HCl is the chemical formula that represents hydrochloric acid. The metal zinc readily reacts with hydrochloric acid to produce hydrogen gas (H2) and zinc chloride (ZnCl2). Every chemical reaction either produces or absorbs heat. In chemistry this effect is described as the reaction enthalpy. The zinc reaction produces the heat and hence has the negative enthalpy. Calculating the enthalpy (heat) is a common assignment in chemistry.
Write down the equation of the chemical reaction between zinc and the hydrochloric acid. Zn + 2HCl = ZnCl2 + H2
Find out the enthalpies of formation for all compounds involved in the reaction using the source given in Resources. Those values are usually given in kilojoules (kJ): Zn = 0 kJ HCl = -167.2 kJ ZnCl2 = -415.1 kJ H2 = 0 kJ Enthalpies of formation of elements such as Zn or H2 are equal to zero.
Add up the enthalpies of formation of the reagents of the reaction. The reagents are zinc and hydrochloric acid, and the sum is 0 + 2 * ( -167.2) = -334.3. Note that the heat of formation of HCl is multiplied by 2 because the reaction coefficient of this compound is 2.
Sum up the enthalpies of formation of the reaction products. For this reaction, products are zinc chloride and hydrogen, and the sum is -415.1 + 0 = -415.1 kJ.
Subtract the enthalpy of the reagents from the enthalpy of the products to calculate the enthalpy (heat) of the zinc reaction; the enthalpy is -415.1 - (-334.3) = -80.7 kJ.
- "Chemistry: Textbook"; Raymond Chang; 2007
- Elmhurst College: Chemical Properties
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