Chemistry 30 notes review of specific heat capacity Thermodynamics the movement of thermal energy o Open system: both matter and energy are allowed to enter or leave the system. o Closed system: energy can enter or leave the system but matter cannot. o Isolated system: neither matter nor energy can to enter or leave the system. There are two types of energy: o Kinetic energy (E k ): energy of motion (of particles or thermal energy). This is the energy type involved in temperature changes. o Potential energy (E p ): energy that is stored (in chemical bonds). This is the energy type involved in a phase change (e.g. solid to liquid). It does not involve a temperature change. Formula to determine the amount of energy gained or released by a substance due to a temperature change: mcdt Where heat transferred (J) m mass of substance (g) c specific heat capacity of substance (J/g o C) Dt temperature change ( o C) Note: Dt t f - t i A negative value indicates an exothermic process (energy released) o In an exothermic process, the total energy used to break bonds in reactants is less than the total energy released when bonds form in the products. A positive value indicates an endothermic process (energy absorbed) o In an endothermic process, the total energy used to break bonds in reactants is greater than the total energy released when bonds formed in the products. Chemistry 30 Lesson 1-01 Page 1
Example 1 Determine the amount of energy required to heat 150 g of water from 12.5 o C to 25.2 o C. Example 2 Determine the energy required to heat 300 g of H 2 O (l) from 10.0 o C to 35.0 o C in an aluminium pot with a mass of 0.750 kg. Note: energy requirements are additive, you are heating the water and the pot. Chemistry 30 Lesson 1-01 Page 2
Chemistry 30 Specific Heat Capacity Problems. 1. How much energy is needed to heat enough water to make a cup of tea (250 ml), if the water is initially at 20.0 ºC and you want to increase the temperature to 85.0 ºC? (Assume that 1.00 ml of water has a mass of 1.00 g.) 2. As the tea in uestion 1 steeps, it cools from 85.0 ºC to 75.0 ºC. How much energy is lost by the tea as it steeps? 3. If 100.0 kj of energy is used to heat 500.0 g of water, what is the temperature change of the water? Chemistry 30 Lesson 1-01 Page 3
4. A 1.00 kg block of ice (c 2.00 ), at -25.0 ºC, is warmed by 35 kj of energy. What is the final temperature of the ice? 5. Imagine that you have a 500 g iron pot (c 0.440 J and a 500 g aluminium pot (c 0.897 ), a 500 g copper pot (c 0.385 J ), J ). You fill each pot with 250 ml of water and heat the water to 100 ºC on a stove. Which pot will keep the water warm the longest? Explain your answer. Pot: Explanation: 6. A 5.0 g sample of an unidentified metal absorbs 71 J of energy as its temperature increases from 125 ºC to 162 ºC. What metal is the sample? (Hint: See the data in uestion 5.) Chemistry 30 Lesson 1-01 Page 4
7. What mass of seawater ( 3.89 ºC? ) is needed to provide 300.0 MJ as it cools from 75 ºC to 33 8. How much water can be heated from its melting to its boiling point by adding 2.75 10 6 J of energy? 9. If 100.0 g of a substance releases 45 kj of energy as it cools from 13.0 ºC to 15.0 ºC, what is the specific heat capacity of the substance? Chemistry 30 Lesson 1-01 Page 5
Chemistry 30 Specific Heat Capacity Problems Answer Key 1. Mass of water, m 250 ml 250 1.00 g ml 1.00 ml 250 g Specific heat capacity of water, c 4.19 J Final temperature, T 2 85.0 C Initial temperature, T 1 20.0 C Change of temperature, DT 65.0 C ö (250 g ) ç 4.19 J (65.0 C) 68 087.5 J @ 6.81 10 4 J It would take 6.81 10 4 J of energy to heat 250 ml of water from 20.0 ºC to 85.0 ºC. 2. Mass of water, m 250 ml 250 1.00 g ml 1.00 ml 250 g Specific heat capacity of water, c 4.19 J Final temperature, T 2 75.0 C Initial temperature, T 1 85.0 C Change of temperature, DT 10.0 C ö (250g ) ç 4.19 J ( 10 C ) 10 475 J @ 1.05 10 4 J The 250 ml of tea loses 1.05 10 4 J as it cools from 85.0 ºC to 75.0 ºC. 3. 100.0 kj 1.000 10 5 J m 500.0 g c 4.19 J D T mc (500.0 g 5 1.000 10 J ) 4.19 ç 47.7327 C 47.7 C 4. 35 kj 3.5 10 4 J m 1.00 kg 1.00 10 3 g c 2.00 J T 1 25.0 C 7.5 C DT /mc (1.00 10 g 4 3.5 10 J 3 ) 2.00 ç g C g 17.5 C DT T 2 T 1 T 2 T 1 + DT 25.0 C + 17.5 C 5. The aluminium pot will keep the water warm the longest. It will store more energy while heating to 100 C because it has the highest specific heat capacity.
6. m 5.0 g 71 J T 2 162 C T 1 125 C DT T 2 T 1 DT 162 C 125 C 37 C c m D T 71 J (5.0g) (37 C) J 0.38378 g g C 0.38 J @ g g C The specific heat capacity of the metal is similar to the specific heat capacity of copper, so the metal is most likely copper. 7. 300.0 MJ 3.000 10 8 J c 3.89 J T 2 33 C T 1 75 C DT 33 C 75 C 42 C m cdt 8 3.000 10 J 3.89 ( 42 C ) 1836210.062 g 1.8 10 6 g 8. 2.75 10 6 J c 4.19 J T 2 100.00 C T 1 0.00 C DT 100.00 C m cd T (100.00 C) 6 2.75 10 J 4.19 6563.25 g 6.56 kg 3 6.56 10 g 9. m 100.0 g 45 kj 4.5 10 4 J T 1 13.0 C T 2 15.0 C DT 28.0 C c m D T 4 4.5 10 J ( 100.0 g )( 28.0 C) 16.071 J @ 16 J