Heat of combustion of alkanols

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Diagram:

alkanol prac

Method:

  1. Set up apparatus as shown.
  2. Light the first spirit burner.
  3. Adjust the height of the vessel so that the tip of the flame just touches the vessel.
  4. Weigh the burner (initial mass) with its liquid contents and record.
  5. Add 200 mL of cold water to the vessel using a measuring cylinder. Place the thermometer in the water and record its initial temperature.
  6. Light the wick and stir the water gently (to ensure uniform heating).
  7. When the temperature has risen 10C extinguish the flame by placing the cap.
  8. Again weigh the burner (final mass). Remove any soot from the bottom of the vessel and replace the water before testing the next alcohol.

Methods to reducing error when determining heat of combustion

  • Ensure the tip of the flame touches the vessel to minimise heat lost to the environment.
  • Use a copper can or perform experiment in a bomb calorimeter to contain as much heat as possible.

Remember there will always be heat loss to the environment thus the energy absorbed by the water is less than the amount of energy released by the fuel combusting. This loss of energy can be considerably large and thus will result in large inaccuracies.

Solubility of Carbon Dioxide, explained in terms of Le Chatelier’s Principle

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2.2.5 Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle

This dotpoint is really just an application of Le Chatelier’s principle using the solubility of carbon dioxide as an example. As such, set yourself in the habit for such questions by starting with the equation, and then working through changes in concentration, pressure, volume and temperature.

Note that when you have soft drink in a glass or open bottle, you can see bubbles rising in it. This is because the carbon dioxide gas is constantly escaping, thereby constantly favouring the backwards reaction in an attempt to minimise the disturbance to the system. In comparison, a closed bottle of soft drink has no bubbles unless you shake it, because it is in equilibrium.

CO2 (g) + H2O(l) −↽−⇀− H2CO3 (aq) + Heat
Using the above equilibrium as a practical example of Le Chatelier’s principle:

  • An increase in the concentration of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the concentration of carbon dioxide.
  • An increase in pressure will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the pressure.
  • An increase in the volume of CO2 (g) will shift the equilibrium to the right, converting carbon dioxide and water into carbonic acid in order to reduce the volume of carbon dioxide. Thus the system will attempt to counteract this change by favouring the backwards reaction.
  • An increase in temperature will shift the equilibrium to the left, converting carbonic acid into carbon dioxide and water in order to reduce the temperature.
  • An increase in temperature will shift the equilibrium to the left, converting carbonic acid into carbon dioxide and water in order to reduce the temperature.

Remember- Le Chatelier’s principle will ensure that equilibrium is reached once again. However, this new point of equilibrium will not be same as the original point of equilibrium, as the impact was only minimised, not completely reversed. This is the reason why opened soft drinks will go ‘flat’ irreversibly. 

From The Student’s Guide to HSC Chemistry.

Le Chatelier’s Principle

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Dot point notes on Le Chatelier’s principle:

−↽− −⇀− represents equilibrium sign (which looks like this: ChemicalEquilibrium)

2.2.3 Define Le Chatelier’s principle

The symbol for equilibrium is −↽−⇀− and simply means that, in a closed system, the rate of the forwards reaction is equal to the backwards reaction. This simple means that the reactants are converting to products at the same rate that the products are converting back into the reactants. Whilst there appears to be no change on a macroscopic level, the system is continually changing on a microscopic level. This process, known as dynamic equilibrium, results in the concentration of the substances in the system remaining constant.

According to Le Chatelier’s principle, if a system at equilibrium is disturbed, then the system will adjust itself in order to minimise the disturbance. However, note that the effects of the disturbance are never fully removed. They are only minimised, or lessened to a degree.

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2.2.4 Identify factors which can affect the equilibrium in a reversible reaction

Le Chatelier’s principle is one which plays a crucial role in the HSC Chemistry course. Thus, a sound understanding of it is important, and it may appear again in this subject depending upon what Option you do. For this reason, a treatment sounder than required for this dotpoint will be provided.

Several factors can affect the equilibrium in a reversible reaction. These disturbances to the system can be in the form of changes in concentration, pressure, volume, or temperature.

Concentration

Imagine a system in equilibrium of four compounds, A, B, C, and D. A + B −↽− −⇀− C + D

The simplest way of visualising changes in concentration is simply seeing Le Chatelier’s principle as working to minimise any changes made to the equilibrium. As more of A or B is added, then the system will try to minimise the change by converting more A and B into C and D. As such, the equilibrium shifts to the right.

