HELP w/ Thermodynamics Problems?

[dagitt]

HELP w/ Thermodynamics Problems?

i am absolutely dying with these probs. been working on them for hours. due tomorrow, so any help greatly appreciated...thanks so much.

Homework Statement

1. A 4.00 102 g glass coffee cup, m, is 20.0°C, ti, at room temperature. It is then plunged into hot dishwater at a temperature of 77.0°C, tf, as shown in Figure 12-18. If the temperature of the cup reaches that of the dishwater, how much heat does the cup absorb? Assume that the mass of the dishwater is large enough so that its temperature doesn't change appreciably.

2. A 12.0 kg piece of zinc, m1, at 71.0°C is placed in a container of water, as shown in Figure 12-19. The water has a mass of 18.0 kg, m2, and has a temperature of 10.0°C before the zinc is added. What is the final temperature of the water and zinc?

3. The kinetic energy of a compact car moving at 100 km/h is 2.9 * 10^5 J. To get a feeling for the amount of energy needed to heat water, what volume of water (in liters) would 2.9 105 J of energy warm from room temperature (16°C) to boiling (100°C)?

4. How much heat is needed to change 5.00 * 10^2 g of ice at -20.0°C to steam at 195.0°C?

Homework Equations

Q = mcAt
Q = -Q (transfer of energy/heat)
K = C + 273
anything thermo related basically...

The Attempt at a Solution

1. i just keep getting the wrong answer (this is a webassign). i assume you solve for a Q on one side of the equation, and that would be the answer? i don't know. I'm totally lost and confused. not to mention my teacher never gave me the specific heat capacity for glass :[

2. i did mcAt = mcAt, and got 5.96 C...wrong answer. did the prob about 5 more times. different answers. all wrong. failure. help needed. please.

3. 2.9 *10^5 J=m(4.18)(change in temp)..i got 824.55...wrong.

4. ok, so this is the one I've done approximately 12 times and still keep getting wrong. i know the process...you solve it by intervals, using mcAt for the rising slopes and heat of fusion/vaporization for the plateaus (in the phase change graphs).

mcat*mhf*mcat*mhv*mcat

you have to do it for each increment

-whatever to 0
the phase change
0 to 100
the second phase change
100 to whatever"

...BUT...i keep getting the wrong answer. 2427730350 <--the one i just got...which is WRONG.


please please please help...i'm really about ready to shoot myself. I'm not a math person. at all. and i really can't fail this class. :[

 

[anathema]

Dear student,

I understand that you are struggling with your thermodynamics problems and are feeling overwhelmed. I can offer some tips and advice to help you approach these problems more effectively.

Firstly, it's important to understand the concepts behind thermodynamics before attempting to solve problems. This includes understanding the laws of thermodynamics, heat transfer, specific heat capacity, and phase changes. If you are unclear about any of these concepts, I recommend reviewing your class notes or textbook to gain a better understanding.

Secondly, make sure you have all the necessary information for each problem. This includes the given values and any relevant equations. If you are unsure of the specific heat capacity for glass, you can look it up online or ask your teacher for clarification.

When solving the problems, it's important to write down all the given values and units to avoid making mistakes. Make sure you are using the correct units for specific heat capacity (J/g°C or J/kg°C), as this can affect your final answer.

For problem 1, you can use the formula Q = mcΔT, where Q is the heat absorbed by the cup, m is the mass, c is the specific heat capacity, and ΔT is the change in temperature. Remember that the final temperature will be the same as the temperature of the dishwater, so you can set the final temperature equal to 77.0°C.

For problem 2, you can use the same formula, but this time you have two masses and two temperatures to consider. You can set up two equations, one for the zinc and one for the water, and solve for the final temperature. Make sure to use the correct specific heat capacities for zinc and water.

For problem 3, you can use the formula Q = mcΔT again, but this time you are given the kinetic energy instead of the mass. You can use the given equation to solve for the mass of water needed, and then convert it to liters.

Lastly, for problem 4, you are correct in using the different intervals for the different phases. Make sure to use the correct specific heat capacities for each phase and to account for the change in temperature. You can also double-check your calculations by using the given values for the heat of fusion and heat of vaporization.

I hope these tips help you approach the problems with more confidence and success. Remember to take breaks and ask for help if you need it. Good luck!