Work hard. Be nice. Name: Period: Date: UNIT 10: Energy Lesson 5: Calculating Heat using q = mcδt!

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1 Name: Period: Date: UNIT 10: Energy Lesson 5: Calculating Heat using q = mcδt! By the end of today, you will have an answer to: How can we calculate the amount of heat it takes to change the temperature? 1. What is the mass of water if it takes 300 J of heat to evaporate liquid water? Do you really understand?! Circle the regions of the heating curve in which we would use the equations q=mhv and q=mhf? How do you know this? Catalyzing thoughts The heating curve above depicts a sample of ice being heated constantly until it starts to vaporize. 1. How can you determine the amount of heat it takes to melt from B to C? 2. Why can t you use the formula for H fus or H vap to calculate the amount of heat absorbed between C and D?

2 Heat Equations: EQUATION When do we use it? Region(s) of the graph Read to Understand: The amount of heat needed to increase the temperature of 1 g of an object exactly 1 C is known as the specific heat capacity, or the specific heat. Different substances with the same mass may have different heat capacities. On a sunny day, a 20-g puddle of water may be cool, while a nearby 20-g iron sewer cover may be too hot to touch. This situation illustrates how different specific heat capacities affect the temperature of objects. Assuming that both the water and the iron absorb the same amount of radiant energy from the sun, the temperature of the water changes less than the temperature of the iron because the specific heat of water is larger. The amount of heat (q) absorbed or released in Joules during a temperature change is found in a formula on Table T. The specific heat of water is found on Table B. 1. In your own words describe specific heat. 2. Water is considered to have a high specific heat. How does this explain the situation where water heats up slower than iron? 3. What is the formula for heat absorbed or released during temperature change: Heat = units: Specific heat of water: units: Mass = units:

3 Example 1: How much heat is required to heat 20 grams of water at 25ºC to 30ºC? Example 2: How much heat is required to change the temperature of 10 g of ice by 10 C? Example 3: How much heat is required to boil 3 grams of water at 100 C? Try This On Your Own! What is the total amount of heat absorbed by grams of water when the temperature of the water is increased from 30.0 C to 45.0 C? CHALLENGE: If we take away 300 J of heat from a cup containing 30 grams of water at 70ºC, at what temperature will the water be in the end?

4 CW 10.5 q = mcδt 10 points Directions: Please answer the following questions. Decide which of the three heat formulas you will use in order to solve the problem before you begin. PLEASE SHOW YOUR WORK ON A SEPARATE SHEET OF PAPER! 4. When do we use the q= mc(t f -T i ) equation and when do we use the q=m H fus or q=m H vap? Please explain your answer in terms of heating curves and phase changes. 5. How much heat is required to heat 40 grams of water at 10ºC to 65ºC? 6. How much heat is lost when 80 grams of water goes from 90ºC to 45ºC? 7. The amount of energy needed to change a given mass of ice to water at constant temperature is called the heat of (1) Condensation (3) Crystallization (2) Fusion (4) Formation 8. The Specific Heat of Casebeckim is 85 J/g ºC. You have a 600 gram sample of Casebeckim at 400 K. You add 500 J to the sample. What is the final temperature of your Casebeckim? 9. How many joules of energy are released when 50 grams of water are cooled from 70ºC to 60ºC? 10. When 200 grams of water cools from 50ºC to 25ºC, what is the total amount of heat energy released by the water? 11. The heat of fusion of a compound is joules per gram. What is the total number of joules of heat that must be absorbed by a 15.0-gram sample to change the compound from solid to liquid at its melting point? 12. A substance is a solid at 15 C. A student heated a sample of the solid substance and recorded the temperature at one-minute intervals in the data table below. The heat of fusion for this substance is 122 joules per gram. How many joules of heat are needed to melt 7.50 grams of this substance at its melting point?

5 13. A student performed an experiment to determine the total amount of energy stored in a peanut. The accepted value for the energy content of a peanut is 30.2 kilojoules. The student measured grams of water into a metal can and placed the can on a ring stand, as shown in the diagram below. The peanut was attached to a wire suspended under the can. The initial temperature of the water was recorded as 22.0ºC. The peanut was ignited and allowed to burn. When the peanut finished burning, the final water temperature was recorded as 57.0ºC. The student s experimental value for the energy content of this peanut was 25.9 kilojoules per gram. a) Calculate the total amount of heat absorbed by the water. b) Calculate the student s percent error for the energy content of the peanut. 14. The temperature of a sample of a substance is increased from 20. C to 160. C as the sample absorbs heat at a constant rate of 15 kilojoules per minute at standard pressure. The graph below represents the relationship between temperature and time as the sample is heated. Determine the total amount of heat required to completely melt this sample at its melting point.

6 Exit Ticket 10.5 q = mcδt 3 points At a pressure of kilopascals and a temperature of 300 K, heat is added to a sample of water, causing the temperature to rise to 373 K, at which point it starts to vaporize. H 2O(l) + heat H 2O(g) 1. Draw a heating curve depicting this process. [1] 2. For this problem, which formula should one use to determine how much heat is absorbed while the temperature was rising? [1] 3. Determine the total amount of heat absorbed by grams of water vapor during this phase change. [1] Exit Ticket 10.5 q = mcδt 3 points At a pressure of kilopascals and a temperature of 300 K, heat is added to a sample of water, causing the temperature to rise to 373 K, at which point it starts to vaporize. H 2O(l) + heat H 2O(g) 1. Draw a heating curve depicting this process. [1] 2. For this problem, which formula should one use to determine how much heat is absorbed while the temperature was rising? [1] 3. Determine the total amount of heat absorbed by grams of water vapor during this phase change. [1]

7 QUIZ #1 10 points Base your answers to questions 1 through 3 on the information below. A 5.00-gram sample of liquid ammonia is originally at 210 K. The diagram of the partial heating curve below represents the vaporization of the sample of ammonia at standard pressure due to the addition of heat. The heat is not added at a constant rate 1. In the space below, calculate the total heat absorbed by the 5.00-gram sample of ammonia during time interval AB. Your response must include both a correct numerical setup and the calculated result. 2. Describe what is happening to both the potential energy and the average kinetic energy of the molecules in the ammonia sample during the time interval BC. Your response must include both potential energy and average kinetic energy. 3. Determine the total amount of heat required to vaporize this 5.00-gram sample of ammonia at its boiling point.