How to Calculate the Molar Heat of Neutralization

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Neutralization reactions occur when you combine two extremely reactive substances together for the purpose of rendering them inactive, or neutral. For example, combining an acid and a base together produces water. Neutralization reactions give off energy, which is known as the heat of neutralization. The molar heat of neutralization is the amount of heat each mole of base added to the acid (or vice versa) causes the reaction to give off. (A mole is a unit chemists use to represent large numbers of molecules.) Once you determine the change in temperature that occurs, the rest is simple.

  1. Weigh Acid

  2. Weigh your acid on an electronic balance. Place an empty beaker on the balance and hit the Tare button to cancel out the weight of the beaker, then pour your acid into the beaker and place it on the balance. Record the mass of your acid.

  3. Find Temperature Change

  4. Calculate the change in temperature that occurs during the reaction using a calorimeter, a piece of equipment that both measures temperature and holds the reactant. Add your base to the calorimeter and place your acid (in its beaker) below the calorimeter's mouth. Insert the calorimeter's thermometer into the acid and read the initial temperature. Add the amount of base your reaction specifies to your acid, then read your calorimeter to determine the change in temperature.

  5. Calculate Heat of Neutralization

  6. Calculate the heat of neutralization using the fomula Q = mcΔT, where "Q" is the heat of neutralization, "m" is the mass of your acid, "c" is the specific heat capacity for aqueous solutions, 4.1814 Joules(grams x °C), and "ΔT" is the change in temperature you measured using your calorimeter. For example, if you start with 34.5 grams of hydrochloric acid at 26°C and its temperature increass to 29.1°C when you add sodium hydroxide to it, compute the heat of neutralization as follows: Q = mcΔT = (34.5 g x 4.1814 J) ÷ ((g x °C) x 3.1°C) = 447.48 Joules.

  7. Determine Molar Heat of Neutralization

  8. Calculate the number of moles of base you add to determine the molar heat of neutralization, expressed using the equation ΔH = Q ÷ n, where "n" is the number of moles. For example, suppose you add 25 mL of 1.0 M NaOH to your HCl to produce a heat of neutralization of 447.78 Joules. (Keep in mind that 1.0 M means one mole per liter.) Since you added 25 mL (25/1000, or .025 L) of NaOH, determine moles as follows: 1.0 mol/L x .025 L = .025 mol. In this example, your molar heat of neutralization, ΔH, is 447.48 Joules per .025 moles of NaOH added — 447.48/.025, or 17,900 Joules per mole.

    Tips

    • If your directions specify as such, add an acid to a base rather than a base to an acid. Weigh the base and calculate the number of moles of acid you add to it.

      Convert the molar heat of neutralization to kilojoules for a more manageable value by dividing it by 1,000. Keep in mind that 1 kJ=1,000 J. For the example above, ΔH expressed using kJ is 17.9 kJ/mol.

References

About the Author

Robert Schrader is a writer, photographer, world traveler and creator of the award-winning blog Leave Your Daily Hell. When he's not out globetrotting, you can find him in beautiful Austin, TX, where he lives with his partner.

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