Question

How much heat is released if 25 g of sodium hydroxide mix with 4 molars of hydrochloric acid?

Answer 1

How much heat is released if 25 g of sodium hydroxide mix with 4 molars of hydrochloric acid?

Solution:

NaOH+HCl=NaCl+H₂O

Use the periodic table to calculate the molar mass of NaOH:

M(NaOH)=23+16+1=40 g/mol.

Determine the number moles of NaOH in 25g of its:

$$v = \frac{m}{M} = \frac{25}{40} = 0.625 \text{ moles}.$$

So, the amount of NaOH is limited. Hence, the much heat is realized in this reaction we determined by amount of NaOH.

A measure of the heat of reaction is the enthalpy change $(\Delta H, kJ/mol)$.

By applying Hess's Law

$\Delta H = \sum \Delta H_{\text{of products}} - \sum \Delta H$ of reactants

The enthalpy of reaction $\Delta H^{\circ}$ will be:

$(\Delta H \text{ of NaCl plus } (\Delta H \text{ of H}_2\text{O}) \text{ minus } [(\Delta H \text{ of NaOH}) \text{ plus } (\Delta H \text{ of HCl})].$

We give the standard enthalpies of formation of compounds are:

$\Delta H$ of NaCl = -411 kJ/mol·K,

$\Delta H$ of H₂O = -241.8 kJ/mol·K,

$\Delta H$ of NaOH = 426.3 kJ/mol·K,

$\Delta H$ of HCl = -92.3 kJ/mol·K.

When it is considered that there is only 0.625 mole of NaOH the enthalpy of reaction $\Delta H^{\circ}$ will be:

$$\Delta H = 0.625\{-411+(-241.8)\cdot[426.3+(-92.3)]\} = -894.5 \text{ kJ}.$$

Answer: -894.5 kJ