Investigation 5
Part 1
In the last investigation we observed changes when matter in its 3 phases heated up and cooled down. What changes did you observe when a volume of gas heated up? Cooled down? What changes did you observe when samples of liquids and solids were heated and cooled? What happens at the particle level to cause expansion and contraction?
When matter gets hot, the particles of matter gain kinetic energy. Kinetic energy is energy of motion. Increased kinetic energy pushes the particles farther apart. Increased distance between particles causes the sample of matter to expand. Contraction occurs when particles lose kinetic energy, resulting in less distance between particles.
If I mix equal volumes of hot and cold water, what will the resulting temperature be? Can you predict the final temperature accurately?
Record a prediction for the temperature of the water mixture. Write your reasoning for your prediction – not just “ because I think so”. Share predictions. With your teams, write a step by step procedure on page 42 (rough draft) for checking your prediction – be specific in your instructions. (3 minutes) Copy the following procedure into your lab books on p. 43. (3 minutes)
Measure equal volumes of hot and cold water (50 ml is a good volume to use). Measure the temperature of the hot and cold water BEFORE mixing. This can be done right in the graduated cylinders. Pour the hot and cold waters into the foam cup. The foam cup can be put into the large clear cup for stability. Measure the resulting temperature.
Use two of your foam cups to get hot and cold water. Use a pipette to get the water in the two graduated cylinders exactly even. Mix the waters in the third foam cup. Put the foam cup in a large clear cup for stability. Measure the temperature of the mixture as soon as the alcohol in the thermometer stops moving.
T hot = temperature of the hot water T cold = temperature of the cold water Prediction = temperature you think the final temperature will be T final = temperature of the mixture
Getters get materials Remember – your measurements must be quick. Any delay will affect the final temperature. You have 10 minutes to complete the investigation. Transcribe your results on the following data table.
Did you predict accurately, or was your prediction high or low? Which groups predicted the final temperature accurately? Can you see a pattern between the starting temperatures and the predicted temperatures?
Equation for 2 equal volumes of water Equation for more than 2 equal volumes of water T f = T h + T c 2 T f = T 1 + T 2 + T 3 …… + T n n
What happens to the hot water when it was mixed with the cold water? What happened to the cold water when it was mixed with the hot water? What happened to the kinetic energy of the particles in the hot water when the hot and cold water mixed? What happened to the kinetic energy of the particles in the cold water when the hot and cold water mixed?
The hot water particles had high kinetic energy. The cold water particles had low kinetic energy. We mixed hot and cold water together to make warm water. The average kinetic energy of the warm water particles was medium. Somehow the kinetic energy of the hot water particles and the cold water particles changed when they were mixed.
Can we figure out how the kinetic energy of particles changes?
Part 2
When hot and cold water are mixed, is the final temperature hotter than the hot water, colder than the cold water, or somewhere in between? Was the final temperature closer to the hot water temperature or the cold water temperature? What do you think might have happened to make the final temperature different from the starting temperatures? Share your ideas.
At the particle level, how were the 2 water samples different? Compare the kinetic energy of the hot water and the final mixture. Compare the kinetic energy of the cold water and the final mixture. Were the water particles in the final mixture the same particles that were in the hot and cold water sample? Or were they different particles? What caused the kinetic energy of the particles to change?
The movement of energy from one place to another is called energy transfer (glossary). Sometimes matter gets hotter as a result of energy transfer, and sometimes matter gets colder. Change of temperature is evidence of energy transfer.
Did the energy transfer from the hot water to the cold water? or from the cold water to the hot water? Where did the energy transfer takes place? Share your ideas.
Important Idea #1 The particles that make up matter are in constant motion. As a result, particles have kinetic energy. The level of kinetic energy of the particles in a sample of matter determines how hot it is. Kinetic Energy =Heat The greater the kinetic energy of the particles in a sample of matter, the hotter it is.
Important Idea #2 Energy transfers as a result of particle collisions. When a fast moving particle collides with a slow moving particle, the one that was going fast slows down, and the one that was going slow speeds up. Energy transfers between particles when they collide. Energy transfer that results from collision is called CONDUCTION.
