Klein Forest High School-Chemistry Jarrod G Collins, M.Ed. Thermochemistry day 2 Klein Forest High School-Chemistry Jarrod G Collins, M.Ed.
Warm up! Heat flows from warm to cold or cold to warm? An endothermic process is one where heat flows into or out of a system? Is ice melting an endothermic or exothermic process?
Quantifying Heat Energy The heat capacity of an object is proportional to the following: Its mass The specific heat of the material So we can calculate the quantity of heat absorbed by an object if we know the mass, the specific heat, and the temperature change of the object.
q = m Cp T = m Cp (Tfinal – Tinitial ) Example How much energy is needed to heat 80 grams of water from 10oC to 55oC at constant pressure? Specific heat of water (CH20 ) = 4.184 J/goC q = m Cp T = m Cp (Tfinal – Tinitial )
Example How much energy is needed to heat 80 grams of water from 10oC to 55oC at constant pressure? Specific heat of water (CH20 ) = 4.184 J/goC q = m Cp T = m Cp (Tfinal – Tinitial ) q = (80grams) ( 4.184 J/goC) (55oC – 10oC) q = + 15062 joules divide by 1000 to get kilojoules 15062 J 1 kJ = q =15.06 kJ absorbed 1000J
Calculating Heat Practice problem 5 g of copper was heated from 20C to 80C. How much energy was used to heat the Cu? Specific heat of Cu (CCu ) = 0.38 J/(gC)
Calculating Specific Heat Capacity What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.1 oC when 207 J of heat is added?
Heat Capacity and Specific Heat Specific heat, quantity of heat transferred, mass of substance, and temperature change: Specific heat = heat transferred mass temperature change Cs = q m T
Calculating Specific Heat Capacity What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.1 oC when 207 J of heat is added? Cs = _____207 J _______ (28.4 g x 8.1 ºC) Cs = 0.8998 J/gºC Cs = q m T
Practice on your own!! DIRECTIONS: Use q = (m)(ΔT)(Cp) to solve the following problems. Show all work and units. How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22°C to 55°C, if the specific heat of aluminum is 0.90 J/g°C? 2. The specific heat of ethanol is 2.46 J/g oC. Find the heat required to raise the temperature of 193 g of ethanol from 19oC to 35oC.
Practice on your own!! 3. How much heat (in J) is given out when 85.0 g of lead cools from 200.0 ºC to 10.0 ºC? (Cp of Pb = 0.129 J/g ºC) 4. How many joules of heat are needed to change 50.0 g of ice at -15.0ºC to steam @120.0 ºC? (Cp of H2O = 4.184 J/g ºC) 5. A 15.75-g piece of iron absorbs 1086.75 joules of heat energy, and its temperature changes from 25ºC to 175ºC. Calculate the specific heat capacity of iron. 6. If it takes 41.72 joules to heat a piece of gold weighing 18.69 g from 10.0 ºC to 27.0 ºC, what is the specific heat of the gold? 7. A certain mass of water was heated with 41,840 Joules, raising its temperature from 22.0 ºC to 28.5 ºC. Find the mass of the water, in grams. (Cp of H2O = 4.184 J/g ºC) 8. Calculate the number of joules given off when 32.0 grams of steam cools from 110.0 ºC to ice at -40.0 ºC. (Cp of H2O = 4.184 J/g ºC) 9. The specific heat of ethanol is 2.46 J/g oC. Find the heat required to raise the temperature of 193 g of ethanol from 19oC to 35oC. 10. When a 120 g sample of aluminum (Al) absorbs 9612 J of energy, its temperature increases from 25oC to 115oC. Find the specific heat of aluminum.