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Gas Laws Law of Combining Gas Volumes The volume of gases taking part in a chemical reaction show simple whole number ratios to one another when those volumes are measured at the same temperature and pressure. Examples The molar ratio of the following reaction to produce ammonia gas is 1:3:2. N 2 (g) + 3H 2 (g) -----> 2NH 3 (g) Since all the reactants and products are gases, the mole ratio is the same as the ratio of the volumes of gases. So, 10mL of nitrogen gas reacts with 10 x 3 = 30mL of hydrogen gas to produce 10 x 2 = 20mL ammonia gas The molar and volume ratios of the following reaction are 2:1:2 since the reactants and products are gases. 2H 2 (g) + O 2 (g) -----> 2H 2 O(g) So, if there is 50mL of hydrogen gas, what are the volumes of oxygen gas and water vapour? 50mL of hydrogen gas would react with 50 x ½ = 25mL oxygen gas to produce 50mL of water vapour

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Gas Laws – Boyles Law At constant temperature: Volume of a given quantity of gas is inversely proportional to pressure: V= 1 / P (E.g. if the volume of a gas is doubled, its pressure is halved.) OR The product of a gass volume and its pressure is a constant : PV = constant, PV = k So, at constant temperature for a given quantity of gas : P i V i = P f V f where, P i and V i are the initial pressure and volume, P f and V f are the final pressure and volume. Note: pressures and volumes must be in the same units of measurement. Ideal vs. Real gases All gases approximate Boyle's Law at high temperatures and low pressures. Ideal Gas - a hypothetical gas which obeys Boyle's Law at all temperatures and pressures Real Gas - approaches Boyle's Law behaviour as the temperature is raised or the pressure lowered.

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Gas Laws – Charles Law At constant pressure, Volume of a given quantity of gas is directly proportional to the absolute temperature : V= T (in Kelvin) (E.g. if the temperature (K) is doubled, the volume of gas is also doubled.) OR The ratio of its volume and the absolute temperature is a constant : V / T = constant, V / T = k So, at constant pressure: V i / Ti = V f / Tf where, T i and V i are the initial temperature and volume, T f and V f are the final temperature and volume. Note: T i and T f must be in Kelvin NOT Celsius. (temperature in Kelvin = temperature in Celsius + 273) (approximately) Ideal vs. Real gases All gases approximate Charles' Law at high temperatures and low pressures. Well above its condensation point, the volume of a real gas decreases linearly as it is cooled at constant pressure. However, as the gas approaches the condensation point, the decrease in volume slows down. At condensation, the gas turns to a liquid and, therefore, does not obey Charles Law Absolute zero (OK) is the temperature where the volume of a gas would theoretically be zero if it did not condense.

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Gas Laws – Ideal Gas Law An Ideal Gas (perfect gas) is one which obeys Boyle's Law and Charles' Law exactly. An Ideal Gas obeys the Ideal Gas Law (General gas equation): PV = nRT where, P=pressure, V=volume, n=moles of gas, T=temperature, R=the gas constant (dependent on the units of pressure, temperature and volume) R = 8.314 J K -1 mol -1 P is in (kPa), V is in (L), T is in (K) R = 0.0821 L atm K -1 mol -1 P (atm), V (L), T (K)

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Gas Laws – Ideal Gas Law Presumptions of an Ideal Gas according to Kinetic Theory of Gases: Gases consist of molecules which are in continuous random motion The volume of the molecules present is negligible relative to the total volume occupied by the gas Intermolecular forces are negligible Pressure is due to the gas molecules colliding with the walls of the container Real Gases deviate from Ideal Gas Behaviour because at low temperatures the gas molecules have less kinetic energy (move around less) so they do attract each other at high pressures the gas molecules are forced closer together so that the volume of the gas molecules becomes significant compared to the volume the gas occupies The Overall Presumption Under ordinary conditions, deviations from Ideal Gas behaviour are so slight that they can be neglected. A gas which deviates from Ideal Gas behaviour is called a non-ideal gas.

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