Four variables are used to describe the condition of a gas. They are pressure , volume , temperature , and the amount of the gas as measured by the number moles. We will examine separately how the volume, temperature, and amount of gas each affect the pressure of an enclosed gas sample.
The Figure below shows what happens when air is added to a rigid container. A rigid container is one that is incapable of expanding or contracting. A steel canister is an example of a rigid container.
Increase in pressure with increase in number of gas particles. The canister on the left contains a gas at a certain pressure. The attached air pump is then used to double the amount of gas in the canister. Since the canister cannot expand, the increased number of air molecules will strike the inside walls of the canister twice as frequently as they did before.
The result is that the pressure inside the canister doubles. As you might imagine, if more and more air is continually added to a rigid container, it may eventually burst. Reducing the number of molecules in a rigid container has the opposite effect and the pressure decreases.
Pressure is also affected by the volume of the container. If the volume of a container is decreased, the gas molecules have less space in which to move around. As a result, they will strike the walls of the container more often and the pressure increases. Figure below shows a cylinder of gas whose volume is controlled by an adjustable piston.
On the left, the piston is pulled mostly out and the gauge reads a certain pressure. On the right, the piston has been pushed so that the volume of the enclosed portion of the container where the gas is located has been cut in half. The pressure of the gas doubles.
Increasing the volume of the container would have the opposite effect and the pressure of the gas would decrease. Decrease in gas volume produced increase in gas pressure. It would be very unadvisable to place a can of soup over a campfire without venting the can. As the can heats up, it may explode. The kinetic-molecular theory explains why.
The air inside the rigid can of soup is given more kinetic energy by the heat coming from the campfire. The kinetic energy causes the air molecules to move faster and they impact the container walls more frequently and with more force.
The increase in pressure inside may eventually exceed the strength of the can and it will explode. An additional factor is that the soup may begin boiling which will then aid even more gas and more pressure to the inside of the can. Shown in the Figure below is a cylinder of gas on the left that is at room temperature K. On the right, the cylinder has been heated until the Kelvin temperature has doubled to K. The kinetic energy of the gas molecules increases, so collisions with the walls of the container are now more forceful than they were before.
As a result, the pressure of the gas doubles. Decreasing the temperature would have the opposite effect, and the pressure of an enclosed gas would decrease. Increase in temperature produces increase in pressure.
Watch the video at the link below and answer the following questions:. How important is it to check the weather? Each day, hundreds of weather balloons are launched. Made of a synthetic rubber and carrying a box of instruments, the helium-filled balloon rises up into the sky. As it gains altitude, the atmospheric pressure becomes less and the balloon expands. At some point the balloon bursts due to the expansion, the instruments drop aided by a parachute to be retrieved and studied for information about the weather.
Robert Boyle , an English chemist, is widely considered to be one of the founders of the modern experimental science of chemistry. He discovered that doubling the pressure of an enclosed sample of gas while keeping its temperature constant caused the volume of the gas to be reduced by half.
An inverse relationship is described in this way. As one variable increases in value, the other variable decreases. Physically, what is happening? The gas molecules are moving and are a certain distance apart from one another.
An increase in pressure pushes the molecules closer together, reducing the volume. If the pressure is decreased, the gases are free to move about in a larger volume. The is a constant for a given sample of gas and depends only on the mass of the gas and the temperature. The Table below shows pressure and volume data for a set amount of gas at a constant temperature. The third column represents the value of the constant for this data and is always equal to the pressure multiplied by the volume.
As one of the variables changes, the other changes in such a way that the product of always remains the same. Volume is plotted on the -axis, with the corresponding pressure on the -axis.
We use and to stand for the initial pressure and initial volume of a gas. After a change has been made, and stand for the final pressure and volume. This equation can be used to calculate any one of the four quantities if the other three are known. A sample of oxygen gas has a volume of mL when the pressure is equal to kPa. The gas is allowed to expand into a 1.
Calculate the new pressure of the gas. Step 1: List the known quantities and plan the problem. It is important that the two volumes and are expressed in the same units, so has been converted to mL.
First, rearrange the equation algebraically to solve for. Now substitute the known quantities into the equation and solve. Step 3: Think about your result. The volume has increased to slightly over 4 times its original value and so the pressure is decreased by about. The pressure is in kPa and the value has three significant figures.
Note that any pressure or volume units can be used as long as they are consistent throughout the problem. How do you bake bread? Everybody enjoys the smell and taste of freshly-baked bread. It is light and fluffy as a result of the action of yeast on sugar.
The yeast converts the sugar to carbon dioxide, which at high temperatures causes the dough to expand. The end-result is an enjoyable treat, especially when covered with melted butter. As a container of confined gas is heated, its molecules increase in kinetic energy and push the movable piston outward, resulting in an increase in volume.
French physicist Jacques Charles studied the effect of temperature on the volume of a gas at constant pressure. The absolute temperature is temperature measured with the Kelvin scale.
The Kelvin scale must be used because zero on the Kelvin scale corresponds to a complete stoppage of molecular motion. The Table below shows temperature and volume data for a set amount of gas at a constant pressure. The third column is the constant for this particular data set and is always equal to the volume divided by the Kelvin temperature. When this data is graphed, the result is a straight line, indicative of a direct relationship, shown in Figure below. The volume of a gas increases as the Kelvin temperature increases.
