Unit 6: Topic 1 - Endothermic and Exothermic Processes Flashcards
Define system and surroundings in the context of thermochemistry.
A thermodynamical system is a body of matter, with specific permeability, in which the chemical reaction occurs. Everything outside of this system is known as the surroundings, so that the system and surroundings combined account for the universe. A system can either be isolated (allowing exchange of neither of energy nor mass), closed (allowing exchange of energy but not mass), or open (allowing exchange of both).
In a given chemical reaction, determine the relationship between temperature change and energy change.
If there is a change in energy in a chemical reaction, then the reaction either absorbs or releases energy. If the reaction releases energy, then that energy, some of which is heat, is released into the surroundings. Since the surroundings gain heat, their temperature increases. Similarly, if the reaction absorbs energy, then heat from the surroundings is lost, and the temperature of the surroundings would decrease.
Define exothermic and endothermic.
An exothermic process releases energy from the system to its surroundings, and an endothermic process absorbs energy from the surroundings.
Determine whether the melting of ice into water is exothermic or endothermic.
The melting of ice into water is endothermic. It takes a certain amount of heat to move any substance from one phase to a more energetic phase. Also, the surroundings will lose heat: if you hold an ice cube and let it melt, your hand should get colder.
Note: if the temperature of the system is higher than the melting point of ice, the reaction is still endothermic, even though it will proceed on its own. This is a concept called spontaneity, to be discussed later.
Is combustion of a hydrocarbon exothermic?
Combustion is the reaction of the hydrocarbon with oxygen. A combustion reaction involves burning of the hydrocarbon, which produces heat energy. So combustion is exothermic.
What does the first law of thermodynamics state?
The first law of thermodynamics states that energy in the universe cannot be created or destroyed. Since the universe is made up of the system and surroundings, if a system loses energy then the surroundings gain this energy and vice versa.
If a reaction is exothermic, what conclusions can be drawn about the reverse reaction?
If a reaction is exothermic, then energy is released from the system to the surroundings. If we were to reverse this process, then we should expect the surroundings to release energy into the system, or for the system to absorb energy. The reverse process would be endothermic.
Explain the steps of a solvation process and describe how this process could be both endothermic or exothermic.
In a solvation process, a solute will dissolve in a solvent. There are three steps to this process.
1. Separate particles of the solute from each other (endothermic)
2. Separate particles of the solvent from each other (endothermic)
3. Combine particles of solute and solvent with each other (exothermic).
Separation of particles is endothermic, because energy is required to break the intermolecular forces between particles. However, combining particles is exothermic because it is the reverse of separating particles. Since there are three steps, we can combine the total energy transferred to determine if the dissolution is exothermic or endothermic. In general, this is exothermic, since the exothermic combination step is dominant (the other two require less energy together than that one releases). However, there are endothermic examples (dissolution of ammonium nitrate is endothermic).
