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Chapter 16 – Solutions

Chapter 16 – Solutions. Mr.Yeung. Objectives – Lesson 2. Cold and hot packs Reaction types – endothermic and exothermic Mmmm yummy Salt on ice Freezing point and boiling point elevation. THINK AT THE MOLECULAR LEVEL!. Hot and Cold packs. What do you know about hot and cold packs?

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Chapter 16 – Solutions

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  1. Chapter 16 – Solutions Mr.Yeung

  2. Objectives – Lesson 2 • Cold and hot packs • Reaction types – • endothermic and exothermic • Mmmm yummy • Salt on ice • Freezing point and boiling point elevation THINK AT THE MOLECULAR LEVEL!

  3. Hot and Cold packs • What do you know about hot and cold packs? • How do you activate the hot and cold packs? • What do you think is happening inside the package? • What kind of chemical reaction is happening?

  4. Reaction types • Exothermic process • Is one that release heat to its surroundings • Ex. When you touch something cold, you release your heat to the object • Endothermic process • Is one that gains heat as the surroundings cool down • Ex. When you touch something cold, the cold object gains the heat from your hands.

  5. Explanation of hot and cold packs • When you break open the hot or cold packs…a chemical reaction occurs.

  6. Salty science demo • 3 de-icers • One is ‘rated’ for -30C • One is ‘rated’ for -15C • One is just regular table salt • Which one do you think will melt the most ice? • Which one do you think will cause the most ‘heat’? • If I put some milk with the ice and the de-icer, which one do you think will ‘freeze’ the milk?

  7. Salty Science demo • What happened? • 2 processes • 1. Melting of the ice • 2. Freezing of the milk

  8. Melting of the ice • When salt is put on to the ice, the NaCl are broken apart into Na+ and Cl- • The salt upsets the equilibrium of the solid ice and mixes with the water molecules • The melted portion creates this brine solution (or salty water) • The salt creates more disorder (lots of disturbances or a big party!) which makes it harder to re-form back to the solid phase • So a lower freezing point is established (or it requires the temperature to be lower to refreeze the liquid back into solid

  9. Results • Extreme De-icer lowered the temperature the most • Medium de-icer was (as expected) medium  • Table salt lowered the temperature the least

  10. So… why? • Let’s take a look at the molecules • Extreme De-icer was MgCl2 • Medium de-icer was CaCl2 • Table salt was NaCl

  11. When the molecules… • When the molecules comes in contact with the ice… • They break apart • Which of the 3 molecules would break apart and cause the most disturbance? • MgCl2 or CaCl2 • Because there are 3 atoms\ • So why did MgCl2 have a lower temperature than CaCl2? • Look at the size of the elements • Mg is a lot smaller than Ca so there would be more molecules in the mixture causing more disorder or disturbance

  12. Freezing of the ice cream • Well now we know that the extreme de-icer would cause the coldest temperatures WHILE melting the ice… • How does it actually freeze the milk?

  13. How does the freezing occur? • The melting of the ice was due to the endothermic process where heat was absorbed from its surroundings • Like a person in a well heated house, the person absorbs the heat in the house. • This results in the lost of heat in the surroundings causing colder surrounding temperatures • When you come into the same well heated house from -30C temperatures, you are cold, but you gain the heat from the house causing the house to become colder. • The heat or energy from the liquid milk was absorbed by the melting of the ice, making it colder than before, thus freezing!

  14. Summary • Endothermic reaction • Forming bonds where it takes in or absorbs heat or energy from its surroundings • Exothermic reaction • Breaking bonds that results in the release of heat or energy • Making ice-cream • 2 processes • Melting ice-cream • Due to the dissociation or breaking of the molecules • Freezing ice-cream • Endothermic process where heat is absorbed from the milk causing it to freeze • Hot and cold packs • Same principle as making ice cream • Chemical reaction that either released heat or absorbed heat from the surroundings.

  15. Journal Questions • How did the liquid milk freeze in the bag? • Explain the entire chemical process that occurs when you put fresh ice in a glass of coke. • How come salt trucks are not used when the temperature dips below • -10C during the winter time?

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