of a substance one degree (Helmenstine, 2007), can be assumed to be equal to that of water, which is 4.184 J/°Cg. After converting the value of Q to kilojoules, the number of moles of water must also be calculated. To do this, the limiting reagent of the reaction must be determined. For example, the neutralization of 50 mL of 2.0M NaOH and 40mL of 2.0M HCl means that the limiting reagent is HCl, since it produces the least amount of moles between the two reactants.  As stated before, this reaction is between two strong electrolytes. This means that the two reactants are completely disassociated into ions in diluted solutions (Tracey, 2007). Therefore, this neutralization reaction would be:

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H+ + Cl- + Na+ + OH-   HOH + Na+ + Cl-  (Helmenstine, 2007)

After eliminating the common ions, the chemical reaction is essentially:

H++ OH-   HOH  (Helmenstine, 2007)

This simply means that H+ is the limiting reactant and since there is a 1:1 ratio between H+ and H2O, the amount of moles of water must be used in the calculations. As a result, the ΔH is the number of kJ/mol of water formed. Thus, the number of moles of H20 is equal to the product of the concentration and volume of the limiting reagent (n=CV) (Helmenstine, 2007). As well, the value of ΔH for ...

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