We are surrounded by exothermic and endothermic reactions. Episode 29 starts by describing the first law of Thermodynamics...
We are surrounded by exothermic and endothermic reactions. Episode 29 starts by describing the first law of Thermodynamics (1:39) as well as the terms system and surroundings (1:53) using the combustion of hydrogen as an example. We take a closer look at the bond breaking and bond formation of the combustion (2:20) and describe the energy diagrams for exothermic (2:51) and endothermic reactions (4:12). Hand warmers and cooling packs are great examples of exothermic and endothermic processes (4:47). But also purely physical changes, like water freezing or ice melting can be described as exothermic or endothermic (5:31).
Question: In Chemistry, we use Enthalpy as a measurement of heat content. What is the sign for enthalpy for an endothermic reaction?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap the energy of chemical reactions.
Lets Zoom Out:
Unit 6 - Thermodynamics
Topic 6.1 and 6.2 - Endothermic and Exothermic Processes and Energy Diagrams
Big idea - Energy
The 4th of July fireworks in the United States, the fireplace burning on a cold winter night or candles casting a romantic light on Valentine’s Day - no matter what time of the year, chemical reactions make celebrations even more special! As we’ve discussed in our very first episode on chemical and physical changes, the change in temperature indicates a chemical change, but can also be tied to a physical change, for example, phase changes. Today, we will take a closer look at the exchange of energy.
Let’s zoom in:
The examples listed in our introduction all release energy in the form of heat. They are exothermic reactions. Far less spectacular in our everyday life are endothermic reactions - reactions that absorb heat. Exothermic and endothermic processes can be seen as phase changes as well as chemical transformations. Before we look at a couple of examples, let’s lay the foundation by defining terms.
The first law of Thermodynamics states that energy is conserved. It can neither be created nor destroyed, only converted from one form of energy to another. That means that the total energy of the universe is constant. If a system loses energy, as happens in exothermic reactions, the surroundings have to absorb the energy. What are the system and the surroundings? In chemistry, our system usually includes our molecules and chemical substances we want to study. The surrounding is everything outside of the system. For example, if we have the combustion of hydrogen with oxygen in a classroom, the hydrogen and oxygen molecules would be the system and the classroom would be the surroundings.
The combustion of hydrogen is an exothermic chemical reaction. It releases large amounts of heat and the formed product, water, has different properties than the reactants. As we know, chemical bonds are broken and new bonds are formed in chemical reactions. In this case, we have to break the hydrogen hydrogen bond as well as the oxygen oxygen double bond. This will require energy. Therefore, to get our chemical reaction started, we need to provide activation energy. In an energy diagram, that is the difference between the reactants and the highest point of the diagram. Once the bonds are broken, new bonds can form. In our case between oxygen and hydrogen. Bond formation releases energy. In exothermic reactions the energy content of the products is lower than the energy of the reactants. The excess energy is being released.
What about endothermic reactions? They are the other way round! In endothermic reactions, you still require activation energy to break the chemical bonds. But, in comparison with exothermic reactions, the products of endothermic reactions have a higher energy content than the reactants. This energy that is added to the system has to come from the surroundings. Therefore, while exothermic reactions release heat and increase the temperature you are measuring, endothermic reactions are characterized by a decrease in temperature of the surroundings.
Let’s look at another example: Hand warmers and cooling packs! The dissolution of ionic compounds in water to form an aqueous solution can be either exothermic or endothermic, depending on the strength of the forces between the ions as well as the ion-dipole forces. Therefore, these processes can be argued to be both physical and chemical. If the dissolution releases energy from the system to the surroundings, the water, the solution heats up - and you have a lovely hand warmer! If you are an athlete, or like our author Sarah prone to burning herself on the oven, you’d prefer a cooling pack. A cooling pack is an endothermic reaction: the system takes in heat from the surroundings, making the surroundings cooler.
So how does that relate to physical changes? When ice is melting or water is boiling, the system has to take in heat. These changes are endothermic processes. On the other hand, when water freezes or vapor condenses, the system has to release heat and the processes are therefore exothermic.
To recap:
According to the first law of Thermodynamics, energy is conserved. During exothermic processes heat is released from the system and absorbed by the surroundings. Fireworks, handwarmers and freezing are examples of exothermic processes. In endothermic processes, heat is absorbed by the system from the surroundings and the temperature of the surroundings decreases. Examples of endothermic reactions are cooling packs for athletes and melting ice.
Today’s Question of the day is about enthalpy.
Question:
In Chemistry, we use Enthalpy as a measurement of heat content. What is the sign for enthalpy for an endothermic reaction?