MHS Chemistry
An Exothermic Chemical Reaction

In this lab, you will be investigating heat in a chemical reaction. Perform each of the following steps in order, filling in the blanks as you go. Show your work in the spaces provided.

Solid potassium hydroxide reacts with aqueous hydrochloric acid (hydrogen chloride) to form aqueous potassium chloride and liquid water. Write the balanced chemical equation for this reaction.

Use a graduated cylinder to obtain 50.0 mL of 0.25 M hydrochloric acid. Pour this into a small beaker or styrofoam cup, and measure the temperature. The temperature is _______________ °C.

Potassium hydroxide absorbs water from the atmosphere extremely quickly. This makes it hard to measure its mass very accurately. It also causes jars left open to change into a solid (useless) chunk. Finally, it will cause severe skin burns from very little contact, so if you think you touch any, rinse it off very well. Very quickly and carefully measure 1.000 grams of potassium hydroxide into a weighing dish, and immediately cap the jar.

Quickly transfer all of the potassium hydroxide pellets into the acid solution in the beaker. Take note of the extreme temperature reached. The [highest, lowest] temperature reached was _______________ °C.

Recall from the first term how to calculate a heat change from a temperature change. Complete the equation:

                  q = _______________

Calculate the heat change (including sign) of the water in this reaction. Assume ("pretend") that the solution acts like pure water. Don't forget the density and specific heat of water!

                  q(water) = _______________ calories

therefore, q(reaction) = _______________ calories

Determine the number of moles of hydrogen chloride present in your starting solution. Recall that you started with 50.0 mL of solution, with 0.25 moles of hydrogen chloride per liter.

                  _______________ mol hydrogen chloride

Determine the number of moles of potassium hydroxide present in 1.000 g.

                  _______________ mol potassium hydroxide

Based on your balanced reaction, is there a limiting reactant? If so, which chemical?

Calculate the heat change per mole of hydrogen chloride actually reacted.

                  q(reaction) = _______________ cal/mol hydrogen chloride

Write down the balanced equation again (from the beginning of this section), but this time include the appropriate amount of energy as a product or reactant.

DISPOSAL The solution produced can be flushed down the drain. Rinse the glassware thoroughly, and leave it in the dishwasher.

[Chapter 12 Notes][MHS Chemistry Page]