“Statistical Analysis of Zinc Coated Washers”

Purpose of the Experiment

Determine how many centimeters of zinc and layers of zinc atomsare 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.

The zinc layer provides protection to the metal from corrosion.

Before & After Hot Dip Galvanizing

The steel is protected by thesurrounding zinc even if it is scratched.

Cathodic Protection

An anodic layer, such as chromium,nickel, copper, or paint, allows corrosion to

grow under its layer when it is scratched.

Anodic Protection

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.

How Does Zinc Protect The Underlying Iron Surface?

http://www.videopediaworld.com/video/41136/Deconstruction-Building-a-House-Galvanizing-Nails 

These potentials indicate the relative thermodynamic tendency for the indicated half-reaction to occur.

The oxidation of zinc is more likely than the oxidation of iron.

**

Zn <--> Zn+2 + 2 e– E = –0.763 voltsFe <--> Fe+2 + 2 e– E = –0.409 volts

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 on the filter paper.

3. Record on datasheet the inner and outer diameters and the height of each washer to the nearest 0.01 cm.

Zinc Coating Experiment

Outer diameter

Inner diameter Height

Vernier Calipers

https://www.youtube.com/watch?v=ZUNoWWw6V10

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)

Zinc Coating Experiment

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.

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.

Zinc Coating Experiment

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.

Height of the washer,

h

Di: Inner diameter

Do: Outer diameter

Total Surface Area = 2r0

2-ri2)+ 2r0h + 2rih

where r = radius = ½ diameter

Calculations

5 individual measurements for each washer

AccurateThe average is

accurate butnot precise.

PreciseThe average is

precise but not accurate.

Accurate & PreciseThe average is both accurate and precise.

Error Analysis of Accuracy & Precision

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).

Percent Error

100%

theor

obstheor

X

XXE

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.

Determination of AccuracyThe accuracy can be determined by looking at the difference between the expected (theoretical) average and the experimental (observed) average.

The Average or Mean Value (xbar)

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.

N

XX

N

ii

1 Ʃ means “sum”

*Estimate of 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,

1

)(1

2

N

XXs

N

ii

The Standard Deviation

N

XN

ii

1

2)(

If the number of values, N, is small, i.e., if N < 30, an estimate of the standard deviation, s, is given by,

The standard deviation is used for large populations, N ≥ 30.

*We will be using the Estimate of the

Standard Deviation because we have a

small data set.

Confidence Limit

X tN

Student’s t-factors are given in tables for different probabilities. (Note: The table in your book is on page 56.)

X

where t is Student’s t-factor.

At the 90 % confidence limit, 90 times 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 .

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

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Washers

Vo

lum

e (c

m)^

3

1

2

3

4

5

Average

Graph #2

Graph #3

Surface Area of Washers

11.3

11.4

11.5

11.6

11.7

11.8

11.9

12.0

12.1

Washers

Su

rfac

e A

rea

(cm

)^2 1

2

3

4

5

Average

Thickness of Washers

020004000

60008000

1000012000

140001600018000

Washers

Th

ickn

ess

(ato

ms) 1

2

3

4

5

Average

Hazards for Zinc ExperimentReactant: 3 M HCl is a corrosive strong acid.

(If spilled, NaHCO3 will be used to neutralize.)

Product: Hydrogen gas is flammable.

Stockroom InformationBefore Experiment Check Out:

5 Zinc coated washers 1 set of Vernier calipers*5 pieces of filter paper 1 piece of Copper wire*

*Return to Stockroom:1 set of Vernier calipers1 piece of Copper wire

Place in “Used Solids” bucket:5 washers with Zinc coating removed5 pieces of filter paper

After Experiment:

Pour in “Zinc Lab” liquid waste carboy:Used HCl with Zn2+ & RinsingsLeftover HCl

Next Week - February 10-13Read: Determining the Empirical Formula of

Copper Chloride pp. 77-94.

Read: Dimensional Analysis Sets #2 & #3 (pp. 24-28)

Turn-In: 1.) Dimensional Analysis Sets #2 & #3 (all problems)

2.) Zinc pp. 65-75

(This includes the Post-Lab).

Note: #2 (p 75) - You need to make 3 graphs!

Monday Only - Extensions for February 10Turn In:

1.) Dimensional Analysis Set #1

2.) Graphing Lab & Survey