Three things are usually required for a chemical explosion: a chemical reaction that occurs very rapidly, a large increase in gas pressure, and a confined-reaction vessel in which the pressure of the gaseous products can increase to a point that the gases break violently out of the container. In this chemistry and physics science fair project, you will use the combustion of ethanol to provide energy for a small explosion. The chemical equation that describes the combustion of ethanol is shown below. (Note: Hover over the equations in this Introduction with your cursor to view enlarged formulas. )
Equation 1: C2H6O+302- 3H2O+2CO2+heat Ethanol: C2H60 Oxygen: 302 Water: H2O Carbon dioxide: CO2 The chemical equation states that ethanol (C2H6O) combines with oxygen (02) to form water (H2O), carbon dioxide (CO2), and energy in the form of heat. It also forms energy in the form of light, but this is small compared to the heat energy. Ethanol and oxygen are the reactants, and water and carbon dioxide are the chemical products of the equation. Note that the equation is balanced, so that the number of atoms of each element is the same on both sides; for example, there are two carbon atoms on both sides.
Oxygen, which forms about 20 percent of our atmosphere, is a ery reactive chemical. When it reacts with ethanol, the reaction proceeds very quickly and produces a substantial amount of heat. But a mixture of oxygen and ethanol is stable unless you provide a “spark” or other source of energy to get the reaction started. Once the two chemicals begin to react, the reaction itself produces enough energy to sustain further combustion. The reaction stops when the reactants are used up. One way to create an explosion is to increase the number of molecules that are in the form of a gas.
If all of the molecules in Equation 1 were gas molecules, there would be a net increase rom four molecules (one ethanol and three oxygen) to five molecules (three water and two carbon dioxide). Creating more gas molecules in a fixed volume will increase the pressure, just as pumping air into a tire or basketball increases its pressure. If the pressure is greater than the container can handle, an explosion can occur. For this science fair project, you will ignite a small amount of ethanol to launch a film canister. The volume of the canister is small enough that there is little danger of getting injured, but it will make quite a pop! Just how big a pop? Watch the video below for a preview.
Some of the ethanol will be in the form of a liquid. Also, some of the water that is a reaction product will be liquid. Thus, some of molecules will not be in the form of a gas because the temperature will be low enough that they will condense to the liquid form. In this video, the Science Buddies Summer Science Fellows demonstrate that a little bit of breath spray and a spark can lead to a fun ‘pop’ and be a gateway to safely explore some explosive chemistry and physics. Increasing the amount of gas molecules is not the only way to increase pressure.
Another way to increase pressure in a container with a fixed volume is to ncrease the temperature. An increase in temperature causes the molecules in the gas to move faster. The collisions of these more energetic molecules against the sides of the container results in a higher pressure. In summary, the pressure in a closed container will increase if more molecules of gas are added, or if the temperature increases. This relationship is captured in the ideal gas equation: Equation 2: PV=nRT P = pressure, in atmospheres (atm) V = volume, in liters (L) n = amount of gas, in moles (mol) R = gas constant, 0. 0082 (L x atm)/(mol x K) T= temperature, in degrees Kelvin (K)
The gas equation states that the pressure times the volume equals the product of the number of moles, the gas constant, and the temperature. The key point about this equation, for the purposes of this science fair project, is that it clearly shows the mathematical relationship between pressure, amount of gas, and temperature. The pressure, P, is directly proportional to n, and T. If you double P in the equation, keeping V, n, and R the same, then T will also double. The same goes for n and P. The equation is precise only for “ideal” gases in equilibrium with their surroundings, which is not the case in an explosion.
But the ideal gas equation is a useful approximation of this real-world experiment. A version of the experimental setup is shown in Figure 1. The apparatus consists of a piece of wood with the top of a film canister glued onto it. The metal ends of a grill spark igniter are passed through a hole in the wood and through the film canister top. The metal ends will deliver a spark when the red button is pushed.
When you add a fuel to the film canister and attach it to the plastic top that is glued to the board, a spark will ignite the fuel and send the canister flying. The fuel used to launch the anister is Binaca breath spray (ethanol and isopropane). A tire pressure gauge is used to measure the maximum pressure created inside the canister when the ethanol gas is ignited. Picture of a film canister projectile. The top of a film canister is glued to a piece of wood. A spark generator is attached through a hole in the wood, to deliver a spark when the red button is pushed.
A pressure gauge is also attached and is connected to the space inside the canister by another hole in the wood. Binaca breath spray is used as fuel to launch the canister. There is electrical tape around the body of the spark unit to prevent a hock. Figure 1. Picture of a film canister projectile. The top of a film canister is glued to a piece of wood. A spark generator is attached, through a hole in the wood, to deliver a spark when the red button is pushed. A pressure gauge is also attached and is connected to the space inside the canister by another hole in the wood. Binaca breath spray is used as fuel to launch the canister. Note the electrical tape around the body of the spark unit to prevent a shock. By timing how long the canister is in the air, you can calculate the launch velocity.
