Unit 2- Practical Scientific Procedures and Techniques
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Calorimetry Cooling Curves
The objective of this experiment is to provide evidence that I can accurately carry out calorimetry to
determine the rate of cooling of different substances.
The theory behind Calorimetry Cooling Curves:
Calorimetry is the process of measuring the heat capacity change of a substance during a chemical or
physical reaction. This type of calorimetry experiment portrays the heat capacity change that occurs
when a substance experiences a physical reaction. Within simple covalent substances (and graphite)
the physical state of is not dependant on the covalent bonds as this is the chemistry of the molecule.
Attractive forces which exist between the molecules are responsible for the state of a substance;
these are called intermolecular forces. There are three main types of intermolecular forces: Van Der
Waal’s, Dipole-Dipole and Hydrogen Bonding.
The type of intermolecular force depends on the electronegativity of the atoms within the molecule.
Electronegativity is the ability of an atom to attract a pair of electrons in a covalent bond. If the
atoms of a molecule have a similar electronegativity (below 0.5), they will attract each other with
equal force; this is called a non-polar molecule. Van Der Waal’s forces exist between non-polar
molecules and are the weakest type of intermolecular force. If the atoms of a molecule have a
significant difference of electronegativity (0.5 to 2.1), the electrons will shift towards the more
electronegative atom meaning that one atom will be slightly negative (delta minus) while the other
is slightly positive (delta plus); this is called a polar molecule. Permanent Dipole-Dipole forces exist
between polar molecules and are stronger than VdW’s forces. When Hydrogen bonds with Oxygen,
Nitrogen or Fluorine, these molecules form polar molecules with the highest difference delta
plus/delta minus. The intermolecular forces that exist between these molecules are referred to as
Hydrogen Bonding.
A substance can only change state from a solid to liquid when it has reached its melting point and
can only change state from a liquid to gas when it has reached its boiling point. At the melting point,
the substance has absorbed sufficient thermal energy to overcome some intermolecular forces. The
substance becomes a liquid because the decrease of attraction between the molecules allows them
to slide over one another. At the boiling point, the substance has absorbed more thermal energy
which breaks most of the intermolecular forces. The substance becomes gas because the lack of
attraction between the molecules allows them to diffuse within the atmosphere.
A substance can only change state from a gas to liquid when it has reached its condensation point
and can only change state from a liquid to solid when it has reached its freezing point. At the
condensation point, the substance has lost enough thermal energy to form some intermolecular
forces. The substance becomes a liquid because the increase of attractive intermolecular forces
between the molecules brings the molecules to a closer proximity. At the freezing point, the
substance has lost more thermal energy which forms enough intermolecular forces for the
molecules to be held in place. The substance becomes a solid because the molecules have enough
attraction between them to keep them in a fixed arrangement which does not allow movement, only
vibration.
Cooling curves support this theory because when enough heat capacity is lost, the substance does
not have sufficient energy to overcome the attractive intermolecular forces and at certain values of
negative heat capacity change, the molecules have enough attractive forces existing between them
to change state.
,Experiment technique:
Procedures such as calibration must be carried out to obtain accurate results. Calibration is the
process of checking a measuring instrument to ensure there are no faults that could affect the
accuracy of the results. You must calibrate the equipment being used throughout the experiment; if
not, the results will have a higher uncertainty. Each piece of equipment has a calibration method
individual to its function:
Piece of equipment Calibration method
Beaker/test tube Rinse out with distilled water to remove any residue. Also, ensure
there are no cracks/chips because if there is product could be lost.
Liquid/Digital Thermometer Part one:
1. Roughly fill a 250cm❑3❑ beaker with at least 150cm❑3❑ of
boiling water.
2. Submerge the tip of the thermometer into the beaker,
ensuring that you hold it in a position that it does not touch
the sides of the beaker. This can affect the accuracy of the
reading because the thermometer will read the heat of the
beaker instead of just the substance.
3. Take readings of the water temperature at minute intervals.
Do this for three minutes.
Part two:
1. Roughly fill at least half of a 250cm❑3❑ beaker with ice.
2. Submerge the tip of the thermometer into the beaker,
ensuring that you do not touch the sides of the beaker.
3. Take readings of the ice temperature at minute intervals.
Do this for three minutes.
Results from the calibration of the thermometers:
Temperature (° C )
st nd
1 reading 2 reading 3rd reading Average
Boiling water Liquid 84.5 90.1 85.5 87.2
Digital 84 91 87 86.8
Ice Liquid 2 3 2 2.3
, Digital 0.5 1.1 1.8 1.1
Task One: To perform the Calorimetry of Steric acid and Paraffin wax by recording the
temperature of the substances as they change state from liquid to solid. I used these
particular substances because both are solid at room temperature meaning I can simply
melt them and then record their temperature as they cool down.
This apparatus set up is the applicable for both Steric acid and Paraffin wax (the equipment of
diagram 2 is not shown to scale).
Apparatus:
25cm❑3❑ Steric Acid
25cm❑3❑ Paraffin Wax
Test tube ×2
Clamp stand
Clamp
Bunsen Burner
Tripod
Gauze
Heat proof mat
300cm❑3❑ Distilled water
250cm❑3❑ Beaker
Liquid thermometer
Stopwatch
Risk Assessment:
Substance Hazard Comment Prevention
Steric -Flammable -When anhydrous it can form -Wear a lab coat, safety
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