25.3 Heat and Thermal Energy

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Presentation transcript:

25.3 Heat and Thermal Energy Specific Heat Capacity Table 25.3 Heat and Thermal Energy Imagine heating a cup of coffee to a temperature of 100°C. Next think about heating up 1,000 cups of coffee to 100°C. The final temperature is the same in both cases but the amount of energy needed is very different. It takes more energy to heat up 1,000 cups than to heat up a single cup. The amount of thermal energy depends on the temperature and also on the amount of matter you have.

Does it take the same amount of energy to heat 1 cup of water as compared to 1 gallon of water No…more energy to heat 1 gallon…..MORE MASS

Do all materials change temperature at the same rate? No some heat faster than others. The SLOWER something heats, the HIGHER Heat Capacity. WATER has a very high HEAT CAPACITY. Takes a lot of energy to change the temperature. METALS have a LOW specific heat…. Doesn’t take a lot of energy to change the temperature. WHERE IS IT BENEFICIAL FOR WATER TO HAVE A HIGH SPECIFIC HEAT?

25.3 Specific Heat The temperature of gold rises quickly compared with water because its specific heat is much less than the specific heat of water.

25.3 Specific Heat The specific heat is the quantity of heat it takes to raise the temperature of one kilogram of material by one degree Celsius.

Which is going to take more energy… Which is going to take more energy….change the temperature 5oC or 100oC? Takes more energy to change it to 100oC. GREATER the temperature change, the more temperature needed.

25.2 Calculate Heat One kilogram of water is heated in a microwave oven that delivers 500 watts of heat to the water. One watt is a flow of energy of one joule per second. If the water starts at 10°C, how much time does it take to heat up to 100°C? 1) You are asked for the time (t) to reach a given temperature (T2). 2) You are given the mass (m) of water, power (P), and initial temperature (T1). The specific heat of water is 4,184 j/kgoC. 3) The heat equation, E = mcp(T2-T1), gives the heat required. Power is energy over time: P = E/t. 4) First, calculate the heat required: E = (1 kg) (4,184 J/kgoC) (100oC - 10oC) = 376,560 joules. Next, recall that 500 watts is 500 joules per second. At 500 J/sec, it takes 376,560÷500 = 753 seconds, or about 12.6 minutes. NOTE: Specific heat often changes with temperature and pressure.

Specific heat (J/kgoC) 25.2 Heat Equation Specific heat (J/kgoC) Heat energy (J) Q = mc(T2-T1) Change in Temperature (oC) Mass (kg)

How much energy is needed to raise 50 grams of water from 25oC to 75oC?

Specific Heat / Heat Capacity

STEPS OF EQUATIONS Determine whether Q is going IN or OUT Determine if ENDOTHERMIC or EXOTHERMIC…which graph? Determine location of energy change (ie. TEMP. CHANGE of S,L,G or melting/freezing or evaporating/condensing) **use appropriate C, Hf or Hv

endothermic = increase in temp.

exothermic (negative heat)

How much heat is required to raise the temperature of water of 50 grams from 20oC to 80oC?

How much heat is required to REDUCE the temperature of water of 50 grams from 80oC to 20oC?

How much heat is required to raise the temperature of water of 50 grams from -20oC to -5.0oC?

How much heat is required to raise the temperature of water of 50 grams from 120oC to 180oC?

How much heat is required to MELT 50 grams of water?

How much heat is required to CONDENSE 75% of 50 grams of water?

#6 130 kcal x 4.18 kJ = 543.4 kJ 1 kcal 534.4 kJ x 1000 J = 543,400 joules 1 kJ convert 130 kcal = 543,400 joules 2.5 kg = 2500 grams