Important Idea #3 Scientists often talk about energy transfer in terms of flow. Energy is NOT a fluid; it is a condition of matter, but energy flow is a nice idea because energy always flows “downhill”. By “downhill” I mean that energy always flows from higher energy levels to lower energy levels. Energy always flows from higher energy to lower energy. Always!!
#1 Kinetic Energy =Heat #2 Energy transfers between particles when they collide. #3 Energy always flows from higher energy to lower energy. Always!!
Heat Transfer by Collision ltimedia/ChemicalInteractions/collision/collis ion.html ltimedia/ChemicalInteractions/collision/collis ion.html Mixing Hot and Cold ltimedia/ChemicalInteractions/mixing_water/ hot-cold-water.html ltimedia/ChemicalInteractions/mixing_water/ hot-cold-water.html
Read Energy on the Move as a class Answer Questions in lab book p. 45 completely and accurately. Turn in. Homework – lab book p. 47
Part 3
Lab book p. 48 Calculating Heat and Calories 1a-d together 1e-h in teams 2 a-d in teams
The amount of heat needed to change the temperature of a mass of water is measured in calories. What is the equation for calculating the amount of heat needed to heat up or cool down a mass of water? Cal = Cal = m(g) X ∆T(C°)
ΔT=T f - T i 39°C-60°C=21°C
Cal = m x ΔT 40 g x -21°C = -840 x cal
ΔT=T f – T i 39°C -25°C =14°C Cal = m x ΔT 60 g x 14°C = 840 cal
same
Lab book p. 49 Calculating Heat and Calories B ( 15 minutes) Show your equations and your work! Work alone; raise your hand if you need help.
1. ΔT=T f – T i Cal = m x ΔT 2. Cal = m x ΔT ΔT=T f – T i 3. ΔT=T f – T i Cal = m x ΔT
The last time we mixed water because we wanted to find out what the temperature of the final mix would be. Today we want to find out how much heat transferred from the hot water and how much heat transferred to the cold water during the mixing. Lab book p. 51
What is the relationship between the volume and the mass of water? 1ml = 1g This is ONLY true for water.
Get about 50 ml of hot water in one cup and 50 ml of cold water in the other. Decide on the mass(ml or g) of the hot and cold water you will use. Measure the water with the graduated cylinders. Record the masses and starting temperatures in the table. Immediately pour the hot water and the cold water in the third cup. Quickly measure and record the final temperature. Getters get equipment. You have 15 minutes to gather data and clean up. Complete lab book p. 51
What happened when you mixed the hot and cold water? What caused the kinetic energy to change? Did energy transfer from the hot water or from the cold water? How many calories were transferred from the hot water? How many calories were transferred from the cold water? What does that tell you about energy transfer?
One of the most important discoveries in science was the fact that energy is CONSERVED. This means that the amount of energy in a system is always the same – no energy is ever created, and no energy is ever destroyed. Energy can, however, transfer from one place to another, and it can be transformed from one kind of energy into another kind of energy.
When we mixed hot water (high kinetic energy) with cold water (low kinetic energy), we got medium temperature water with a medium amount of kinetic energy. The amount of energy in the system did not change, but it did transfer from one place to another.
When hot and cold water were mixed, particle from the hot water started to collide with particles from the cold water. Every time particles with high kinetic energy from the hot water hot a particle with low kinetic energy from the cold water, the kinetic energy of both particles changed: the hot water particle slowed down and the cold water particle sped up.
This transfer of kinetic energy continues until all of the water particles throughout the volume of water were moving at about the same speed. The final average speed of the particles was faster than the particles in the original cold water, and slower than the particles in the original hot water.
Energy transfer continues as particles continue to bang into one another. But the number of particles gaining energy will be the same as the number of particles losing energy. The temperature (average kinetic energy) will hold steady. When a system is in balance, and there is not net energy transfer going on, the system is in EQUILIBRIUM. When you mixed hot and cold water, the final temperature was its EQUILIBRIUM temperature.
Mixing Hot and Cold Energy Flow