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##### Chapter 14

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**The Behavior of Gases**Chapter 14**Section 1**Properties of Gases**Compressibility**• Compressibility – a measure of how much the volume of matter decreases under pressure. • Gases are easily compressed because of the space between the particles.**Factors Affecting Gas Pressure**• The amount of gas, volume, and temperature are factors that affect gas pressure. • Pressure (P) in kilopascals (kPa) • Volume (V) in liters (L) • Temperature (T) in Kelvin (K) • Number of moles (n) in mole (mol)**Amount of Gas**• You can use kinetic theory to predict and explain how gases will respond to a change of conditions. • As you add more gas particles the pressure increases.**Amount of Gas**• Once the pressure exceeds the strength of the container the container will burst.**Aerosol cans depend on the movement of gas from a region of**high pressure to a region of low pressure. • Pushing the spray button creates an opening between the inside of the can and the outside.**The gas flows through the opening to the lower pressure**region outside. • The movement of the gas propels the paint out of the can until the gas can no longer propel paint out.**Volume**• You can raise the pressure exerted by a contained gas by reducing its volume. • The more a gas is compressed the greater the pressure.**Temperature**• As a gas is heated, the temperature increases and the average kinetic energy also increases.**When the volume of a container is held constant and the**temperature increases and the pressure increases.**Section 2**The Gas Laws**Boyle’s Law: Pressure and Volume**• If the temperature is constant, as the pressure of a gas increases, the volume decreases.**Boyle’s law– states that for a given mass of gas at a**constant temperature, the volume of the gas varies inversely with pressure.**Charles’s Law: Temperature and Volume**• As the temperature of an enclosed gas increases, the volume increases, if the pressure is constant.**Charles’s law– states that the volume of a fixed mass of**gas is directly proportional to its Kelvin temperature if the pressure is kept constant.**How can you tell from the picture that there is a fixed**amount of gas in the cylinder? Describe what is happening in the cylinder as it’s being heated.**Gay-Lussac’s Law: Pressure and Temperature**• As the temperature of an enclosed gas increases, the pressure increases, if the volume is constant.**Gay-Lussac’s law– states that the pressure of a gas is**directly proportional to the Kelvin temperature if the volume remains constant.**P**V Boyle’s Law Charles’s Law Gay-Lussac’s Law T**The Combined Gas Law**• Combined gas law – describes the relationships among the pressure, temperature, and volume of an enclosed gas.**The combined gas law allows you to do calculations for**situations in which only the amount of gas is constant.**Boyle’s Law**P V T Charles’s Law Gay-Lussac’s Law**Section 3**Ideal Gases**Ideal Gas Law**• The combined gas law is good when the amount of gas does not change – this does not always stay constant though.**To calculate the number of moles of a contained gas requires**an expression that contains the variable n. • The number of moles is directly proportional to the number of particles and can be introduced into the combined gas law by dividing each side by n.**Ideal gas constant – (R) has the value of 8.31**(L•kPa)/(K•mol). • Ideal gas law – includes the variables of P, V, T, and n. • P is the pressure (units of kPa) • V is the volume (units of L) • T is the temperature (units of K) • n is the number of moles (units of mol)**Ideal Gases and Real Gases**• An ideal gas is one that follows the gas laws under all conditions of temperature and pressure. • Real gases differ most from an ideal gas at low temperatures and high pressures.**Section 4**Gases: Mixtures and Movements**Dalton’s Law**• Partial pressure – the contribution of each gas in a mixture makes to the total pressure. • In a mixture of gases, the total pressure is the sum of the partial pressures of the gases.**Dalton’s law of partial pressures – states that at**constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases.**Example:**• Determine the total pressure of a gas mixture that contains oxygen, nitrogen, and helium. The partial pressures are: PO2 = 20kpa, PN2 = 46.7kPa; and PHe = 26.7kPa.**Graham’s Law**• Diffusion – tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.**Bromine gas is put in a cylinder and after several hours you**can see how the gas has diffused.**Effusion – a gas escapes through a tiny hole in its**container. • Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.**Thomas Graham’s Contribution**• Scottish chemist Thomas Graham studied rates of effusion in the 1840’s • Relates to KE = ½ mv2. • Kinetic energy of the particles (KE) is related to the mass (m) and their velocity (v).**Graham’s law of effusion – states that the rate of**effusion of a gas is inversely proportional to the square root of the gas’s molar mass.**Example:**• Determine the rate of effusion for helium compared to nitrogen. • This result tells me that the helium effuses/diffuses faster than the nitrogen at the same temperature.