Enthalpy of neutralization is always constant for a strong acid and a strong base: this is because all strong acids and strong bases are completely ionized in dilute solution. Enthalpy changes in neutralization are always negative-when an acid and alkali react, heat is given out.
Aim:
To determine the enthalpy of neutralization of strong acid (hydrochloric acid) and strong base (sodium hydroxide).
Theory:
Heat is evolved during neutralization of an acid with alkali. The neutralization reaction of a strong acid with a strong base is essentially the combination of one equivalent of hydrogen ions with one equivalent of hydroxyl ions.
Enthalpy of neutralization is the heat evolved when one gram equivalent of the acid is completely neutralized by a base in dilute solution.
| Also Read: Enthalpy of Neutralization of Strong Acid and Strong Base Viva Questions |
The chemical reaction is given below.
H+ + OH– → H2O + 13.7 kcal
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) → Na+(aq) + Cl–(aq) + H2O + 13.7 kcal
13.7 kcal of heat is liberated out and is the heat of neutralization for all strong acids and bases. Hess in 1840 obtained a constant value of 13.7kcal as the heat of neutralization in almost all the cases of strong acids and strong bases. This constancy of heat of neutralization of a strong acid and strong bases is explained on the basis of ionic theory.
Materials Required:
- Wide polythene bottle
- Rubber cork
- Thermometer
- Stirrer fitted with a cork
- Measuring cylinder
- 0.2M Hydrochloric acid
- 0.2M Sodium hydroxide solution
Apparatus Setup:
Procedure:
- Calculate the water equivalent of the calorimeter.
- Place 100ml of 0.2M hydrochloric acid solution in it.
- Now record the temperature of the acid solution.
- Take another separate vessel and place 100ml of 0.2M sodium hydroxide solution in it.
- Note down the initial temperature of sodium hydroxide taken.
- Wait till both the solution attains the same temperature.
- Now transfer 100ml of sodium hydroxide solution into the hydrochloric acid quickly.
- Fit the cork immediately which has a thermometer and a stirrer in the mouth.
- Stir well the solution and note down the temperature after small intervals of time.
- Keep noting down the temperature till the temperature becomes constant.
- Record the highest temperature reached.
- Calculate the heat evolved when the two solutions are mixed by ratio proportion method.
Observation and Inference:
| Initial temperature of the acid and base | t1oC |
| Final temperature after neutralization | t2oC |
| Change in temperature Δt | (t1-t2)oC |
| Mass of the mixture solution after neutralization | 200g |
| Water equivalent of calorimeter | W g |
Calculations:
Enthalpy change during neutralization of 100ml of 0.2M HCl
Therefore, the enthalpy change during neutralization of 1000ml of 1M HCl
Results and Discussions:
The enthalpy of neutralization of HCl with NaOH is ________ kJ.
Precautions:
- Due to radiation, some heat is lost to the environment.
- The solution density is supposed to be 1g / ml.
- Hydrochloric acid and sodium hydroxide ionization is assumed to be 100 %.
- The solution’s specific heat is taken as 4.189J/g
- The mixture of HCl and NaOH should be stirred well.
- Water equivalent calorimeter or beaker should be calculated accurately.
Frequently Asked Questions on Enthalpy of Neutralization of Strong Acid and Strong Base
Define the enthalpy of solution.
Enthalpy of solution or solution heat is expressed in kJ / mol, and when a solution is formed it is the amount of heat energy that is released or absorbed.
Find out whether neutralization reaction is exothermic or endothermic reaction?
It examines the neutralization of a strong acid with a strong base and finds that it always adds heat to the environment. It is important to understand that bonds are made and energy is released into the environment during an exothermic reaction. So neutralization reaction is exothermic in nature.
What is meant by exothermic and endothermic reactions ?
An exothermic reaction is known as a chemical reaction in which heat is released. An endothermic reaction is called a chemical reaction in which heat energy is absorbed.
Why is potassium nitrate used in powdered form?
Potassium nitrate used in powdered form because the dissolution will be faster in powdered form.
What is meant by water equivalent of the calorimeter?
A calorimeter’s water equivalent is nothing but the amount of water that absorbs the same amount of heat that is absorbed by the calorimeter.
Why do Strong Acids that React with Strong Alkalis Produce Similar Values?
The reaction between a strong acid and a strong alkali produces a neutralization reaction in which water and a salt are formed. In this type of reaction, the acid and alkali are both fully dissociated into their component ions. The similarity in the values of the pH and pOH values is a result of the fact that these two measures are both based on the concentrations of hydrogen ions and hydroxide ions in a solution.
xperiment to understand the Enthalpy of Neutralization of Strong Acid and Strong Base
In this experiment, the enthalpy of neutralization of a strong acid (HCl) and a strong base (NaOH) will be determined. The reaction between HCl and NaOH will produce water and sodium chloride (NaCl). The heat released or absorbed in this reaction can be used to determine the enthalpy of neutralization.
The experiment will be performed by measuring the temperature change of a solution of HCl and NaOH before and after the reaction has occurred. The enthalpy of neutralization can be calculated using the following equation:
ΔH = (molar mass of water) x (enthalpy of fusion of water)
The experiment was performed using a solution of HCl and NaOH with a concentration of 1.0 M. The results are summarized in the table below.
The enthalpy of neutralization was calculated to be -57.2 kJ/mol.
Note on graphical analysis
To get the best value for the temperature change (ΔT) you should take multiple readings before and after mixing the reactants and then plotting a graph versus time.
Above are
two graphs from simple calorimeter experiments (picture on right).
On the left are typical results from an exothermic reaction e.g. metal plus acid or metal plus metal salt displacement reaction.
On the right are typical results from an endothermic change e.g. when certain salts dissolve in water.
The initial readings give you a baseline, but the reaction may take a few seconds or a few minutes, and so you cannot get an immediate true ΔT. However by drawing a baseline for the initial temperature and extrapolating back to the start of the reaction (e.g. at 1.5 minutes) you can then estimate the real temperature change.
What happens is quite simple, but it leads to inaccuracy:
For exothermic reactions the system will continuously lose heat once the reaction has started, so the temperature starts to fall once the reaction is complete, so extrapolating back up gives the true temperature rise.
In this case ΔT (corrected) = 28.4 - 20.4 = 8.0oC
For endothermic reactions the system will continuously gain heat once the reaction has started, so the temperature starts to rise once the reaction is complete, so extrapolating back down gives the true temperature fall.
In this case ΔT (corrected) = 19.5 - 11.2 = 8.3oC
Without allowing for these unavoidable experimental circumstances, you will always measure too low a temperature change.
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