How much heat in kilojoules is evolved or absorbed in the reaction of 1.00 g of Na with H2O? Is the reaction exother- mic or endothermic? 2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g) ΔH° = -368.4 kJ

Verified step by step guidance

1. First, calculate the molar mass of sodium (Na). The atomic mass of Na is approximately 22.99 g/mol.
2. Next, determine how many moles of Na are present in 1.00 g of Na using the formula: moles = mass (g) / molar mass (g/mol).
3. Using the stoichiometry of the balanced chemical equation, identify that 2 moles of Na react with 2 moles of H2O to produce 2 moles of NaOH and 1 mole of H2, releasing 368.4 kJ of heat. Therefore, the reaction of 1 mole of Na would involve half of this heat change.
4. Calculate the heat evolved or absorbed when 1.00 g of Na reacts by multiplying the moles of Na calculated in step 2 by the heat change per mole of Na derived in step 3.
5. Determine whether the reaction is exothermic or endothermic by analyzing the sign of ΔH°. A negative ΔH° indicates that the reaction is exothermic, meaning heat is released.

Key Concepts

Thermochemistry

Thermochemistry is the study of the heat energy associated with chemical reactions and phase changes. It involves understanding how energy is absorbed or released during these processes, which is quantified in terms of enthalpy change (ΔH). In the given reaction, the negative ΔH value indicates that heat is released, characterizing the reaction as exothermic.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It allows us to determine the amount of heat evolved or absorbed per mole of reactant. In this case, knowing the molar mass of sodium (Na) helps convert the mass of sodium used in the reaction to moles, which can then be used to calculate the total heat change.

Exothermic vs. Endothermic Reactions

Exothermic reactions release heat to the surroundings, resulting in a temperature increase in the environment, while endothermic reactions absorb heat, leading to a temperature decrease. The sign of ΔH indicates the nature of the reaction; a negative ΔH, as in this case (-368.4 kJ), signifies that the reaction is exothermic, meaning heat is evolved when sodium reacts with water.