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ChemCatalyst. Which will melt more of a snowman, a cup of water at 90 °C or a large barrel of water at 20 °C? Explain your thinking. Key Question. You will be able to:. What is the difference between temperature and heat?. define a calorie
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ChemCatalyst Which will melt more of a snowman, a cup of water at 90 °C or a large barrel of water at 20 °C? Explain your thinking.
Key Question You will be able to: What is the difference between temperature and heat? define a calorie complete simple calculations involving heat transfer for water samples describe basic differences between thermal energy, heat, and temperature
Prepare for the Lab Watch simulation: Mixing two ideal gases In your groups: Measure what happens when you mix two water samples of different temperatures. Design experiments to determine the effects of mixing water of different masses and different temperatures.
Discussion Notes The final temperature of a mixture depends on both the mass and the temperature (average kinetic energy) of the samples. Heat and temperature are different concepts. Thermal energy refers to the amount of energy in a sample. Thermal energy: The total kinetic energy associated with the mass and motions of the particles in a sample of matter.
Discussion Notes (cont.) The calorie is a unit of energy used to measure heat transfer and to express thermal energy. q = m(1 cal/g °C)∆T heat transferred = (mass)(1 cal/g °C)(change in temperature) The value 1 cal/g °C applies only to water. Calorie: The amount of energy it takes to raise the temperature of one gram of water by one Celsius degree.
Sample calculation - Finding heat energy in Calories Suppose you need to heat some water: How many calories of energy does it take to heat a 75mL sample of water for cocoa from 25℃ to 38℃? • Convert volume to mass in grams (remember, DH2O = 1g/1mL) 75mL H2O = 75 g H2O • Determine the change in temperature (ΔT = Tfinal - Tinitial) 38℃ - 25℃ = 13℃ • Use the equation for heat transfer to solve for q • q = m(1 cal/g℃)ΔT • q = 75g (1 cal/g℃) 13℃ 975 calories are needed to raise water temperature Your Turn: How many calories of energy does it take to heat a 2L sample of water for a footbath from 25℃ to 38℃?
Example Calculation - Conservation of Energy You mix 300 mL of water at 60℃ with 100 mL of water at 20℃ Find the final temperature of the mixture • The final temperature will be a weighted average of the two initial temperatures. (Meaning, each temperature is “weighted” by multiplying it by the mass of each sample that is mixed). 300g *(60℃) + 100g *(20℃) = 50℃ 400g Show that the heat transferred from the hot water is equal to the heat transferred to the cold water • The calories of energy transferred is given by the equation: q = m(1 cal/g℃)ΔT Hot water: q = (300g)(1 cal/g℃)(50℃- 60℃) = -3000 cal Cold water: q = (100g)(1 cal/g℃)(50℃- 20℃) = 3000 cal The negative sign means energy was transferred from the hot water (lost). The positive sign means energy was transferred to the cold water (gained)
Wrap Up What is the difference between temperature and heat? • Temperature is a measure of the average kinetic energy of the atoms and molecules in a sample. • The thermal energy is a measure of the total kinetic energy of all the atoms and molecules in a sample and is dependent on both the mass of the sample and its temperature. • Thermal energy can only be measured as heat transfer between two samples of known temperature and mass. • Heat transfer, q, is measured in calories. The amount of energy required to raise 1 g of water by 1 ° C is 1 calorie. For water, q = m(1 cal/g °C)∆T.
Check-In You have one beaker containing 500 g of water at 75 °C. You also have a beaker with 2000 g of water at the same temperature. Which will melt more ice? Explain your answer in terms of thermal energy, heat transfer, and temperature.
Homework questions How many calories of energy do you need to transfer for each of the following changes? • Raise the temperature of 1 g of water by 5℃ • Raise the temperature of 2 g of water by 5℃ • Raise the temperature of 9 g of water by 35℃ Which will warm a child’s inflatable pool more: adding 500g of water at 50℃ or 100g at 95℃? The temperature of the water in the pool is 20℃. Explain your reasoning. Suppose you mix two water samples: 300g of water at 20℃ and 200g water at 50℃. What do you expect the final temperature to be? Complete the following table:
