Enthalpy of Acid-Base Reaction

Identification of the Specific Heat Capacity for a Calorimeter and of the Enthalpy of an Acid-Base Reaction

Abstract
The purpose of this lab was to first, determine the specific heat capacity of a homemade calorimeter, and second, to calculate the enthalpy of reaction for an acid-base reaction between 6M KOH and 6M HNO3. To determine the specific heat capacity of the calorimeter, two differing temperatures of water were measured and volume was measured and mixed within the calorimeter. The enthalpy of reaction for an acid-base reaction was found by these steps: measure volume and the temperature for both the acid and the base; mix the acid and base within the calorimeter while recording the change in temperature with a temperature probe; and calculate the enthalpy of reaction from the data found. The specific heat capacity was found to be and average of 20.0 J/(g(°C)). The enthalpy of the reaction was found to be -64.5 kJ/mol. These values show that the specific heat of the calorimeter is nearly unimportant when calculating the enthalpy when compared to other data.
Introduction
The purposes of this experimentation was to determine the specific heat capacity constant for a homemade coffee cup calorimeter and then to investigate an acid-base reaction within the coffee cup calorimeter and calculate the heat of reaction. The outcomes of this lab are useful for other scientists making a coffee cup calorimeter. For example, the data found from this experimentation can be used to compare between other similar experiments to help determine the accuracy of data discovered. This and many other heat of reaction can demonstrate to researchers that in an acid-base reaction the specific heat capacity constant of a coffee cup calorimeter is so small when compared to the total heat of the reaction.
Calorimetry, in its most primitive form, works with the heat of reactions. This also brings forth the topic of endothermic and exothermic reactions. When working with children it is difficult for them to understand what the enthalpy of reaction means. Therefore, reactions are explained in the most basic for of endothermic and exothermic. This can help teach kids about the different types of reactions that take place in the world and in the classroom.
In other situations, such as chemical engineering, the enthalpy of reaction can be used to determine the amount of energy in certain reactions. This can help to choose which method may be better suited or uses less energy to create a product. Knowing these things can help reduce cost and maximize product.
In this paper, the methodology and calculations when determining the specific heat capacity constant and the heat of an acid-base reaction will be explicitly described. Detailed lab procedures and results will be explained.
Experimental
The first phase of this experiment was to establish the specific heat capacity constant of a homemade coffee cup calorimeter. The materials needed for this were 50 mL of water of a hot and cold temperature each, one or two Styrofoam cups, a lid made of cardboard or the like, a method to stir the solution, be it a magnetic plate and stir bar or a stir rod, a strong acid, a strong base, and a temperature probe. The strong acid and strong base will be used in the second phase of the experiment. Two calorimeters were assembled using one Styrofoam cup each and a piece of cardboard for the lid. The cardboard was cut to cover the opening of the cup and hang just a bit over the edge. A hole was put in the lid for the probe to sit comfortably. Water was cooled using ice to nearly 10 °C. Water was also heated to a high temperature near 70°C. The water was heated by hot plate and by Bunsen burner in this situation.
Step one was place about 50 mL of cool water in the calorimeter. Measure the temperature of the cool water. Heat a new sample of water in a beaker over a hot plate or Bunsen burner. Next add about 50 mL of hot water to the calorimeter. Keep the calorimeter on the magnetic plate with the stir bar inside. Temperature should be constantly measured with the probe and the final temperature should be recorded. The final volume of the calorimeter contents is around 100 mL. This operation should be performed three times for each calorimeter. From this information, the specific heat capacity constant can be calculated using this equation:
Ccal= -(mHcwater(TTotal-TH)+mCcwater(TTotal-TC)
(TTotal-TC)

