Studio_9_HessLaw_modified-1

Studio_9_HessLaw_modified-1 - Chem. 25: Studio #8 _ _Heat...

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Chem. 25: Studio #8 _____ __Heat of Neutralization and Hess’s Law NAME:________________________ STUDIO:________________________ HEAT OF NEUTRALIZATION OF ACIDS AND BASES In the main, it’s not a good thing to run a chemical reaction on your skin. At one time, some people proposed that if you spilled an acid on your skin, the quickest way you could get rid of the hazard was to rinse with a base, thereby neutralizing the acid and destroying it. That’s a bad idea, however, for several reasons, not the least of which is that the neutralization reaction is quite exothermic and the heat liberated is sufficient to cause a major thermal burn in addition to the chemical burn caused by the initial exposure to acid. (Should you be exposed to acid, just wash with copious water and forget about the suggestion to neutralize it.) The bottom line in this lab: what is the heat associated with a neutralization reaction? Remember the simple way we are defining acids and bases: acids increase the hydrogen ion concentration of a solution; bases, the hydroxide ion concentration. Here is a classic neutralization reaction of a strong acid (HCl (aq)) and a strong base (NaOH (aq)) written in the form of molecular, complete ionic and net ionic equations: HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) Na + (aq) + Cl - (aq) + H 2 O (l) H + (aq) + OH - (aq) H 2 O (l) The driving force for this process is the formation of molecular water, and the forming of two covalent bonds gives off a significant amount of energy. The heat of a reaction can be determined by carrying out the reaction in a thermally insulated vessel called a calorimeter. The heat released by the reaction may be calculated by measuring the increase in temperature, T , of the calorimeter in which the reaction is run. To do this calculation, you must know the heat capacity of the calorimeter (sometimes called the calorimeter constant), which can be determined by supplying a known amount of heat ( q calor ) to the device and observing the temperature change. C c a l o r q c a l o r T = (1) The calorimeter used in this studio is a simple ‘coffee cup calorimeter,’ made by nesting one Styrofoam cup in another. In this set-up (pictured on the next page), the system consists of the dissolved and the Styrofoam cups. They are both the container for the reaction and the insulation that keeps all the heat within the aqueous medium. 1
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Fig. 1: a coffee cup calorimeter The heat capacity of the calorimeter is determined in this experiment by measuring the temperature change that occurs when a known amount of hot water is added to a known amount of cold water in the calorimeter. The heat lost by the hot water is equal to the heat gained by the cold water plus the calorimeter; assume that the calorimeter is perfect and that no heat is lost into the laboratory. To express this mathematically, let T 1 equal the temperature of the calorimeter containing 50 mL of cold water. Let
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Studio_9_HessLaw_modified-1 - Chem. 25: Studio #8 _ _Heat...

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