Problem 45 - Chemistry The Molecular Science

Question

Energy is stored in the body in adenosine triphosphate, ATP, which is formed by the reaction between adenosine diphosphate, ADP, and dihydrogen phosphate ions. \(\mathrm{ADP}^{3-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{PO}_{4}^{2-}(\mathrm{aq}) \longrightarrow \mathrm{ATP}^{4-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)\) $$ \Delta_{\mathrm{r}} H^{\circ}=20.5 \mathrm{~kJ} / \mathrm{mol} $$ Is the reaction endothermic or exothermic?

Step-by-Step Solution

Answer

The reaction is endothermic.

Step 1: Understand the Concept of Reaction Enthalpy Change

The change in enthalpy (\(\Delta_{\mathrm{r}} H^{\circ}\)) of a reaction determines whether it's endothermic or exothermic. A positive \(\Delta_{\mathrm{r}} H^{\circ}\) indicates that energy is absorbed, classifying the reaction as endothermic. Conversely, a negative \(\Delta_{\mathrm{r}} H^{\circ}\) indicates an exothermic reaction where energy is released.

Step 2: Identify Given Enthalpy Change

In the problem, it's given that \(\Delta_{\mathrm{r}} H^{\circ} = 20.5 \, \mathrm{kJ/mol}\). Since this value is positive, it suggests that the reaction absorbs energy from the surroundings.

Step 3: Conclude the Reaction Type

Based on the positive enthalpy change, we conclude that the reaction ADP with dihydrogen phosphate to form ATP and water is endothermic because it absorbs energy.

Key Concepts

Endothermic Reaction

Understanding endothermic reactions is crucial in the study of thermodynamics. These reactions absorb energy from their surroundings, causing the surroundings to cool down. In a chemical reaction, endothermicity is indicated by a positive enthalpy change (\(\Delta_{\mathrm{r}} H^{\circ}\)). Unlike exothermic reactions, which release energy, endothermic processes require an input of energy to proceed. This energy is absorbed in the form of heat.

Some common examples of endothermic reactions include:

• Photosynthesis, where plants absorb light energy to convert carbon dioxide and water into glucose.
• The melting of ice, which absorbs heat from the environment to transition from a solid to a liquid state.

In the context of the given chemical equation, the reaction between adenosine diphosphate (ADP) and dihydrogen phosphate ions to form adenosine triphosphate (ATP) and water is endothermic, as evidenced by its positive enthalpy change of \(20.5 \, \mathrm{kJ/mol}\), suggesting that energy is absorbed from the surroundings.

Adenosine Triphosphate

Adenosine triphosphate, or ATP, is often referred to as the "energy currency" of the cell. This is because it stores and provides energy necessary for many biological processes. ATP is formed by the bonding of adenosine diphosphate (ADP) with an inorganic phosphate group.

The structure of ATP consists of:

• Three phosphate groups
• Ribose, a sugar molecule
• Adenine, a nitrogenous base

These components allow ATP to store energy effectively. When ATP is broken down into ADP and an inorganic phosphate, it releases energy, which cells use for various activities such as muscle contraction, nerve impulse propagation, and chemical synthesis.

In the discussed exercise, ATP is synthesized from ADP and dihydrogen phosphate ions. This synthesis process requires an input of energy, classifying the operation as endothermic. This process highlights ATP’s role as a crucial energy-transferring molecule in living organisms.

Energy Absorption

Energy absorption occurs when a system takes in energy from its surroundings. In chemical reactions, sometimes this energy absorption is required to drive the reaction forward. The reaction between ADP and dihydrogen phosphate ions to form ATP is one such example.

In this scenario, energy absorption can be depicted by the positive reaction enthalpy (\(\Delta_{\mathrm{r}} H^{\circ} = 20.5 \, \mathrm{kJ/mol}\)). This value indicates that 20.5 kJ of energy per mole is absorbed from the surroundings into the reaction mixture.

• This intake of energy can often result in a decrease in temperature of the surroundings unless energy is continuously supplied.
• The absorbed energy is stored within the chemical bonds formed during the reaction.

For biochemical reactions like ATP synthesis, energy absorption is crucial as it allows the storage of energy which can be utilized by the body when needed. Understanding how energy is stored and transferred in such reactions is fundamental to fields like biochemistry and physiology.