Conversely, if more of C or D is added, increasing the concentration of the products, then the system will convert more C and D into A and B, shifting the equilibrium to the left.

Note that a system can only minimise a disturbance. It cannot completely undo it.

Pressure

Imagine a system in equilibrium of four compounds, A, B, C, and D. Unlike the example used to illustrate changes in concentration, the four compounds in this example are gases, and the number of moles of A is two rather than one.

2 A(g) + B(g) −↽−⇀− C(g) + D(g)

Determining the affect of changes in the pressure of a system is simply an exercise in counting moles of gases. In the equilibrium above, there are three moles of gas on the left side, and 2 moles of gas on the right. Any increase in pressure will result in the system trying to relieve the pressure by ‘leveling’ the moles of gas within the system. As such, in the above system, an increase in pressure will lead to a shift in the equilibrium to the right. This occurs simply because the system is essentially counteracting the fact that three moles of gas are becoming two moles of gas.

Conversely, a decrease in pressure will shift the above equilibrium to the left in an attempt to increase pressure once again.

Changes in pressure affect only gases. Increasing the pressure in the following system will lead to equilibrium shifting to the right, as there are two moles of gas on the left side and only one on the right.

A(g) + B(g) −↽−⇀− C(g) + D(s)

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2.2. ACIDIC OXIDES AND THE ATMOSPHERE 

Any change in volume in a gaseous equilibrium is simply a change in pressure. As such, treat increases in volume as decreases in pressure, as there are more moles of gas in the fixed space, and treat decreases in volume as increases in pressure.

Temperature

The effect of Le Chatelier’s principle with changes in temperature can often be confusing. However, simply thinking of heat as either a product or reactant greatly simplifies any problems, as shown in the equilibrium below, where the reaction is endothermic (Absorbs heat in order for the reaction to occur) rather than exothermic (Releases heat).

A + B + H e a t −↽− −⇀− C + D

In the above endothermic equilibrium, an increase in temperature will result in the system working to reduce the temperature by shifting the equilibrium to the right, converting A and B into C and D in order to reduce temperature.

Conversely, a decrease in temperature will shift the equilibrium to the left, converting C and D into A and B in order to produce more heat.

In the case of an exothermic reaction, the equation will be of the form A + B −↽− −⇀− C + D + H e a t

As shown above, treating heat energy as an actual item in the equilibrium is a much simpler method of thinking of a problem. Simply determine whether a reaction is exothermic forwards, i.e. the heat is placed on the right, or endothermic forwards, i.e. the heat is placed on the left.

Remember- Changes in concentration, pressure, volume and temperature will all disturb a system in equilibrium.

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From the Student’s Guide to HSC Chemistry. Licensed for free distribution under the GFDL.  

Practicals – reliability, accuracy, validity and errors

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ACCURACY (exactness)

Most measurements contain some uncertainty. Accuracy refers to the exactness of a measurement.
We can measure a small distance with a metre rule or with much greater accuracy using a micrometer.

RELIABILITY (dependability)

Reliability refers to the consistency with which we can confirm a result. Consistency is usually achieved by repetition.

VALIDITY (fairness)

A procedure is valid if it tests what it is supposed to be testing. A procedure is invalid if the method of the experiment is incorrect or partially incorrect.
In a valid experiment all variables are kept constant apart from those being investigated, all systematic errors have been eliminated and random errors have been reduced by taking multiple measurements.
In determining validity, students should consider the degree to which evidence supports the assertion or claim being evaluated. This may be done by making comparisons or conducting further experiments.

first-hand information and data

secondary information and data

Accuracy Instruments should be precise and calibrated. Sources should be reputable?
Reliability All tests should be repeated a significant number of times. Information obtained should be consistent with information from other reputable sources.
Validity Experiments should test the hypothesis that is proposed.The experimental method must be correct?All variables should be identified and controlled. Information should be gathered in an unbiased and professional manner.Findings must relate to the hypothesis or problem.

ERRORS

The two different types of error that can occur in a measurement are:

1. Systematic error – this occurs to the same extent in each measurement. EG when the needle of a voltmeter is not correctly adjusted to zero when no voltage is present.

2. Random/Human error – this occurs in any measurement as a result of the variations in measurement technique. Eg parallax error, limit of reading.