The apparatus drawn was used in a student experiment to measure the heat of reaction (ΔH) for the reaction between 1.0 M hydrochloric acid (HCl) solution and 1.0 M sodium hydroxide (NaOH) solution - Leaving Cert Chemistry - Question 3 - 2006
Question 3
The apparatus drawn was used in a student experiment to measure the heat of reaction (ΔH) for the reaction between 1.0 M hydrochloric acid (HCl) solution and 1.0 M s... show full transcript
Worked Solution & Example Answer:The apparatus drawn was used in a student experiment to measure the heat of reaction (ΔH) for the reaction between 1.0 M hydrochloric acid (HCl) solution and 1.0 M sodium hydroxide (NaOH) solution - Leaving Cert Chemistry - Question 3 - 2006
Step 1
What is meant by heat of reaction?
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Answer
The heat of reaction (ΔH) refers to the energy change when one mole of a reactant is completely reacted according to the balanced chemical equation. It indicates whether the reaction is exothermic (releases heat) or endothermic (absorbs heat).
Step 2
How do the temperature measurements taken during the experiment provide evidence that the reaction between HCl and NaOH is exothermic?
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The temperature measurements show an increase during the reaction, indicating that heat energy is released. This rise in temperature is evidence of an exothermic reaction, as energy is transferred from the reaction mixture to the surrounding environment.
Step 3
What is the advantage of using a cup made from polystyrene?
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Polystyrene cups are excellent insulators. They minimize heat loss to the environment, allowing more accurate temperature measurements during the reaction, which is essential for determining the heat of reaction.
Step 4
How would you have obtained a reasonably accurate value for the change in temperature?
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To obtain an accurate change in temperature, one could allow the temperature to equalize in advance by preheating the solutions to similar temperatures. An accurate thermometer should be used (calibrated for a short range of 0.1 degrees). The temperatures should be measured quickly, and it is important to record the highest temperature reached during the reaction.
Step 5
Calculate (i) the number of moles of hydrochloric acid (HCl) in 50 cm³ of 1.0 M hydrochloric acid
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The number of moles of hydrochloric acid can be calculated using the formula:
ext{moles} = rac{ ext{volume in litres} imes ext{concentration (M)}}{1000}
Calculate (ii) the heat in kJ which would be produced if a solution containing 1 mole of hydrochloric acid reacted fully with sodium hydroxide
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Answer
For 1 mole of hydrochloric acid reacting with sodium hydroxide, the reaction produces 56 kJ of heat energy. This value represents the heat generated from the complete reaction based on stoichiometry.
Step 7
Calculate (iii) the heat of reaction (ΔH) for the reaction in kJ mol⁻¹
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The heat of reaction (ΔH) can be calculated by dividing the heat released (2.8 kJ) by the number of moles of HCl reacting:
