We use cookies to give you the best experience possible. By continuing we’ll assume you’re on board with our cookie policy

Determining the Approximate Strength of H-bond Essay Sample

essay
  • Pages: 5
  • Word count: 1,193
  • Rewriting Possibility: 99% (excellent)
  • Category: chemistry

Get Full Essay

Get access to this section to get all help you need with your essay and educational issues.

Get Access

Introduction of TOPIC

To determine the approximate strength of hydrogen bond formed between ethanol molecules by measuring the enthalpy change on mixing it with cyclohexane.

Principle

Ethanol contains -OH group which can form hydrogen bond among its molecules but cyclohexane hasn’t. On mixing, cyclohexane will break the hydrogen bond among ethanol molecules. Breakage of hydrogen bond consumes energy thus the temperature lowers and hydrogen bond strength of ethanol can hence be calculated.

Chemicals

ethanol, cyclohexane

Apparatus

10ml pipette, pipette filler, test-tube, cotton wool, thermometer, beaker

Experimental set-up

Procedure

1.> Weigh a test-tube and the weight is recorded.

2.> The initial temperature of ethanol, cyclohexane and test-tube are recorded in Table 1.

3.> A test-tube is placed into a beaker and is surrounded with cotton wool.

4.> A thermometer is then put into the test-tube.

5.> Pipette 4 cm3 of ethanol into the test-tube.

6.> Pipette about 9 cm3 of cyclohexane into the same test-tube.

7.> The mixture is stirred gently & carefully with the thermometer and the lowest temperature is recorded.

Results

Mass of the test-tube: 14.44 g

Table 1 shows the temperature change after mixing two substances:

ethanol

cyclohexane

test-tube

Initial temperature /?

15.5

16.5

18

Lowest temp. after mixing /?

13.8

?T /?

– 1.7

– 2.7

– 4.2

Table 2 shows the data of the substances:

Molecular mass

Density /kgdm-3

Specific heat capacity kJkg-1K-1

Volume of solution added /cm3

ethanol

46

0.81

2.44

4

cyclohexane

84

0.78

1.83

9

test-tube

/

/

0.78

/

Energy absorbed to break hydrogen bond

= energy absorbed from test-tube (E1) + energy absorbed from ethanol (E2) + energy absorbed from cyclohexane (E3)

Energy = mc?T

E1 = [14.44 1000].[0.78].[4.2] kJ

= 0.04731 kJ

E2 = [(0.81.4) 1000].[2.44].[1.7] kJ

= 0.01344 kJ

E3 = [(0.78.9) 1000].[1.83].[2.7] k

J = 0.03469 kJ E = E1 + E2 +

Sorry, but full essay samples are available only for registered users

Choose a Membership Plan
E3

= 0.09544 kJ

Strength of hydrogen bond of ethanol = E no. of moles of ethanol

= E [(0.81.4) 46]

= 1.36 kJmol-1

Precaution

1.> The test-tube should be used instead of beaker because the smaller in size of test-tube can reduce heat loss or heat gain from surroundings. For same reason, there is cotton wool between test-tube and beaker.

2.> Since both ethanol and cyclohexane are easily vapourize, the volume of substances added may be affected, so the transfer process and the

experiment should be done as fast as possible.

3.> The ethanol – cyclohexane mixture should be stirred gently & carefully with the thermometer, to ensure heat is evenly distributed.

4.> Ethanol & cyclohexane are flammable, it should be ensure that there is no fire around.

Discussion

Hydrogen bond is defined as the the intermolecular force (or intramolecular force) formed between H atom attached to F, O, or N and the lone pair electrons on F, O or N. The more electropositive (?+) on H, the stronger is the hydrogen bond.

In this experiment, there are 2 assumptions made – it is assumed that no heat loss to (or gain from) the surroundings and all hydrogen bonds between ethanol molecules are broken.

It should be noticed that among ethanol molecules, hydrogen bonds can be present due to the presence of lone pair electrons on O attached to H:

However, among cyclohexane molecules, there is only relatively much weaker van der Waal’s force but no H-bond:

After mixing these two substances, hydrogen bonds formed between ethanol molecules are broken because cyclohexane molecules separate ethanol molecules and hence interrupt its hydrogen bonds from forming:

To ensure that all the hydrogen bonds are broken, excess cyclohexane is added.

