Thermal Energy Answer Key

Thermal Energy- ANSWER KEY 1. A 10.0 g piece of copper wire, sitting in the sun reaches a temperature of 80.0˚C. How many Joules are released when the copper cools to 40.0˚C? The specific heat capacity of copper is 0.377 J/(g.˚C). First: write down your givens Mass (m) = 10 g Initial temperature (Ti) = 80˚C Final temperature (Tf) = 40 ˚C Specific heat capacity = 0.377 J/(g.˚C) The formula to find heat (Q) (or thermal energy) = m c Δ T Remember that Δ T = Tf - Ti Next: Plug your givens in your formula Q= (10) (0.377) (80- 40) Q= 150.8 Joules 2. The specific heat capacity of water is 4.184 J/(g.˚C). How much thermal energy is required to change the temperature of 700.0g of water from 25.6˚C to 75.4 ˚C? Givens: c = 4.184 J/(g.˚C) m = 700 g The formula for thermal energy is Q = (700) (4.184) (75.4-25.6) Q= 145, 854.24 Joules Tf= 75.4˚C Q= m c ΔT Ti = 25.6˚C 3. How much thermal energy is released when a 201g piece of blown glass at an initial temperature of 150˚C is cooled to 25˚C? The specific heat capacity of glass is 0.837 J/(g.˚C). Givens: c = 0.837 J/(g.˚C) m = 201 g The formula for thermal energy is Tf= 25˚C Ti = 150˚C Q= m c ΔT Q = (201) (0.837) (25-150) Q= - 21,029.625 Joules 4. If 2077 J are released to change the temperature of a block of ice initially at a temperature of -20˚C to 0˚C, find the mass of the ice. Ice has a specific heat capacity of 2.077 J/(g.˚C). Givens: c = 2.077 J/(g.˚C) m=?g Tf= 0˚C Ti = - 20˚C Q= -2077 J (Q is negative because energy is RELEASED) The formula for thermal energy is Q= m c ΔT -2077 = (m) (2.077) (0- (-20)) - 2077 = m (41.54) m= -2077/ 41.54 m= - 50 Since we are finding mass, answer should be a positive answer. m= 50 grams 5. Mercury has a specific heat capacity of 0.139 J/(g.˚C). How many Joules are required to change the temperature of a 50.0 g sample of Mercury from 20.7˚C to 100.4˚C. Givens: c = 0.139 J/(g.˚C) The formula for thermal energy is Q = (50) (0.139) (100.4 – 20.7) Q= 553.915 Joules m = 50 g Q= m c ΔT Tf= 100.4˚C Ti = 20.7˚C 6. A balloon is filled with 0.5 g of air, which has a specific heat capacity of 1.046 J/(g.˚C). How many Joules are required to change the temperature from 20˚C to 30˚C? Givens: c = 1.046 J/(g.˚C) m = 0.5 g The formula for thermal energy is Tf= 30˚C Ti = 20˚C Q= m c ΔT Q = (0.5) (1.046) (30-20) Q= 9.96 Joules 7. A 25g block of sugar requires 781 Joules to change the temperature from 25˚C to 50˚C. What is the specific heat capacity of sugar? Givens: c = ? J/(g.˚C) m = 25 g The formula for thermal energy is Tf= 50˚C Ti = 25˚C Q= 781 Joules Q= m c ΔT 781 = (25) (C) (50-25) 781= c (625) C= 781/625 C= 1.2496 J/(g.˚C) 8. When a 400.0 g f ammonia is cooled from a temperature of 25˚C to 10˚C, 13,054 Joules of thermal energy are released. Find the specific heat capacity of ammonia. Givens: c = ? J/(g.˚C) m = 400 g The formula for thermal energy is Tf= 10˚C Ti = 25˚C Q= 13,054 Joules Q= m c ΔT 13,054 = (400) (c) (10-25) 13,054 = c (400)(-15) 13,054 = c (6000) -------> c = 13,054/6000 = 2.176 J/(g.˚C)