Attractive Forces
What is an attractive forces and where are they found?
It is a force that makes objects drawn to each other. They are found in solids, liquids and gases
On a heating curve, which segments represent a phase change? What is happening to the substance's temperature during these segments?
The flat portions. The temperature is staying the same.
Which equation, Q=mc(delta)T or Q=mL, is used to calculate the heat released when a substance cools down without changing phase?
Q=mc(delta)T, because the temp. is changing and there is no phase change.
Define thermal conductivity (k) and explain what a high value indicates about a material's ability to transfer heat energy.
Thermal conductivity is the ability of a material to conduct heat. A high value indicates that it conducts heat rapidly.
How much heat (Q) is required to boil (vaporize) 10g of liquid water at 100 C? The latent heat of vaporization for water is 2,260 J/g.
Q = mL= (10.0 g)(2,260 J/g) = 22,600 J
In terms of attractive forces, how does a solid differ from a gas?
A solid has high attractive forces and gases are so low that its almost as if they have none.
What are two things that are happening during the sloped parts of the heating curve?
The temperature is increasing and the particles are moving faster and further apart.
Calculate the amount of heat (Q) released when 50.0 g of liquid water cools from 80.0C to 20.0C (Assume the specific heat capacity of liquid water is 4.18 J/gC).
-12540 J
Why might a metal object feel colder to the touch than a wooden object, even if both are at the same room temperature?
We feel cold as the loss of heat. So when something feels colder it is because it is conducting heat faster away from you.
Determine the mass of a substance that releases 8,000 J of heat when it freezes. The substance's latent heat of fusion (L) is 160 J/g. Is this exothermic or endothermic?
Answer: m = Q / L= 8,000 J/160 J/g = 50 g
What are two ways in which solids differ from gases. Explain each
Shape, volume, compression, attractive forces, flow.
Explain why the temperature remains constant during the flat segments of a heating curve, in terms of energy and attractive forces between particles.
The temperature remains constant because all the energy added is going into overcoming the attractive forces and change phase.
Is the process of freezing (liquid to solid) endothermic or exothermic? Justify your answer by explaining what happens to the heat energy
It is exothermic because water has to lose energy in order to become a solid.
You want to fry some eggs. You have an aluminum and iron pans. Which one should you choose and why?
Material Thermal Conductivity (W/mK)
Aluminum - 237
Iron - 80
Since you want rapid heat transfer for frying an aluminum pan would be better since it transfers heat faster. If you wanted to cook something more slowly, the iron would be better.
A 200.0 g sample of liquid water is heated, raising its temperature from 25.0 C to 85.0 C. If the specific heat capacity of liquid water is 4.18 J/gC, calculate the amount of heat absorbed.
Q = (200g)(4.18J/gC)(85C - 25C) = 50,160J
How are particles arranged in a liquid? How do the attractive forces affect their movement?
Particles are not in a fixed arrangement but flow around each other. They have more freedom of movement because they has less attraction between particles.
Compare the heating curves for water and rubbing alcohol. If alcohol has weaker attractive forces, how would this affect its boiling point and the length/duration of its boiling (vaporization) segment compared to water?
It would boil faster than water if it has weaker attractive forces.
If 100.0 g of a substance requires 15,000 J of energy to completely melt at its melting point, which equation would you use to find the substance's Latent Heat of Melting (L). Write the specific equation and identify the necessary variables.
Q = mL, 15000J = 100g (L)
A hiker wants a thermal blanket that keeps them warm. Should they choose a blanket made of Material A (k - 0.5) or Material B (k - 150) Justify your choice by discussing the effect of k on the rate of energy transfer.
If you want to be warm, then the one with the lower thermal conductivity so it doesn't allow heat to escape, but if they want to be cooler then the material with the higher thermal conductivity so that it can remove trapped heat from their body.
Finding Mass: If 1,254 J of heat are absorbed by an unknown solid, causing its temperature to increase by 50C. Given the specific heat capacity of the solid is 0.5 J/gC, what is the mass of the sample?
m = 1254J/(50C)(0.5 J/gC) = 50.16g
Draw the particles in a solid and a liquid
Particles in solid are close and in a regular arrangement. They are vibrating. Liquids are a bit further apart and vectors show movement.
A student mistakenly believes that a longer flat segment on a heating curve means a substance has weaker attractive forces. Explain why this belief is incorrect and state the actual relationship.
A longer flat segment means stronger attractive forces because it is going to take more energy for a longer period of time to overcome all the attractive forces.
A 550g block of metal has an original temperature of 15.0°C and has a specific heat value 0.45 J/g∙°C. What will be the final temperature of this metal if 450 J of heat energy are added?
16.82C
Explain how the thermal conductivity of a material justifies its choice for a cooking pot versus its choice for a pot handle27. For each use, specify whether a high or low k is desired and why.
The pot should be a material with higher thermal conductivity so that heat transfers quickly, but the handle should have a lower thermal conductivity so it doesn't burn your hand.
Calculate the heat released when 75.0 g of liquid ethanol cools from 60.0 C to 20.0 C. The specific heat capacity of liquid ethanol is 2.44 J/gC. Is this exothermic or endothermic?
Q = (75.0g)(2.44 J/gC)(20.0C - 60.0 C) = -7,320 J
Exothermic process