Determining the composition of pennies with redox reactions
Introduction:
The understanding of oxidation and reduction must be clear to carry out this lab. With the understanding of these concepts we can calculate or hypothesize for the properties of each element or compound. Oxidation involves the gain of electrons of hydrogen or the loss of oxygen or decrease in oxidation state. If zinc completely reacts with HCL, then the theoretical yield of copper should be equivalent to the actual yield.
Purpose:
In this lab, we will determine the percent composition of a modern (post-1982) penny by using a strong acid to react and dissolve the zinc core, leaving only the copper coating. Once only copper remains, we will compare its mass to the…show more content… Use metal shears to cut the edges of the coin in several places. 2. Weigh the penny and record the mass. 3. Under the hood, place the penny into a 150-mL beaker and add approximately 20 mL of concentrated hydrochloric acid. 4. When the coin stops producing gas bubbles, decant the acid into another 150-mL beaker. Record the reaction time. 5. Wash the penny in distilled water. Then rinse with acetone. When the penny is dry, weigh and record the mass of the copper shell. 6. Calculate the percentages of copper and zinc in the penny.
Notes
Pennies that have been made after 1982 are a composite of zinc and copper. The copper is plated on top of the zinc. What if we could reverse this composite by placing the zinc on top of the copper? The zinc can be removed from the penny by cutting the coin and creating a reaction between the zinc and the concentrated hydrochloric acid:
Zn(s) + HCl(aq) -> Zn2+(aq) + Cl - (aq) + H2(g)
Copper does not react with hydrochloric acid. After removing the zinc, reweigh the penny and obtain the mass of copper that is present. The remaining copper can be plated with zinc and the brass alloy produced. This process entails first creating a reaction between zinc and 6M sodium hydroxide:
Zn(s) + 2 OH-(aq) -> ZnO22-(aq) + H2 (g)
Data:
mL of ZnCl2 | 10 mL | Moles of ZnCl2 | 0.10 mol | mL of NaOH | 18.5 mL | Mass of Zn(OH)2 | 0.741 g | Mass of Zn | 2.38 g |
