Lavoisier and Dalton
Chemical Reactions
Note to Instructors: This lab can be done solo as a DEMONSTRATION
Purpose:
To observe some typical chemical reactions and to note that the mass during a chemical
reaction does not change. Also, to summarize simple chemical changes in terms of
balanced chemical equations.
Materials:
Top loading balance, 400 ml beakers, 6 - 8 test tubes/w test tube rack, 0.1M AgNO3,
0.1M NaCl, 0.1M Pb(NO3)2, 0.1M K2CrO4, 6M NH3, 6M HCl solutions.
Safety Precautions:
Wear safety glasses.
Hazardous Waste:
Silver and Chromium are toxic metals. Also, Chromium is a carcinogen. The
contents of all test tubes to which Silver and Chrome has been added should be disposed
of in the heavy metal waste container.
INTRODUCTION
Chemical equations represent what transpires in a chemical reaction. A chemical formula is an expression
showing the chemical composition of a compound in terms of the symbols for the atoms of the elements
involved. We use chemical formulas to indicate how new compounds are formed by chemical
combinations of other compounds. What products are formed? Products can be identified by their
chemical and physical properties. You will be able to observe a reaction by change of colors, precipitates
(a solid) that are formed, or a gas (bubbles) is given off.
In 1808 John Dalton, an English scientist and school teacher, presented the atomic theory that
started the modern era of chemistry. One of his first hypotheses states, a chemical reaction involves only
the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.
His second hypothesis states that to form a compound we need not only atoms of the right kinds of
elements but the correct numbers of atoms. His final hypothesis, law of conservation of mass, says that
matter can neither be created nor destroyed.
PROCEDURE
A.
Conservation of mass.
Put 5 ml of each of the following solutions into separate clean test tubes. Mark the test tubes with a grease
pencil for chemical identification. Place all test tubes into a 400 ml beaker. Place beaker with test tubes on
a top loading balance and weigh to the greatest accuracy possible to the nearest 0.01g. Combine
chemicals*.
*
Possible chemical combinations:
1.
Silver nitrate and sodium chloride
2.
Lead nitrate and sodium chloride
3.
Potassium chromate and lead nitrate
4.
Silver nitrate and potassium chromate
Keep a record as to what chemical combinations are in what test tube.
Place beaker with all test tubes back on the balance and weigh to the greatest accuracy. Does the weight
remain constant?
B.
Other chemical reactions.
To the test tube (# 1) containing the silver nitrate and sodium chloride, add 1 - 2 ml 6M NH3. What
happened?
To the test tube (# 2) containing lead nitrate and sodium chloride, add 1 - 2 ml 6M NH3. Does the ppt
dissolve in ammonia?
Place 5 ml of potassium chromate into a clean test tube, add 1 - 2 ml 6M HCl . What happened?
Then add 1 - 2 ml 6M NH3. Does the color change?
C.
Balancing equations.
Reactants and products will always be given, balance the equation by inserting suitable coefficients to
precede the formulas. The goal is to make the number of atoms of each element the same on both sides of
the equation. (Do not change any of the subscripts in the formulas because that would change the identity
of the substances).
Hints:
1.
Look for elements that appear only once on each side of the equation and with equal numbers (of
atoms) on each side. The compounds containing these elements must have the same coefficients.
Example:
Fe + HCl  FeCl2 + H2
Fe, H, and Cl appear only once on each side of the equation, but Fe but only Fe appears once on both sides.
Thus Fe and FeCl2 must have the same coefficient.
Next: to equalize either the number of Cl and H atoms on both sides of the equation.
Example: Fe + 2 HCl  FeCl2 + H2
Now Check: Reactant
Fe (1)
H (2)
Cl (2)
2.
Products
Fe (1)
H (2)
Cl (2)
If the preceding step does not apply, look for elements that appear only once on each side of the
equation but in unequal number (of atoms). Balance these elements first.
Example:C2 H6 + O2  CO2 + H2O
The number of carbon atoms are not the same on both sides of the equation.
C2H6 + O2  2CO2 + H2O
To balance the carbon atom we place a 2 in front of the CO2.
To balance the H atoms, we place a three in front of the H2O
C2H6 + O2  2CO2 + 3 H2O
Now the C and H atoms are balanced, but not the O. There are seven O on the right hand side and two on the
left. How many pairs of O2 are needed to make seven O? Answer: 7/2O2 = 7O
So,
C2H6 + 7/2O2  2CO2 + 3 H2O
Now to multiply the entire equation by 2 to convert 7/2 to 7.
2(C2H6 + 7/2O2  2CO2 + 3 H2O)
2C2H6 + 7O2  4CO2 + 6 H2O
Now check:
Reactants
Products
C (4)
H (12)
O (14)
3.
C (4)
H (12)
O (14)
Usually, the remaining elements can usually be balanced by inspection.
Name: _______________
Date: ________
Balance the following equations:
__AgNO3 + __NaCl  __AgCl + __NaNO3
__Pb(NO3)2 + __NaCl  __PbCl2 + __NaNO3
__Pb(NO3)2 + __K2CrO4  __PbCrO4 + __KNO3
__K2CO3 + __Ba(NO3)2  __BaCO3 + __KNO3
__Cu + __S8  __Cu2S
__Zn + __HCl  __ZnCl2 + __H2