MISSED THIS? Watch IWE: Measuring AHrxn in a Coffee- Cup Calorimeter, Read Section 7.7. You can click on the Review link to access the section in your e Text. Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction: NH₂NO3(s) → NH; (aq) + NO3(aq) In order to measure the enthalpy change for this reaction, 1.25 g of NH, NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 °C and the final temperature (after the solid dissolves) is 21.9 °C. Part A Calculate the change in enthalpy for the reaction. (Use 1.0 g/mL as the density of the solution and 4.18 J/(g-°C) as the specific heat capacity.) Express the enthalpy change in kilojoules per mole to two significant figures. 1ΨΕΙ ΑΣΦ ΔΗ, = Request Answer kJ/mol
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Given that
Mass of NH4NO3 = 1.25 g
Molecular weight of NH4NO3 = 80.04 g/mol
Volume of the solution = 25.0 mL
Mass of the solution = Volume * density = 25.0 mL * 1.00 g/mL = 25.0 g
Number of moles of NH4NO3 = mass/mol.wt. = 1.25 g/(80.04 g/mol) = 0.015617 mol
Heat capacity of the solution = 4.18 J/(g. °C)
Change in temperature = ΔT = 21.9 – 25.8 = –3.9 °C
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