Heat reactions...

Question

A student dissolves 11.1 g of sodium hydroxide NaOH in 250g of water in a well-insulated open cup. She then observes the temperature of the water rise from 21.0°C to 32.7°C over the course of 7.4 minutes.Use this data, and any information you need from the ALEKS Data resource, to answer the questions below about this reaction:

NaOH(s)→  Na+(aq)+  OH−(aq)

You can make any reasonable assumptions about the physical properties of the solution. Be sure answers you calculate using measured data are rounded to the correct number of significant digits.

Is this reaction exothermic, endothermic, or neither?

1. If you said the reaction was exothermic or endothermic, calculate the amount of heat that was released or absorbed by the reaction in this case (kJ).
2. Calculate the reaction enthalpy ΔHrxn per mole of NaOH (kJ/mol).

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Answer 1

The reaction is exothermic. This is known because the temperature increases, which indicates an exothermic reaction.

q = mcΔT = (250 g + 11.1 g) (4.184 J/g ^oC) (32.7^oC - 21.0^oC) = (261.1 g) (4.184 J/g^oC) (11.7^oC)

≅ 1.28 x 10⁴ J = 12.8 kJ

(12.8 kJ/11.1 g NaOH) (40.0 g NaOH/mol) = 46.1 kJ/mol. Since it's an exothermic reaction,

then ΔΗ = -46.1 kJ/mol