“Statistical Analysis of
Zinc Coated Washers”
Purpose of the Experiment
Determine how many centimeters
of zinc and layers of zinc atoms
are present on the surface of a
galvanized steel washer.
What is Corrosion?
Natural process of deterioration of metals and
alloys in a corrosive environment.
Results in an actual decrease in the thickness or
size of the original metallic structure.
“A poorly protected
surface can be a big
mistake - So don’t
compromise ...
Galvanize!” ☺
Corroded
Galvanized
The most common corrosion reaction is the
rusting of iron in water
4 Fe + 6 H2O + 3 O2 <--> 4 Fe(OH)3
The oxidation portion of the reaction results
in the actual loss of metal
Fe <--> Fe+2 + 2 e- Step one (OX)
Fe+2 <--> Fe+3 + e- Step two (OX)
Fe <--> Fe+3 + 3 e- Overall Oxidation half-reaction
The reduction portion of the reaction
drives the process of corrosion
O2 + 2 H20 + 4 e- <--> 4 OH- Reduction half-reaction
What is Galvanizing?
The process of
galvanizing consists
of coating metals,
such as iron, with a
thin protective layer of
zinc.
Before &
After
Hot Dip Galvanizing
The zinc layer
provides protection to
the metal from
corrosion.
Anodic Protection
An anodic layer, such as chromium,
nickel, copper, or paint, allows corrosion to
grow under its layer when it is scratched.
Cathodic Protection
The steel is protected by the
surrounding zinc even if it is scratched.
How Does Zinc Protect The Underlying Iron Surface?
Zinc coatings prevent corrosion of the protected metal by forming a
barrier, and by acting as a sacrificial anode if this barrier is damaged.
When exposed to the atmosphere,
zinc reacts with oxygen to form zinc
oxide, which further reacts with
water molecules in the air to form
zinc hydroxide.
Finally zinc hydroxide reacts with
carbon dioxide in the atmosphere
to yield a thin, impermeable,
tenacious and quite insoluble dull
gray layer of zinc carbonate which
adheres extremely well to the
underlying zinc, so protecting it
from further corrosion.
http://www.videopediaworld.com/video/41136/Deconstruction-Building-a-House-Galvanizing-Nails
The oxidation of zinc is more likely than the oxidation of iron.
*
*
These potentials indicate the relative thermodynamic
tendency for the indicated half-reaction to occur.
Zn <--> Zn+2 + 2 e– E = –0.763 volts
Fe <--> Fe+2 + 2 e– E = –0.409 volts
Zinc Coating Experiment
1. Get 5 washers, filter paper & Vernier calipers from the stock room.
2. Keep washers in the same order during the experiment. This can be
done by writing numbers 1 to 5 on the filter paper.
3. Record on the filter paper the inner and outer diameters and the height
of each washer to the nearest 0.01 cm. Have TA verify your values.
Then record them on your datasheet.
Vernier Calipers
Outer diameter
Inner diameter
https://www.youtube.com/watch?v=ZUNoWWw6V10
Height
Zinc Coating Experiment
4. Weigh and record the mass of each washer to the nearest 0.001 gram.
(Making sure to weigh the washers on the same balance each time.)
To Weigh: Place filter paper on the balance. Tare. (Set the balance to zero.)
Place the washer on the filter paper. Record weight.
Zinc Coating Experiment
5. In an 100 ml beaker, get ~60 ml of 3M HCl.
6. Attach copper wire to one of the zinc washers.
7. Dip the washer in the beaker of HCl and observe the reaction.
Zn (s) + 2 HCl
→
H2(g) + ZnCl2 (aq)
8. Allow the reaction to continue until bubbling stops, approximately 90 sec.
(The surface will change from shiny silver to a dull gray.)
9. Remove the washer from the acid and rinse with distilled water over a waste
beaker.
Zinc Coating Experiment
10. Use paper towel to dry the washer.
11. Place dried washer on
corresponding filter paper.
12. Weigh the washer on the same
balance used before.
13. Record the mass.
14. Repeat the same procedure for
other washers.
15. Dispose the washers in the used
solids container.
16. Dispose the exhausted HCl in the
liquid waste container.
Significant Figures Review
All non-zero digits are significant, for example, 123 has three significant
figures, 230 has two significant figures, and 300 has one significant figure.
Zeros between non-zero digits are significant, for example, 12.507 has five
significant figures.
Zeros to the left of the first non-zero digit are not significant, for example,
1.02 has three significant figures, 0.12 has two significant figures, and 0.012
also has two significant figures.
If a number ends in zeros to the right of the decimal point, those zeros are
significant, for example, 2.0 has two significant figures and 2.00 has three
significant figures.
Throughout Chemistry 2, the proper number of significant figures must
be used in all laboratory reports and on all examinations.
Failure to do so will result in the loss of credit.
For more practice with significant figures go to web.mst.edu/~gbert
Calculations Using Significant Figures
Significant figures in additions and subtractions
Decimal places are overwriting the significant figure rule.
The answer should have the same number of decimal places as the
quantity with the least decimal places. For Example,
3.7 m + 9.40 m = 13.1 m
tenths place + hundredths place  tenths place
2.354 L + 1.2 L = 3.6 L
3.67 kg + 12.498167 kg = 16.17 kg
Significant figures in multiplications and divisions
The product or quotient should have the same number of
significant digits as the quantity with least significant figures. For
Example,
(0.023 m) x (3.40 m) = 0.078 m2
2 sigfigs x 3 sigfigs  2 sigfigs
56.90125 s / 2.45 s = 23.2
Calculations
Di: Inner diameter
Total Surface Area =
2p(r02-ri2)+ 2pr0h + 2prih
Height of
the washer,
h
where r = radius = ½ diameter
Do: Outer diameter
5 individual measurements for each washer
1. Volume of Zinc Coating = Mass of Washer Reacted / Density of Zinc
where dzinc=7.14 g/cm3
2. Total Surface Area of Zinc = Total Surface Area of Washer
3. Thickness of Zinc Coating = Volume of Zinc Coating / Total Surface Area of Zinc
4. Thickness of Zinc in Atoms, where Diameter of Zn atom = 268 pm, 1 m = 1012 pm
5. Error Analysis & Statistics: Find the Mean (Average), Standard Deviation and
Confidence Interval (98%) for Volume, Surface Area and Thickness of Zinc in
Atoms.
Error Analysis of Accuracy & Precision
Accurate
Precise
Accurate & Precise
The average is
accurate but
not precise.
The average is
precise but
not accurate.
The average is both
accurate and precise.
Systematic or Determinate Errors: Shifts in the measured values from the
true values which reduces the accuracy of a result. (An example of a
systematic error is misreading a graduated cylinder).
Random or Indeterminate Errors: Shifts in the measured values from the
true values which influences the precision or scatter of the result. (Examples
of random errors might be the imprecision among multiple readings).
Determination of Accuracy
The accuracy can be determined by looking at the difference between
the expected (theoretical) average and the experimental (observed)
average.
Percent Error
%E  
Xtheo r  Xobs
100
X theo r
The percent error is the absolute value of the quantity of the theoretical
value minus the observed value divided by the theoretical value and
multiplied by one hundred.
The Average or Mean Value (xbar)
N
X 
X
i 1
i
Ʃ means “sum”
N
The average or mean of a set of numbers, Xi, is found by adding the
numbers and dividing by the number of values, N.
Thus the average of 3, 5, 7, 3, and 5 = 23 / 5 or 4.6.
The Standard Deviation
The standard deviation, a measure of the spread of N values, Xi,
about the average value, , a measure of precision, is given by,
N

