A chemical reaction involves the transfer of energy, usually heat, between the reacting substances (the system) and everything else (the surroundings). Heat is either absorbed from the surroundings or released into them as atoms rearrange to form new molecules. An endothermic process occurs when the system absorbs heat energy, causing the temperature of the surroundings to decrease.
The Definitive Answer: Positive or Negative
Endothermic reactions are positive when considering the change in enthalpy (\(Delta H\)) of the system. Enthalpy quantifies the total heat content of a system at a constant pressure. The change in enthalpy (\(Delta H\)) tracks the flow of energy during a reaction.
The sign convention assigns a positive value to energy gained by the system and a negative value to energy lost. Since an endothermic reaction absorbs heat from the surroundings, the system gains energy, resulting in a positive \(Delta H\). For example, an enthalpy change of \(+50\) kilojoules per mole (\(text{kJ/mol}\)) indicates that \(50 text{ kJ}\) of heat has been taken in by the system per mole of reaction.
Why Energy Change is Positive
The positive sign arises because the change in any scientific quantity is calculated as the final state minus the initial state. For a chemical reaction, the initial state is the energy of the reactants, and the final state is the energy of the products. Therefore, the change in enthalpy is calculated as \(Delta H = H_{text{products}} – H_{text{reactants}}\).
For an endothermic reaction, the system must absorb energy to drive the chemical transformation. This means product molecules hold a greater amount of stored energy than the initial reactant molecules. Since the final energy (\(H_{text{products}}\)) is larger than the initial energy (\(H_{text{reactants}}\)), the subtraction yields a positive number. The absorbed heat energy converts into chemical potential energy stored within the product bonds.
The Opposite Reaction: Exothermic Change
The opposite of an endothermic process is an exothermic reaction, defined by a negative change in enthalpy (\(Delta H\)). An exothermic reaction releases heat energy into the surroundings. This release of energy causes the temperature of the surroundings to increase.
The negative sign for \(Delta H\) occurs because the products have a lower total energy content than the reactants. When the reaction takes place, the excess energy is expelled into the surroundings, resulting in a net loss of energy from the system. Combustion of natural gas is a common example, where bonds are rearranged to form products like carbon dioxide and water, releasing heat energy represented by a negative \(Delta H\) value.
Examples of Endothermic Processes
Endothermic processes produce a noticeable cooling effect as they draw heat from their surroundings. A prime example is the instant cold pack used for sports injuries, which contains ammonium nitrate and water separated by a barrier. When activated, the ammonium nitrate dissolves in the water, a process that requires heat energy to break the salt’s ionic bonds.
This energy is absorbed from the surrounding water and the pack’s exterior, causing a rapid drop in temperature. Another example is photosynthesis, where plants convert carbon dioxide and water into glucose and oxygen. Photosynthesis is endothermic because it requires a continuous input of light energy from the sun to drive the reaction. This light energy is absorbed by chlorophyll and converted into chemical energy stored in the glucose molecules.