Description
PLEASE READ THESE FORMAT CAREFULLY AND FOLLOW THEM STRICTLY:
Format of Laboratory Reports
1. Title page and Abstract: The front page of the report should display the title of the experiment, your name, the name of any experimental partners, and the date on which the report is submitted, and a brief abstract. An abstract is typically 50 to 100 words, starting with the purpose of the study, then summarizing the main results of the study with the corresponding data included, and also stating any significant conclusions.
2. Introduction: This should start at the top of the second page. The introduction presents the theory and motivation of the experiment briefly. The object of the experiment should be clearly stated. Theoretical equation used in treating the data should be included in this section.
3. Experimental Method: The section provides a brief description (in your own words) of the experimental method used to obtain the data. Indicate any significant deviations from the prescribed procedures. Do not include detailed procedures copied from the laboratory manual. Include all equations needed to calculate the data from the experimental measurements (i.e. Calculations of concentration from absorbance, molar enthalpy from temperature change).
4. Results: The section on results should present experimental results in an orderly fashion using table and graphs. Tabulated output of data recorded in the laboratory notebook and/or hardcopy or graphic of tabular data obtained from the instrument or computer interfaced to the instrument. Be certain to include units and uncertainties for any measured values. Include sample calculations when appropriate (i.e. whenever a calculation was performed). Unless specifically requested, do not derive the equations but use references to indicate the source. Include a brief summary of the error analysis.
5. Discussion/Conclusion: This section should interpret the results in terms of the theory presented in the introduction and known molecular properties where possible — be certain to indicate whether the objective(s) of the experiment was accomplished. Compare results with literature values when possible (you may need to locate these values — they will not always be given to you). Answer all questions given additionally for some experiments, with each question clearly labeled. If appropriate, suggest modification to the experimental procedure that could improve the precision and/or error.
6. References: This section includes citations of all sources to which YOU referred, including this laboratory manual and/or other sources of literature values.
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Explanation & Answer
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Surface Tension – Tensiometer
Title Page and Abstract
This lab was conducted to determine the surface tension of liquids. The tensiometer
method was used to conduct the test. The liquids comprise of three pure liquids, that is, water,
ethanol, and toluene, and the derivatives of diluting ethanol. Pure water, ethanol, and toluene had
an experimental surface tension of 64.1 mN/m, 18.6 mN/m, and 23.5 mN/m respectively. The
percent experimental errors in these values were 0.7%, 3%, and 4% for water, ethanol, and
toluene respectively. The literature values were significantly higher than the experimental values.
The percent difference between the experimental and literature values was 12%, 17%, and 18%
respectively. A strong correlation was found to exist between surface tension obtained using
Parochor and tensiometer method. The correlation had a coefficient of determination of 0.96.
The excess surface tension was found to be 5.1mN/m
Introduction
Objective
This experiment was conducted to study the determination of the surface tension of
liquids by the tensiometer method.
Theory
In the initial applications of thermodynamic concepts, the surface effects are usually
ignored. The rationale behind this assumption is based on the knowledge that the surface
molecules to the ratio of the interior molecules are quite small. Some of the scenarios where this
ratio is considered are for the small drops and thin films. The abundance of interior molecules
ensures complete surrounding of each molecule by other molecules thus resulting in a balanced
force arrangement. However, for surface molecules, the case is different. There is a resultant
inward attraction.
The surface tends to minimize its surface energy by arranging itself to have the smallest
area possible. As a result, the gas and liquid bubbles tend to be spherical. Surface energy is the
work that is required to increase the area by one cm2. The work-done (W) by as film is defined
by Equation 1.
𝑊 = −𝛾𝐿𝑑𝑥…………………………….Equation1
Where; L is the film width, dx is the change in length, and 𝛾 is the force per unit length,
known as surface tension.
Two methods are used in the determination of surface tension, namely, capillary rising
and DuNuoy methods. The DuNuoy method was used for this experiment. In the experiment, a
platinum-iridium ring is pulled from a liquid by application of a known force. Ideally, the force
required to break surface film is defined by Equation 2.
𝐹 = 4𝜋𝑅𝛾……………………………………….Equation 2
Where; R is the mean radius of the ring obtained by doubling the perimeter to cater for
the two boundaries between the wire and liquid, that is, one on the inside and the other
outside the ring.
The validity of Equation 2 depends on two conditions, namely, the liquid has zero
constant angle and the ring holds a thin cylindrical shell of liquid before breaking occurs. The
liquid shape is a function of the volume of the liquid held up and the dimensions of the ring. A
correction factor (𝐹𝛾 ) defined in Equation 4 is introduced to correct for the shape of liquid thus
modifying Equation 2 to Equation 3.
𝐹𝐹𝛾 = 4𝜋𝑅𝛾……………………………………….Equation 3
𝐹𝛾 = 0.7250 +
0.01452𝑃
[ 𝐶 2 (𝐷−𝑑)
+ 0.04534 −
1.679
𝑅
𝑟
1
2
] ………………….Equation 4
Where P is the scale reading × Cv, C is the circumference or rind in cm, D is the density
of water at 250C in g/mL, d is the density of air at 250C in g/mL, R is the radius of the
ring and r is the radius of the wire. The value of (D-d) is taken as 0.997. Cv is the
calibration constant for the DuNuoy tensiometer defined in Equation 5.
𝑊𝑔
𝐶𝑣 = 2𝐿×𝑑𝑖𝑎𝑙 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 …………………………………….Equation 5
Where; L is the ring circumference, W is the calibrating weight, and g is the acceleration
due to gravity constant. The constant is applied to all tensiometer readings.
Based on the modified Equation 3, the surface tension, 𝛾, is defined as Equation 5.
𝐹𝐹
𝐹
𝛾 = 4𝜋𝑅𝛾 = (4𝜋𝑅) 𝐹𝛾 = 𝑃𝐹𝛾 ………………………………Equation 6
In 1923, Macleod observed that
𝛾
𝜌𝑙 −𝜌𝑣
= 𝐶…………………………………..Equation 7
Where; C is a constant over a considerable range of temperature, 𝜌𝑙 and 𝜌𝑣 are isobaric
densities of liquid and saturated vapor. Multiplying both sides of Equation 7 with the molecular
weight of the substance yields Equation 8.
𝑀𝛾
𝜌𝑙 −𝜌𝑣
= 𝑀𝐶 = [𝑃] = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 …………………….Equation 8
Where the constant [𝑃] is known as parachor. Neglecting the vapor density yields
Equation 9.
1
𝑀𝛾4
𝜌𝑙
= [𝑃]………………………...