2
(
X


)
 i
i 1
N
The standard deviation is used for large populations, N ≥ 30.
*Estimate of the Standard Deviation
If the number of values, N, is small, i.e., if N < 30, an estimate
of the standard deviation, s, is given by,
N
s
2
(
X

X
)
 i
i 1
N 1
*We will be using the
Estimate of the
Standard Deviation
because we have a
small data set.
Confidence Limit
X
t
N
where t is Student’s t-factor.
At the 90 % confidence limit, 90

timesX out of 100 the true value will
be
within
±1.64
of
the
experimental results.
The confidence limit defines an
interval about the average that most
likely contains .
Student’s t-factors are given in tables
for different probabilities. (Note: The
table in your book is on page 56.)
Graphs of the Volume, Surface Area and Thickness of Zinc in atoms.
Page 75 #2: In Excel (or any program that will make graphs),
make 3 column graphs where the washers and the average
washer are listed on the x axis of all three. On the y axis of graph
#1 show their volumes; on graph #2 their surface areas; and, on
graph #3 their thicknesses. Do not forget to include units.
Graph #1
Volume of Washers
Volume (cm)^3
0.006
0.005
1
0.004
2
3
0.003
4
0.002
5
0.001
Average
0.000
Washers
Important Note: Each student must make their own graphs. If both lab partners submit
identical graphs, then each lab partner will receive a zero score for the graphs or 0/12 pts.
Surface Area of Washers
Graph #2
Surface Area (cm)^2
12.1
12.0
1
11.9
2
11.8
3
11.7
4
11.6
5
11.5
Average
11.4
11.3
Washers
Thickness of Washers
Thickness (atoms)
Graph #3
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
1
2
3
4
5
Average
Washers
Hazards for Zinc Experiment
Reactant: 3 M HCl is a corrosive strong acid.
(If spilled, NaHCO3 will be used to neutralize.)
Product: Hydrogen gas is flammable.
Stockroom Information
Before Experiment Check Out:
5 Zinc coated washers
5 pieces of filter paper
1 set of Vernier calipers*
1 piece of Copper wire*
After Experiment:
Place in “Used Solids” bucket:
*Return to Stockroom:
5 washers with Zinc coating removed
1 set of Vernier calipers
5 pieces of filter paper
1 piece of Copper wire
Pour in “Zinc Lab” liquid waste carboy:
Used HCl with Zn2+ & Rinsings
Leftover HCl
Next Week – Out of Class Feb. 9-11
Lewis Dot Activity – Handout provided in class.
Read pp. 1-8 and Complete the Datasheets pp. 9-23.
Read: Using Exponential Notation & Significant Figures (Text pp. 35-52)
Read: Determining the Empirical Formula of Copper Chloride
(Text pp. 77-94)
Read: Dimensional Analysis Sets #2 & #3 (Text pp. 24-28)
Next In Class Session - February 16-18
Turn-In: 1.) Lewis Dot Activity – pp. 9-23
2.) Exponential Notation – pp. 41-42
& Significant Figures – pp. 51-52
3.) Dimensional Analysis Sets #2 & #3 (all problems)
4.) Zinc pp. 65-75 (This includes the Post-Lab).
Note: #2 (p 75) - You need to make 3 graphs!
Note: Bottom p65 – You need to make
a separate calculations page for *ed calculations.