Q=mcDt. Chemistry 30 notes review of specific heat capacity. Thermodynamics the movement of thermal energy

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1 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

2 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 kg. Note: energy requirements are additive, you are heating the water and the pot. Chemistry 30 Lesson 1-01 Page 2

3 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 kj of energy is used to heat g of water, what is the temperature change of the water? Chemistry 30 Lesson 1-01 Page 3

4 4. A 1.00 kg block of ice (c 2.00 ), at 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 J and a 500 g aluminium pot (c ), a 500 g copper pot (c 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

5 7. What mass of seawater ( 3.89 ºC? ) is needed to provide MJ as it cools from 75 ºC to How much water can be heated from its melting to its boiling point by adding J of energy? 9. If 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

6 Chemistry 30 Specific Heat Capacity Problems Answer Key 1. Mass of water, m 250 ml g ml 1.00 ml 250 g Specific heat capacity of water, c 4.19 J Final temperature, T C Initial temperature, T C Change of temperature, DT 65.0 C ö (250 g ) ç 4.19 J (65.0 C) J It would take J of energy to heat 250 ml of water from 20.0 ºC to 85.0 ºC. 2. Mass of water, m 250 ml g ml 1.00 ml 250 g Specific heat capacity of water, c 4.19 J Final temperature, T C Initial temperature, T C Change of temperature, DT 10.0 C ö (250g ) ç 4.19 J ( 10 C ) J The 250 ml of tea loses J as it cools from 85.0 ºC to 75.0 ºC kj J m g c 4.19 J D T mc (500.0 g J ) 4.19 ç C 47.7 C kj J m 1.00 kg g c 2.00 J T C 7.5 C DT /mc ( g J 3 ) 2.00 ç g C g 17.5 C DT T 2 T 1 T 2 T 1 + DT 25.0 C 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.

7 6. m 5.0 g 71 J T C T C DT T 2 T 1 DT 162 C 125 C 37 C c m D T 71 J (5.0g) (37 C) J g g C 0.38 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 MJ J c 3.89 J T 2 33 C T 1 75 C DT 33 C 75 C 42 C m cdt J 3.89 ( 42 C ) g g J c 4.19 J T C T C DT C m cd T ( C) J g 6.56 kg g 9. m g 45 kj J T C T C DT 28.0 C c m D T J ( g )( 28.0 C) J