Phase two of the experiment was to determine the enthalpy of reaction for the specific acid-base reaction, 6M HNO3 and 6M KOH. This reaction takes place:
KOH(aq)+ HNO3(aq) → H2O(l)+ KNO3(aq)
OH-(aq)+ H+(aq) → H2O(l)
The volume of both the acid and base solutions prior to reaction should be near 50 mL each. Measure the initial temperature and measure out 50 mL of KOH into the calorimeter. Next measure the initial temperature of the HNO3 and measure out 50 mL of this solution as well. In this experiment, the KOH was the solution put into the calorimeter first at 50 mL, and then the HNO3 was added to the calorimeter at 50 mL. During this process the temperature was recorded constantly with the probe and the solution was stirred on a magnetic plate with a stir bar.
Results
The results of the this experiment are as follows for the specific heat capacity section of the lab work,
Hot Plate Data
VC (mL) VH (mL) TC (°C) TH (°C) TTotal (°C)
49.7 49.9 9.1 70.1 40.1
50.1 49.4 6.8 68.8 38.8
50.2 49.7 11.6 67.5 37.5
Bunsen Burner Data
VC (mL) VH (mL) TC (°C) TH (°C) TTotal (°C)
50.0 50.0 22.7 70.0 47.5
50.0 50.0 6.4 78.0 40.0
50.0 50.0 7.3 27.7 16.7
50.0 50.0 6.2 60.6 32.4
50.0 50.0 4.4 71.4 36.9
From this data the specific heat of the calorimeter was calculated for each trial and an average was taken for each. The calculation will be demonstrated using the correct equation for the first hot plate trail.
Ccal= -((49.9)(4.184)(40.1-70.1))+((49.7)(4.184)(40.1-9.1))
(40.1-9.1)

This action was performed for all the trial and an average was taken. The result was the specific heat of the calorimeter was found at 20.0 J/(g (°C)).
The second part of the experiment used 6M HNO3 and 6M KOH. The trials were performed using manual stirring and magnetic plate and stir bar methods. The data collected from this was as follows:

Manual Stirring
VKOH(mL) VHNO3 (mL) TKOH (°C) THNO3(°C) TFinal(°C)
50.0 50.0 22.0 22.1 66.4
50.0 50.0 22.2 22.2 66.5
50.0 50.0 22.8 22.3 66.8

Magnetic Stir Plate
VKOH (mL) VHNO3(mL) TKOH (°C) THNO3 (°C) TFinal (°C)
49.9 50.0 22.6 22.6 66.7
50.1 50.2 22.6 22.7 66.5
50.0 50.2 23.0 22.8 66.8
Graph 1

These graphs show the solution of KOH in the calorimeter before HNO3 and then the peak is the highest temperature of the solution once the HNO3 was added to the calorimeter. The highest temperature is used in calculations of enthalpy, or heat, of reaction. Calculations for enthalpy of reaction are as follows,
6M KOH= (X mol/0.05 L) = 0.30 mol
ΔT = Tfinal – Tinitial = 44.1 °C
Once this information was gathered, the heat of reaction, q, was computed. qcal = ccal(ΔT) = 882.0 J qH2O = (mass)(cH2O)(ΔT) = ~ -18469.9 J qrxn = -qH2O - qcal = -19351.89 J
ΔH = (qrxn/mol) = (-64506.3 J/mol)/ 1000 = -64.5 kJ/mol
Through the calculations above, the enthalpy of the acid-base reaction was found to be -64.5 kJ/mol.
The solution in the end was acid because KOH was the limiting reactant. The solution was neutralized with NaHCO3 and washed down the sink.
Discussion
It is highly possible that error occurred while performing this lab. One error that could have been possible was heat lost to the surroundings. During all calculations and it was assumed that no heat was lost to the surroundings or to the calorimeter. Another challenge that appeared in the lab with the calorimeters themselves; the lids were not very secure and one calorimeter had a hole in it that was patched up by duct tape.
It can be concluded that the q of the calorimeter was a negligible value. When comparing the enthalpy of reaction values with other experiments, it was found that all of the values were relatively close in range. This was true even when the range of the heat of reaction for the calorimeter was from -80 to 100 J/K. This shows that the heat of reaction for the calorimeter is a very small, nearly negligible, when compared to the q of water.
Conclusion
From the data collected during this lab the average specific heat value of the calorimeter was calculated to be 20 J/(g(°C)). This number, when compared to other experiments c values, had a very large range. But the value for the enthalpy of reaction was found to be -64.5 kJ/mol, and the other values were relatively close to this value. From this, it can be deduced that the c value for the calorimeter is very insignificant when compared to the over all enthalpy of reaction value.
There could have been error when calculating the specific heat capacity for the calorimeter. This is true because it was assumed that no heat escaped from the calorimeter to the surroundings and that the calorimeter itself was the same temperature as the first solution put into it.
Therefore when calculating the enthalpy of the reaction, and the calorimeter’s specific heat value is unknown, an approximate value can be found. When performing a neutralization reaction, there will be a similar result for many outcomes.