Once you have the launch velocity, you can alculate the kinetic energy of the canister. The kinetic energy of the canister is the energy due to its mass and velocity. The formula for calculating the kinetic energy is given below. You can also calculate the maximum potential energy, which is determined by how high the canister flies; the higher it goes, the higher the potential energy. The equation for potential energy is given below. If you shoot the canister straight up, it will have zero velocity for a very brief time at the highest point as it stops going up and starts coming down. At this moment, the kinetic energy is at its lowest and the potential energy is at its highest.
Several basic physics formulas for ballistic projectiles are shown below. The projectiles are assumed to be shot 90 degrees (for best height) or at a 45-degree angle (for best range). The formulas are approximations because they assume there is no air resistance. “Time of flight” is the time it takes the canister to return to the same level from which it was launched. For the equations below: V = Launch velocity, in meters per second (m/s) g = acceleration due to gravity, 9. 8 m/s2 Time of flight = time to return to launch height, in seconds (s) H = maximum height, in meters (m) m = mass of the canister, in kilograms (kg) KE = kinetic energy, in joules () PE = potential energy, in joules () Equations to use when launching the canister straight up: Equation 3: V=12xgxtimeofflight The velocity of the canister at launch equals one-half of the product of the acceleration due to gravity (9. 8 m/s2) and the time of flight. Equation 4: H=12V2g The maximum height of the canister is one-half of the launch velocity squared, divided by the acceleration due to gravity (9. 8 m/s2).
Equations to use when the canister is shot at a 45-degree angle: Equation 5: V=0. 71xgxtimeofflight The launch velocity (45 degrees) equals 0. 1 times the acceleration due to gravity, times the time of flight. Equation 6: H=V24g The maximum height (45 degrees) equals the launch velocity squared, divided by 4 times the acceleration due to gravity. (You can get launch velocity from Equation 5). Equation 7: Range=V2g The range (45 degrees) equals the launch velocity squared, divided by the acceleration due to gravity. Equations for the energy of the canister. Assume the mass of the canister is 4 grams (g). Equation 8: KineticEnergy=12mV2 The kinetic energy of the canister at the time of launch is equal to one-half its mass, times the launch velocity squared.
Equation 9: PotentialEnergy=mgH The potential energy of the canister at its maximum height equals its mass times g, times the height. Note from Equation 3 that you can determine launch velocity if you know the flight time. Time of flight is easy to measure; just use the stopwatch to time the interval from launch to landing. Once you have calculated the launch velocity, you can calculate the maximum height the canister flies using Equation 4. Remember that the equations are approximations and do not take into account the air resistance.
You will use the trajectory equations to determine the launch velocity of the canister, its maximum height, and its kinetic and potential energy. The kinetic energy of the canister gives you a minimum value for the chemical energy of the combustion reaction. It is a minimum value because energy is lost as heat, friction, and in other ways. As you work through the procedure, consider where the energy is flowing. The overall flow of energy when the canister is launched straight up follows this sequence: Chemical energy: Energy released as heat causes an abrupt pressure rise in the canister.
Kinetic energy: The canister is launched upward. Potential energy: This reaches a maximum at the maximum height. It returns to zero when the canister lands. Kinetic energy: The potential energy is converted to kinetic energy as the canister falls back to Earth. When the fuel is ignited, the combustion of the alcohol will create a sharp rise in the pressure inside of the canister. One goal of this science fair project is to measure the pressure in the canister. You can then estimate the temperature. The situation inside the canister is far from ideal, so the estimates will be very rough.
Another goal is to measure the volume change over the course of the explosion, using a balloon attached to the canister. Now you’re ready to have a blast! Terms and Concepts Chemical explosion Chemical reaction Gas pressure Combustion Ethanol Chemical equation Reactant Product Balanced equation Condense Ideal gas equation Mole (mol) Kelvin (K) Proportional Launch velocity Kinetic energy Potential energy Questions How does the combustion of ethanol in the film canister compare to the combustion of gasoline in a car engine? How does the combustion of ethanol in the film canister compare to the metabolism of ethanol by yeast?
Will any concentration of ethanol gas in air form an explosive mixture, or are there upper and lower limits in its concentration? The alcohol used in most breath sprays is “specially denatured. ” What does it mean to denature the alcohol? What would a graph of pressure in the canister vs. time after spark ignition look like? What would a graph of temperature in the canister vs. time after spark ignition look like? Where does the energy from the combustion of ethanol end up? Is it true that when the kinetic energy of a canister launched straight up is at its maximum, the potential energy is near zero, and vice versa?
