What is Energy
Energy is the ability to do work. In other words, it measures the system’s ability to do something. There are different types of energies that measure the ability to do different things. Energy cannot be created or destroyed. Instead, it is converted between these different forms.
It is measured in joules (J). You may also commonly see kilojoules (kJ). 1 kJ = 1000 J. Some equations related to energy will be in joules, while others will be in kilojoules, so pay attention to your units.
Kinetic Energy: The energy of motion. It considers both the mass of the object and speed at which the object is traveling.
KE = ½ mv²
– m is mass in kg
– v is velocity in m/s
As the equation reveals, an object with more mass has more KE. Furthermore, since KE is energy of motion, an object with a greater velocity has more KE.
Potential Energy: Stored energy that has the “potential” to be converted into other forms of energy. There are many types of potential energy, such as gravitational potential energy (an object’s ability to fall).
Chemical Potential Energy: Potential energy stored in bonds that can be converted into other types of energy. Energy changes in chemical reactions are due to the release or absorption of chemical potential energy.
Ex. Gasoline has lots of chemical potential energy. When gasoline is burned, the bonds are broken, releasing that energy. This is why we use gasoline to power cars.
Internal Energy: The sum of the kinetic and chemical potential energy in a system. Often, it is represented by U or sometimes just E.
Temperature: A measure of the average kinetic energy of the molecules in a system. The key word is average. A cold pool has lots of particles, so it may have a higher total kinetic energy than a small hot coffee pot. However, the individual coffee pot particles on average have more KE, so the coffee pot is hotter.
In chemistry we often use ˚C for temperature. We also use Kelvin (K).
˚C = K + 273.15
Note that a change in celsius is the same change in kelvin. If something gets 2 ˚C warmer, it also gets 2 K warmer.
As we mentioned, energy is transferred between different forms. For example, when chemical potential energy is released, it can cause nearby particles to speed up, increasing their kinetic energy. In that case, chemical potential energy was converted to KE.
Work
Work is a force exerted over a displacement that creates a change in mechanical energy.
W = Fd
– W = work (J)
– F = force (in Newtons)
– d = displacement (in meters)
Work can be positive or negative. If you were to push a cart forward, increasing its KE, you are doing positive work. If your friend is on the other side of the cart, and they slow it down, they are decreasing the cart’s KE, so they are doing negative work.
Heat
Heat is an idea that encapsulates the energy that mechanical work cannot account for. Again, energy can’t be created or destroyed, it is only transferred between forms.
However, most times, the work that an object has done does not match up to the total change in energy. That’s because work is not the only way to transfer energy. Energy can also be transferred as heat in an interaction.
For example, when you put your hot chicken in the fridge, the chicken cools down and loses energy. However, no force or displacement occurred, so no work could have been done. The chicken’s change in energy was due to heat, not work. In some processes, energy changes can be due to both.
∆E = W + q
– ∆E = change in energy
– W = work
– q = heat
Heat tends to flow from objects of higher temperature to objects of lower temperature. Take the previous example. Through heat, energy is transferred from the chicken to the fridge in an attempt to reach thermal equilibrium: the state in which two objects are at the same temperature.
The chicken gets cooler (loses energy), so negative heat was done on it. The fridge gets warmer (gains energy), so it has positive heat was done on it.
Chemistutor 