Transcript
Effect of Temperature on the Solubility
Of a Salt in Water
Student name:
Lab partner:
CHE 214, Section 02
March 4, 2013
Instructors:
Introduction and Background
Solubility occurs when salt ions of the positive and negative charge attract with the liquid water molecules (H+ and O- ions). On the other words, when any organic salt is placed in the water solvent, the hydrogen bond H+ in the water will break and attract with negative ions in the salt and also the water oxygen ions O- will attract with the positive salt ions. However, the solubility of the solid (salt) influence by several factors such as intermolecular forces (molecule polarity) and temperature the physical and chemical properties of the solute and solvent. Temperature is one of the important factors that contribute in the solubility of the salt in solutions. As the temperature of the liquid water (solvent) increases, the energy of the water molecules will increases which cause the liquid particles to absorb energy (endothermic) and move faster, collide with the salt ions and thus dissolution the salt. The purpose of this lab, however; was to determine experimentally the effect of temperature on the solubility of the solid benzoic acid using recirculation water bath, magnetic stirrer and digita pen thermometer .Therefore, This lab divided into two parts. Firstly, the solubility of the solid was calculated as actual temperature of the solutions was recorded 49.5oC. Then, the volume of the solid benzoic acid (5mL) was used in titrating mixed with (50mL) of distilled water and three drops of phenolphthalein indicator to turn the colorless benzoic acid to pale pink and thus recorded and titrated three times of the acid with NaOH at various volume and different temperatures.
Secondly, the solubility of solid benzoic acid in grams was computed at various temperatures after adjusting the water bath in order of decreasing temperature (45, 40, 35 and 30) oC to allow the liquid to saturate with the solid. Then, the differential heat of solutions was calculated as the solubility literature value of benzoic acid in 100g of water at different temperatures was determined. The following equations were used to compute the data;
The solubility of the solid was calculated using the equations as shown below;
K= [solute]/ [solid] = x
ln (x2/x1) = -Ho/R (1/T2-1/T1)
Where x is the concentration of the solute
K is the equilibrium constant
ln (S2/S1) = -Ho/R (1/T2-1/T1)
Where Ho is the enthalpy in kJ/mol
R is the gas constant (8.314 J /mol.K)
T is the temperature in K
S is the solute solubility in g
The enthalpy of the solution, however, was computed using the equation below;
ln S = -Ho/ RT + b
Where b is the intercept of the solution
In conclusions, to all the equations shown above the calculations, observations result and graphs will illustrate each one in their own way.
Results and Calculations
Table1. Calculation of the Concentration of Benzoic Acid at different temperatures
Recirculating water Bath Temperatures oC
Water Density
(g/L)
Concentration of Benzoic Acid (M)
0.01M NaOH Titrate in (mL)
Trial #1 Trial#2 Trial#3
Average titrating volume (mL) of 0.01 NaOH
49.5
988.21
0.01584
8.0
7.95
7.8
7.92
45
990.12
0.01540
7.71
7.73
7.65
7.70
40
992.21
0.01246
5.70
6.50
6.50
6.23
35
994.14
0.01166
5.80
5.90
5.80
5.83
Sample volume conversions of the sodium hydroxide at 49.5oC:
1L = 1000mL1076325303530
438150150495V NaOH = 7.92 mL 1L
1000 mL
= 0.00792 L
The solubility of Benzoic Acid in 100g of water for various temperatures was calculated as shown below:
C1V1=C2V2
Where C1 is the concentration of the sodium hydroxide NaOH in mol/ L
C2 is the Concentration of the Benzoic Acid in mol/L
V1 is the volume of NaOH in L
V2 is the volume of benzoic Acid in L
The volume of benzoic acid that was used in titrating = 5.0mL
C2 = C1V1 / V2
= (0.01M* 0.00792 L)/ 0.005L
= 0.01584 M
The density of water at various temperatures that obtained from CRC handbook had converted as shown below:
1704975094297547625152400171450= 988.21 Kg 1000 g 1 m3
m3 1Kg 1000 L
= 988.21 g/L
Table2. Calculation of the solubility of Benzoic Acid in Molarity and grams per 100g of H2O
Recirculating water bath Temperature
K
MW (g/mol)
Volume of water added to benzoic acid (mL)
Volume of benzoic acid used in titrating (mL)
Solubility of benzoic acid (g)
322.7
122.13
50
5.0
0.1064
318
122.13
50
5.0
0.1034
313
122.13
50
5.0
0.0837
308
122.13
50
5.0
0.0783
The density of the benzoic acid was computed as shown below:
Density of benzoic acid at 49.5oC
C7H6O2 = m/V n= m/MW
= (n x MW)/ (n/C) C (Molarity) = n/V
= MW x C1
= 122.13 g/mol * 0.01584 M
= 1.9345 g/L
Where n is the number of mole of Benzoic Acid
m is the mass of substance in g
MW is the molecular weight of Benzoic Acid in g/mol
C is the concentration in molarity (mol/L)
V is the volume in L
Therefore the density of the benzoic acid at (45, 40 and 35)oC were (1.8808,1.5217 and 1.4240) g/L respectively
Therefore the solubility (S) of Benzoic Acid in table 2 at 49.5oC was determined as below:
n= m/M
c= n/V n = C x V
m benzoic acid = n x M
= C x V x M
= 0.01584 M * 0.055 L* 122.13g/mol
= 0.1064g
Table 3 Showing calculations of the Enthalpy using the inverse of the temperature in kelvin against the natural logarithm of solubility
# of benzoic acid tittered
ln solubility
Inverse of temperature (1/k)
1
-2.2405
0.0031
2
-2.269
0.00314
3
-2.4805
0.00319
4
-2.547
0.00325
The enthalpy of the solution obtained from the graph was calculated in kJ / mol as shown:
ln S = -Ho/ RT + b
Where -Ho/ R is the slope
b is y-intercept
b = 4.8242
(-Ho/ R) = -2087.3
Ho = - [8.314 J/mol.K *(-2087.3)]
= 17.4 kJ/ mol
The literature value for the heat of solutions (enthalpy) was computed using the equations shown below:
Solubility of benzoic acid in water given from the lab manual were (0.290g at 20oC and 1.155g at 60 oC)
ln (S2/S1) = -Ho/R (1/T2-1/T1) K= oC+273.15
H = - [ln (S2/S1) x R/ (1/T2-1/T1)] = 20oC +273.15= 293.15 K
