Lesson 3

 In SI Metric the temperature scale is defined as Kelvin temperature scale.  The degree unit is the Kelvin (K). The symbol for the unit is K, not o K.  Kelvin temperatures must be used in many gas law equations in which temperature enters directly into the calculations.

 The Celsius and Kelvin scale are related unit for unit. One degree unit on the Celsius scale is equivalent to one degree unit on the Kelvin scale. The only difference between these two scales is the zero point.  The zero point on the Celsius scale was defined as the freezing point of water, which means that there are higher and lower temperatures around it.

 The zero point on the Kelvin scale - called absolute zero – it corresponds to the lowest temperature that is possible. It is units lower than the zero point on the Celsius scale.

 So this means that 0 K equals o C and 0 o C equals K. Thermometers are never marked in the Kelvin scale.  If we need degrees in Kelvin the following relationships are to be used. T K = t c or t c = T K

 Cold Saunas are available a high end resorts. Instead of heating the sauna and causing the people inside to sweat, a cold sauna is cooled down to -110 ° C. What would the temperature be in Kelvin?  T K = t c  T K =  T K = K

 Liquid nitrogen (sometimes abbreviated LOX) is used in liquid-fuel rockets. Its boiling point is -183 o C. What is this temperature in Kelvin’s?  A substance is heated from 300 K to 315 K. What is the change in temperature expressed in °C.

 Perform the following conversions a)24°C to Ka)50 K to °C a)30 °C to Ka) K to °C a) °C to Ka) K to °C.

 A clinical thermometer registers a patient's temperature to be o C. What is this in Kelvin’s?  The coldest permanently inhabited place on earth is the Siberian village of Oymyakon in Russia. In 1964 the temperature reached a shivering o C. What is this temperature in Kelvin’s?

 Helium has the lowest boiling point of any liquid. It boils at 4 K. What is this in o C?

 The direct relationship between the volume of a gas and the temperature of the gas (on the Kelvin temperature scale) is known as Charles Law. According to this law,  as the temperature of a gas increases, the volume increases proportionally, provided that the pressure and the amount of gas remains the same.

 However, as the graph above shows, the volume extrapolates to zero at a temperature of o C. If this temperature were taken as the zero of a temperature scale then all negative temperatures could be eliminated.

 Such a temperature scale is now the fundamental scale of temperature in the SI. It is called the absolute scale, the thermodynamic scale, and the Kelvin scale. Temperature on the Kelvin scale, and only on the Kelvin scale, is symbolized by T.

Charles’s Law can be written as V 1 T 2 = V 2 T 1

 The volume of a sample of gas is 23.2 cm 3 at 20 o C. If the gas is ideal and the pressure remains unchanged what is its volume at 80 o C?

G V 1 = 23.2 cm 3 T 1 = 20 o C = K T 2 = 80 o C = K S V 2 = (23.2 cm 3 x K) K = cm 3 RV 2 = ?P Therefore, when the temperature increased by 60 K it increased the volume from 23.2 cm 3 to cm 3 AV 1 T 2 = V 2 T 1

 Compressed oxygen is widely used in hospitals and retirement homes. To make it easier to transport, the oxygen is cooled. How many degrees Celsius would the gas be if 200 L’s at 23 degrees Celsius is compressed to 40 L?

G V 1 = 200 L T 1 = 23 o C = K V 2 = 40 L S T 2 = (40 L x K ) 200 L = K = o C RT 2 = ?P Therefore, when the gas is compressed for transport it is cooled to o C. AV 1 T 2 = V 2 T 1 T 2 = V 2 T 1 / V 1

 Questions:  page 432 #  page 434 #  Temperature and pressure law next

 When a gas is heated the pressure also increases, if we assume the volume and the amount of gas remains the same, the quotient of the two variables (p/T) has a constant value (k).  Because it remains the same it allows us to compare different sets of temperatures and pressures.

 This is known as the pressure and temperature law or the Gay-Lussac’s Law.

 An aerosol can of paint contains a pressure of 138 kPa at 20 °C. The can is left in a parked car and is heated to 50 ° C. What it the final pressure in the can?

G P 1 = 138 kPa T 1 = 20 o C = K T 2 = 50 o C = K S = 152 kPa R P 2 = ?P Therefore, when the paint can is heated an additional 30 ° C the pressure increases to 152 kPa. A

 page 432 #  page 434 #  Page 435 # 22-25