5  Thermodynamics & Fossil Fuels

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5.1 Fire!

5.2 Thermo Speak

5.3 Heat IN/OUT - Enthalpy Change

5.5 Calorimetry

# Heat IN/OUT - Enthalpy Change

In thermodynamics we are tracking (accounting for) energy in/out of the system. We want to track it and account for it and quantify it. There are some terms that help with this and we should learn them and use them.

### Enthalpy is Heat

Heat is the flow of energy from one region to another in the form of randomized kinetic movement of molecules. In general, if no other change is occurring (no chemical or physical changes and no work being done), heat flow results in a temperature change. Common sense wins here. More heat into a system or substance means the system or substance will increase in temperature. The opposite is true as well... if heat leaves a system, the temperature of the system will decrease. Common sense, right? You want to boil water to make a hot beverage - you HEAT it... you find something that "gets hot" and then you utilize all that generated energy to heat your water up. You have to put your water in thermal contact with the "hot" object and that object will bleed off energy into the water as heat. The result is that the water heats up and the temperature climbs. Let's establish some science-y terms to help formalize all this (names and symbols).

First, temperature is the symbol $$T$$ and is measured in degrees Celsius (°C) or Kelvin (K)- or heck, even degrees Fahrenheit (°F) if you must. Heat is a thermodynamic term which is given the symbol $$q$$ and is measured in joules (J) or calories (cal). A joule is the SI unit of energy and we will try to stick with it. Now here is the thermodynamic term - the energy within any substance that ends up flowing as heat is called enthalpy and its symbol is $$H$$. Imagine all the energy within a substance - it will depend on the actual atoms within the substance, the amount of the substance, and the conditions under which it exists - such as temperature and pressure. "All" the energy will be a combination of kinetic and potential energy. When a change occurs (chemical or physical), any energy that flows as heat during constant pressure is the enthalpy, $$H$$. So start thinking about all the matter around you as having specific amounts of enthalpy within it. That enthalpy can go up or down and monitoring temperature is a great way to track it.

So when heat DOES flow the enthalpy must be changing. The enthalpy will have an "initial" amount ($$H_{\rm i}$$) and then it will have a "final" amount ($$H_{\rm f}$$). The change in enthalpy will therefore be equal to the heat flow. The equation is fairly simple. For any heat flow at constant pressure...

$$\Delta H = H_{\rm f} - H_{\rm i} = q$$

The thing to see in that equation is that the heat flow is equal to the change in enthalpy: ($$\Delta H = q$$).

### IN = Endothermic | OUT = Exothermic

Anytime heat flows INTO a sytem, the process is called endothermic. Anytime heat flows OUT of a system, the process is called exothermic. The science-y way to say it is the following:

• An endothermic process is one in which heat is absorbed by the system.
• An exothermic process is one in which heat is released by the system.

How do we measure heat in/out of the system? The next section on calorimetry will discuss that.