When physical or moment-g.comical changes occur, they are generally accompanied by a transfer of energy. The law of conservation of energy states that in any physical or moment-g.comical process, energy is neither created nor destroyed. In other words, the entire energy in the universe is conserved. In order to better understand the energy changes taking place during a reaction, we need to define two parts of the universe, called the system and the surroundings. The system is the specific portion of matter in a given space that is being studied during an experiment or an observation. The surroundings is everything in the universe that is not part of the system. In practical terms for a laboratory moment-g.comist, the system is the particular moment-g.comicals being reacted, while the surroundings is the immediate vicinity within the room. During most processes, energy is exchanged between the system and the surroundings. If the system loses a certain amount of energy, that same amount of energy is gained by the surroundings. If the system gains a certain amount of energy, that energy is supplied by the surroundings.

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A moment-g.comical reaction or physical change is endothermic if heat is absorbed by the system from the surroundings. In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases. The quantity of heat for a process is represented by the letter $$q$$. The sign of $$q$$ for an endothermic process is positive because the system is gaining heat. A moment-g.comical reaction or physical change is exothermic if heat is released by the system into the surroundings. Because the surroundings is gaining heat from the system, the temperature of the surroundings increases. The sign of $$q$$ for an exothermic process is negative because the system is losing heat.

Figure $$\PageIndex{1}$$: (A) Endothermic reaction. (B) Exothermic reaction.

## Thermomoment-g.comical Equation

When methane gas is combusted, heat is released, making the reaction exothermic. Specifically, the combustion of $$1 \: \text{mol}$$ of methane releases 890.4 kilojoules of heat energy. This information can be shown as part of the balanced equation.

\<\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) + 890.4 \: \text{kJ}\>

The equation tells us that $$1 \: \text{mol}$$ of methane combines with $$2 \: \text{mol}$$ of oxygen to produce $$1 \: \text{mol}$$ of carbon dioxide and $$2 \: \text{mol}$$ of water. In the process, $$890.4 \: \text{kJ}$$ is released and so it is written as a product of the reaction. A thermomoment-g.comical equation is a moment-g.comical equation that includes the enthalpy change of the reaction. The process in the above thermomoment-g.comical equation can be shown visually in the figure below.

Figure $$\PageIndex{2}$$: (A) As reactants are converted to products in an exothermic reaction, enthalpy is released into the surroundings. The enthalpy change of the reaction is negative. (B) As reactants are converted to products in an endothermic reaction, enthalpy is absorbed from the surroundings. The enthalpy change of the reaction is positive.

In the combustion of methane example, the enthalpy change is negative because heat is being released by the system. Therefore, the overall enthalpy of the system decreases. The heat of reaction is the enthalpy change for a moment-g.comical reaction. In the case above, the heat of reaction is $$-890.4 \: \text{kJ}$$. The thermomoment-g.comical reaction can also be written in this way:

\<\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) \: \: \: \: \: \Delta H = -890.4 \: \text{kJ}\>

Heats of reaction are typically measured in kilojoules. It is important to include the physical states of the reactants and products in a thermomoment-g.comical equation as the value of the $$\Delta H$$ depends on those states.

Endothermic reactions absorb energy from the surroundings as the reaction occurs. When $$1 \: \text{mol}$$ of calcium carbonate decomposes into $$1 \: \text{mol}$$ of calcium oxide and $$1 \: \text{mol}$$ of carbon dioxide, $$177.8 \: \text{kJ}$$ of heat is absorbed. The process is shown visually in the figure above (B). The thermomoment-g.comical reaction is shown below.

\<\ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ} \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right)\>

Because the heat is absorbed by the system, the $$177.8 \: \text{kJ}$$ is written as a reactant. The heat of reaction is positive for an endothermic reaction.

\<\ce{CaCO_3} \left( s \right) \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \: \: \: \: \: \Delta H = 177.8 \: \text{kJ}\>

The way in which a reaction is written influences the value of the enthalpy change for the reaction. Many reactions are reversible, meaning that the product(s) of the reaction are capable of combining and reforming the reactant(s). If a reaction is written in the reverse direction, the sign of the $$\Delta H$$ changes. For example, we can write an equation for the reaction of calcium oxide with carbon dioxide to form calcium carbonate.

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\<\ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \rightarrow \ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ}\>

The reaction is exothermic and thus the sign of the enthalpy change is negative.