Conservation of Mass in a Chemical Reaction

CONSERVATION OF MASS IN A CHEMICAL REACTION

LAB #3

INTRODUCTION:

In this chemical reaction the elements are re-arranged into newcompounds, however none of the atoms of any element are destroyed or in any way lost.

Each atom that entered the reaction as a reactant MUST be accounted for in the products. The reaction is:

Pb(NO3)2(aq) + K2Cr2O7(aq) PbCr2O7(s) + 2KNO3(aq)

The reactants

Before Reaction

Input

The Products

After the reaction

Output

Notice that each atom from the reactants is accounted for in the products, for example, there is onePb as a reactant and one Pb as a product. No mass should be lost in a chemical reaction; itmust all be accounted for. As you do the lab you will notice that the physicalpropertiesof the reactant substances are different than product substances.

PURPOSE:

1) To examine chemical reactions.

2) To use table S to name and identify elements.

3) To use table E to name chemical groups.

4) To introduce phases of matter.

MATERIALS.

-a micropipette of Pb(NO3)2(aq)

-a micropipette of K2Cr2O7(aq)

-beam balance

- 100 ml beaker

PROCEDURE (micro scale):

1)Place the pipette of K2Cr2O7(aq)and the pipette of Pb(NO3)2 (aq) in the beakerand record the mass. This is the mass of the system before the reaction.

2)Expel the solutions from the pipettes into the beaker and mix by swirling.

3)Observe any changes, record in your lab journal.

4)Record the mass of the beaker with the solution WITH the empty pipettes in thebeaker. This is the mass of the reaction after the reaction.

PROCEDURE (standard scale)

1)Using a graduated cylinder, measure 5.0 ml. of K2Cr2O7(aq) and place it into asmall test tube. Measure 5.0 ml of Pb(NO3)2(aq) in a clean graduated cylinderand place it into another small test tube.

2)Place the test tube of K2Cr2O7(aq)and the test tube of Pb(NO3)2(aq) in the beakerand record the mass. This is the mass of the system before the reaction.

3)Transfer the solutions from the test tubes into the beaker and mix by swirling.

4)Observe any changes, record in your lab journal.

5)Record the mass of the beaker containing the solution WITH the empty test tunesin the beaker. This is the mass of the system after the reaction.

QUESTIONS:

1)Prove that the above reaction is balanced; list the number of each element that isfound before and after the reaction (across the arrow).

2)Define solution, precipitate and identify these in your observations.

3)Using table S, write the name of each element that appears in the reaction (hint,each element has only one capitol letter in its symbol)

4)Using table E, identify any polyatomic groups in the reaction.

5)Define solid (s), aqueous (aq)

Lab Report Notes

TITLE: See page 1

ABSTRACT: Summary of the lab. Consult the handout on writing a lab report if you are not sure what goes here.

INTRODUCTION: DO NOT COPY THE INTRODUCTION!!!Your introduction should discuss:

A) What a chemical reaction is.

B) What are the ways you can tell a chemical reaction has occurred.

C) Why equations must be balanced.

METHODS AND MATERIALS. See page 1. The procedureshould again be past tense, impersonal, and written in complete sentences.

RESULTS:

A) Data table:

Micro Scale (Procedure 1) / Standard Scale (Procedure 2)
Mass of Reactants (Trial 1)
Mass of Products (Trial 1)
Mass of Reactants (Trial 2)
Mass of Products (Trial 2)

B) List all observations you made during the 4 trials (what did the materials look like BEFORE and AFTER the reaction.

C) Answer questions 1-5.

DISCUSSION:

First discuss what the Law of Conservation of Mass is. Then discuss how it applies to this lab.

Second, explain how you know if a chemical reaction takes place. What happened in this lab.

Lastly, give some insight as to why some groups did not get equal values during their trials.