Notice that the line goes exactly toward the origin, meaning that as the absolute temperature of the gas approaches zero, its volume approaches zero. However, when a gas is brought to extremely cold temperatures, its molecules would eventually condense into the liquid state before reaching absolute zero.
The temperature at which this change into the liquid state occurs varies for different gases. Now we use and to stand for the initial volume and temperature of a gas, while and stand for the final volume and temperature. The direct relationship will only hold if the temperatures are expressed in Kelvin.
Temperatures in Celsius will not work. A balloon is filled to a volume of 2. Find the new volume of the balloon. The temperatures have first been converted to Kelvin. The volume increases as the temperature increases. The result has three significant figures. Perform the calculations at the web site below:. How much propane is in the tank? Propane tanks are widely used with barbeque grills.
You can buy gauges that measure the pressure inside the tank to see how much is left. The gauge measures pressure and will register a higher pressure on a hot day than it will on a cold day.
So you need to take the air temperature into account when you decide whether or not to refill the tank before your next cook-out. When the temperature of a sample of gas in a rigid container is increased, the pressure of the gas increases as well.
The increase in kinetic energy results in the molecules of gas striking the walls of the container with more force, resulting in a greater pressure. The French chemist Joseph Gay-Lussac discovered the relationship between the pressure of a gas and its absolute temperature. A graph of pressure vs. As a gas is cooled at constant volume its pressure continually decreases until the gas condenses to a liquid.
The gas in an aerosol can is under a pressure of 3. It is dangerous to dispose of an aerosol can by incineration. The pressure increases dramatically due to large increase in temperature. Work on the problems found at the web site below:. What keeps things cold? The modern refrigerator takes advantage of the gas laws to remove heat from a system. Compressed gas in the coils see above is allowed to expand.
This expansion lowers the temperature of the gas and transfers heat energy from the material in the refrigerator to the gas. As the gas is pumped through the coils, the pressure on the gas compresses it and raises the gas temperature. This heat is then dissipated through the coils into the outside air. As the compressed gas is pumped through the system again, the process repeats itself. To this point, we have examined the relationships between any two of the variables of , , and , while the third variable is held constant.
However, situations arise where all three variables change. The combined gas law expresses the relationship between the pressure, volume, and absolute temperature of a fixed amount of gas. For a combined gas law problem, only the amount of gas is held constant. What will be the new gas volume? Use the combined gas law to solve for the unknown volume. STP is K and 1 atm. The temperatures have been converted to Kelvin. Both the increase in pressure and the decrease in temperature cause the volume of the gas sample to decrease.
Since both changes are relatively small, the volume does not decrease dramatically. It may seem challenging to remember all the different gas laws introduced so far. For example, consider a situation where a change occurs in the volume and pressure of a gas while the temperature is being held constant. In that case, it can be said that. Look at the combined gas law and cancel the variable out from both sides of the equation.
Work on the problems at the link below:. How much air do you put into a tire? A flat tire is not very useful.
It does not cushion the rim of the wheel and creates a very uncomfortable ride. When air is added to the tire, the pressure increases as more molecules of gas are forced into the rigid tire. How much air should be put into a tire depends on the pressure rating for that tire. Too little pressure and the tire will not hold its shape. Standard pressure is one atmosphere atm — the pressure exerted by the atmosphere on Earth at sea level. Temperature, pressure, amount and volume of a gas are interdependent, and many scientists have developed laws to describe the relationships among them.
Boyle's law is named after Robert Boyle, who first stated it in Boyle's law states that if temperature is held constant, volume and pressure have an inverse relationship; that is, as volume increases, pressure decreases, according to the University of California, Davis' ChemWiki. Increasing the amount of space available will allow the gas particles to spread farther apart, but this reduces the number of particles available to collide with the container, so pressure decreases. Decreasing the volume of the container forces the particles to collide more often, so pressure is increased.
A good example of this is when you fill a tire with air. As more air goes in, the gas molecules get packed together, reducing their volume.
As long as the temperature stays the same, the pressure increases. In , Joseph Louis Gay-Lussac, a French chemist and physicist referenced data gathered by his countryman, Jacque Charles, in a paper describing the direct relationship between the temperature and volume of a gas kept at a constant pressure.
This law states that the volume and temperature of a gas have a direct relationship: As temperature increases, volume increases, when pressure is held constant. Heating a gas increases the kinetic energy of the particles, causing the gas to expand.
In order to keep the pressure constant, the volume of the container must be increased when a gas is heated. The table shows some of the properties of gases and why they are like this: Property Reason They flow and completely fill their container The particles can move quickly in all directions They can be compressed squashed The particles are far apart and have space to move into Gas pressure The particles in a gas move quickly in all directions, but they do not get far before they bump into each other or the walls of their container.
Particles in a gas colliding with their container wall. They flow and completely fill their container. The particles can move quickly in all directions. They can be compressed squashed. The particles are far apart and have space to move into.
0コメント