However, in this experiment, hydrogen bond strength obtained is not reliable because we can’t ensure that all hydrogen bonds are broken. If we want to have a more reliable result, we can improve the experiment by several methods:

< 1.> Repeat the experiment by adding more cyclohexane until the temperature becomes constant which indicates hydrogen bonds are no longer present.

< 2.> Since ethanol is a polar molecule whereas cyclohexane is non-polar, they are immiscible. To ensure all hydrogen bonds are broken, it’s better to use 2 substances which are miscible, with only ethanol can form hydrogen bond.

< 3.> Ethanol used should be as pure as possible, say, 99% to eliminate the effect of hydrogen bonds formed between water molecules.

When hydrogen bonds are broken, energy is absorbed, temperature drops. Since hydrogen bond is much stronger than van der Waal’s force, the slightly increase in temperature caused by formation of van der Waal’s force between ethanol & cyclohexane molecules can be compensated by the highly decrease in temperature caused by breaking of hydrogen bond among ethanol molecules. Hence, the approximate value of hydrogen bond strength of ethanol can be obtained by this calorimetric method.

Conclusion

From this experiment, we can conclude that hydrogen bond is present between ethanol molecules due to the presence of hydroxyl group, –OH. The approximate strength of hydrogen bond of ethanol can be measured by using simple calorimetric method.

We can write a custom essay on

Determining the Approximate Strength of H-bond Ess ...
According to Your Specific Requirements.

Order an essay

You May Also Find These Documents Helpful

Inorganic Chemistry

Transition elements contain ions that present in aqueous solution as complex ions. A metal ion with a molecule called a ligand are called complex ion and it attached through coordinate covalent bonds. A complex salt is an ionic compound but it differs in the fact that there are these covalent bonds attaching the metal to the ligand. For example in iron(II) ion, it exists in water as Fe(H2O)62+. In this ion, water molecules are arranged about the iron atom with their oxygen atoms bonded to the metal by donating electron pairs to it. A coordination compound is a compound that have either of complex ions and other ions of a neutral species or opposite charge. As an example of coordination compound is the compound K4[Fe(CN)6] which contains the complex ion Fe(CN)6 4- and four K+ ions. Some of the transition elements have biological activity and their role in human nutrition...

General chemistry - Titration

Introduction Titration is defined as technique whereby one solution of accurately known concentration also known as the titrant is added slowly into another solution of unknown concentration until a neutralization reaction is reached. There are several types of titration methods in chemistry. Firstly, is the acid-base titration method which was carried out during this experiment. In an acid-base titration, the acid/base with a known concentration and fixed volume will be added into the conical flask with a pipette and this would be the solution that is to be titrated. This solution is also known as the titrand. The base/acid with a known concentration but no fixed volume would then be added into the burette. This solution is called the titrant [1]. The point where neutralization is reached is usually indicated by an indicator while in some reactions the solutions are self-indicating. Hence, a small amount of acid-base indicator will be...

Ammonium hydroxide solution

In the strong acid-weak base titration, 0. 1 M ammonium hydroxide ( NH4OH ) was titrated with 0. 1 M hydrochloric acid ( HCl ). 2-3 drops of phenolphthalein were added as indicator into the conical flask which contains 25mL of hydrochloric acid. Ammonium hydroxide was then titrated slowly into the conical flask and the conical flask was swirled gently to ensure the reaction is reacted completely. The titration stops when the colour of the titrand changed from colourless to pale pink. The average titrant (29. 55 mL) of 0. 1 M of ammonium hydroxide was added into the conical flask. By using screened methyl orange as indicator, the average titrant (26. 45 mL) of 0. 1 M of ammonium hydroxide was added to change the colour of the titrand from red to grey in conical flask. In weak acid-strong base titration, 0. 1 M sodium hydroxide ( NaOH )...

Popular Essays

logo

Emma Taylor

online

Hi there!
Would you like to get such a paper?
How about getting a customized one?