= - [ln (1.155g/0.290) * 8.314 J/mol.K / (1/333.15-1/293.15)
= 28.05kJ/mol
The significance of the values of Ho obtained experimentally and H literature was used to compute the percentage error:
Relative % error= [Literature value – experiment obtained value/ Literature value] x100%
= [(28.05-17.4)/28.05] *100%
= 38%
Graph of the natural log of the Benzoic acid solubility against the reciprocal of absolute Temperature K-1
Discussion
The purpose of this experiment was to determine the effect of the temperature on the solubility of the solid benzoic acid at various temperature measurements. The first part of the experiment was to determine the solubility of solid benzoic acid at very high temperature 49.5oC in order to prepare a saturated solution since the higher the temperature the more salt will dissolve. Three volume (mL) and temperatures reading were observed and recorded after using 5 mL of benzoic acid mixed with 50mL of water and three drops of phenolphthalein indicator and titrated with standard 0.01 M sodium hydroxide for three times till the solution reached the end point and turn to pale pink. However, three different temperatures reading scale in decreasing order (45, 40 and 35) oC were taken to determine the solubility of the solid and observe the relationship and effect of the temperature on the solid acid solubility. For example, the solubility of benzoic acid at the highest temperature 49.5oC was 0.1064g whereas the solubility at the lowest temperature reading scale 35oC was 0.0783g which demonstrates that as the temperature of the solutions decreases the solubility of the acid decreases and hence the highest the temperature the more solid will dissolve.
Therefore, the temperature is directly proportional to the solubility of the solid. When increasing the temperature of the solid benzoic acid the acid molecules gain energy, the entropy will increases, the particles moves faster and thus collide with each other. As the water was added to the acid, the acid ions attracted with the water molecules and hence the acid particle dissociated and thus the solid dissolved. However, solutions can consider to be saturated and at equilibrium when a sufficient amount of solid are dissolved in the solvent such water. Thus, in order to determine and verify that the solution was saturated and at equilibrium at the start of the experiment was by adding more solid benzoic acid to the solutions. If the added solid was dissolved in the water solvent then the solutions was not at it saturated point. Whereas, the solutions saturated point can be determine , if the added solid benzoic acid did not dissolved in the solvent and precipitate in the bottom of the beaker.
There are many techniques used to measuring the solubility of the solid such as dissolutions, PH meter and the solvent evaporation. One of the important and widely used in laboratory is the PH metric techniques. A solid start to precipitate and dissolved when the solid added to the water solvent cause a change in the PH reading of the solution. Solutions saturated point is observed when a solution of strong acid and base are added to the solution mixture. Thus, the solubility of the solutions will be determined as soon as the PH of the solution measured. The sources of experiment errors that contributed in obtaining high percentage of errors of about 38% were the volume of the benzoic acid used and the temperature reading. When the solution of benzoic acid was heated using circulating water bath, the temperature of the solutions was rescored using digital pen thermometer as soon as the string bar was turned off without wait for the temperature of the thermometer to stable. Also, the volume of the benzoic acid that was withdrawing using a glass cotton plugged tube was not accurate. As the volume of benzoic acid transferred using to the Erlenmeyer flask some of the volume was spilled or dropped in the counter which means that the volume of benzoic acid that used in titrating was not 5.0 mL. However, when the sample solutions was titrating with sodium hydroxide standard solution, the volume of the solution that was recorded as the solution turned into moderate pink which means that the solution had more sodium hydroxide than the supposed volume. For all these reasons above, the value enthalpy of the solid (17.4 kJ/mol) was significant comparing with the literature value (28.05 kJ/mol). Thus, the relative percentage error of the experimental was slightly high which can be reduced by taking more temperature reading of the solubility to obtained more accurate result and using sufficient amount of sodium hydroxide as titrating the solutions sample.
Conclusions
Solubility of the solid is the number of the solute that dissolves in 100g of the water solvent. However, solubility can be affected by many physical and chemical properties such as polarity, density, pressure and temperature. Temperature is one of the important factors that affect the solubility of an organic salt such as benzoic acid. When a solution of solid added to the water is heated up, the temperature of the solution will increases, thus the particles will gain kinetic energy. Since the water is polar molecular, the salt ions will attract the solvent molecules which will favour the endothermic dissolution reaction and hence the solid will dissociate. Therefore, as the temperature of the solutions increases the solubility of the solid will increases too which conclude that the relationship between the temperature and the solubility is linearly proportional.
.
References:
Laboratory Manual for Thermodynamics II, Effect of Temperature on the solubility of a Salt in Water, 2013, page 11-13.
The 92st edition of the CRC Handbook of Chemistry and Physics, 2011-2012
Frank, H. ‘Solubility’, Analytical Chemistry, February 21, 2013. http://en.wikipedia.org/wiki/Solubility
eHow, Methods for Measuring Solubility,1999-2013
http://www.ehow.com/info_8792380_methods-measuring-solubility.html
