8th Grade Science Professor Shai

Synthesis of Zinc Chloride

Section 6.4 Page 118

Introduction

The theme of this experiment is to exhibit the law of constant proportions.  Namely, that compounds are composed of substances using the same proportions of mass.  In this case, a fixed amount of hydrochloric acid will be mixed with zinc to produce zinc chloride.

Goal

To synthesize zinc chloride from zinc and hydrochloric acid, and measure the ratio of the reacting zinc to the zinc chloride produced.  Each group will have 10 cc of acid and varying mass of zinc.  With a small amount of zinc, the zinc will all be used up and there will be acid leftover after the reaction.  With a large amount of zinc, all the acid will be used up, and there will be zinc leftover after the reaction.  The goal is to find out how much zinc reacts, and how much zinc chloride is produced.

Method

First Day

1.  Get 10 cc of hydrochloric acid from me and keep it in a graduated cylinder.

2.  Get some zinc from me - you will receive anywhere from .5 to 4 grams.  Mass it to make sure you know how much you have.  Mass of zinc to start =

3.  Put the zinc in test tube, and have the test tube sit inside a beaker of cold water.  Pour the acid into the test tube.  The reaction should give off a lot of heat - hence the beaker of cold water.  It also produces hydrogen.  The reaction ends up with a liquid in which zinc chloride is dissolved, and perhaps some excess zinc that will be visible.  Let it sit overnight.

4.  Discussion -- we will discuss the ideas behind this experiment the rest of the day.

Next Day

1.   - Mass an evaporation dish and pour the liquid from the reaction into it.  If any zinc remains, do not pour it into the dish.  Leave the excess zinc in the test tube.
Mass of evaporation dish =

2.  Rinse the test tube (and zinc if any) with 5 cc water and pour the water into the evaporation dish.  This is done in order to get everything (excess acid, zinc chloride, etc) except the zinc into the dish.

3.  Dry and mass the leftover zinc, if any.  Calculate the mass of zinc that reacted.

Mass of test tube and zinc =                      Mass of test tube =             
Mass of remaining zinc =
Mass of test tube and zinc - Mass of test tube =   
Mass of reacting zinc = Mass of zinc to start - Mass of remaining zinc =

4.  Now gently heat the evaporation dish until the liquid (acid, if any, and water) is gone and there is only precipitate (zinc chloride) left.  Danger:  You must wear goggles!  If acid splatters on your face or eyes you will be injured.  Be careful not to let any substance splatter out when you are heating.  If you heat too long, the liquid will all evaporate and the solid precipitate of zinc chloride will start to melt.  You can do this on purpose to make sure the liquids are gone, but then let the melted zinc chloride cool back to solid to be sure.  You must make sure the zinc chloride is dry because otherwise, when you mass it you will be massing it along with extraneous liquids.

5.  Mass the zinc chloride and the dish as soon as it cools to the touch.  Leaving it too long, will allow it to absorb water from the air and mess up your mass.

Mass of zinc chloride and dish =
Mass of zinc chloride produced = Mass of evaporation dish - Mass of zinc chloride and dish =

6.  Calculate the ratio of the mass of zinc that reacted to the mass of zinc chloride produced.

Ratio = Mass of reacting zinc / Mass of zinc chloride produced  =

Conclusions

Questions:

How would your ratio calculation be affected if you did not completely dry the zinc chloride before massing it?  Explain.





How would your ratio calculation be affected if you did not wash the zinc and test tube before massing the zinc?  Explain.




How would your ratio calculation be affected if you splattered a lot of solution out when you were heating the evaporation dish?  Explain








Book Problems:  Page 120: 10-13, and Page 122:  14-15.