6. Given that the AH for CaO, H3PO4, Ca3(PO4)2 and H2O, are, respectively, -635, -1267, -4126, and – 286 kJ/mole, (a) calculate the AH in kJ for the reaction: 3CAO + 2H3PO4→ Ca3(PO4)2 + 3H2O (b) calculate how much heat is released or absorbed when 15g of CaO reacts with excess phosphoric acid
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.
![### Thermodynamic Calculations: Enthalpy Change of a Reaction
**Problem Statement:**
Given that the \( \Delta H^\circ \) for \( \text{CaO} \), \( \text{H}_3\text{PO}_4 \), \( \text{Ca}_3(\text{PO}_4)_2 \), and \( \text{H}_2\text{O} \) are, respectively, -635 kJ/mole, -1267 kJ/mole, -4126 kJ/mole, and -286 kJ/mole,
(a) calculate the \( \Delta H \) in kJ for the reaction:
\[ 3\text{CaO} + 2\text{H}_3\text{PO}_4 \rightarrow \text{Ca}_3(\text{PO}_4)_2 + 3\text{H}_2\text{O} \]
(b) calculate how much heat is released or absorbed when 15 g of \( \text{CaO} \) reacts with excess phosphoric acid.
**Answers:**
**(a) Calculation of \( \Delta H \) for the Reaction**
To find the enthalpy change \( \Delta H \) for the given reaction, use the standard enthalpy of formation values for the reactants and products:
\[
\Delta H_{\text{reaction}} = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})
\]
For the given reaction:
\[ 3\text{CaO} + 2\text{H}_3\text{PO}_4 \rightarrow \text{Ca}_3(\text{PO}_4)_2 + 3\text{H}_2\text{O} \]
Standard enthalpy of formation values:
- \( \Delta H_f^\circ \) for \( \text{CaO} \) = -635 kJ/mole
- \( \Delta H_f^\circ \) for \( \text{H}_3\text{PO}_4 \) = -1267 kJ/mole
- \( \Delta H_f^\circ \) for \( \text{Ca}_3(\text{PO}_4)_2 \) = -4126 kJ/mole
- \( \Delta H_f^\circ \](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F09cc29b2-b8f1-46a4-8709-ee61d656c9a7%2Fa787b653-e46a-4b1b-a30f-13b173ff1c5e%2Fedwvhkc_processed.jpeg&w=3840&q=75)
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