Collapsing Can Demo

area of surface of Coke can = 0.031 m²

pressure on this area = 3.1 E 3 N (about 680 lb)

1 atm = 1.0 E 5 N/m²

if we can reduce pressure by as little as 75% there would be a 500 lb difference between pressure inside and outside the can

1) when can is inverted in water bath - the water seals the opening and cools the can

2) as can cools vapor condenses - reduction of pressure inside can

3) can is sufficiently weak and water sufficiently viscous that can collapses before it fills with water

4) must use all aluminum can

Physical Changes in State (see examples in packet)

ice (0° C) + heat à water vapor (100° C)

36 g 25 920 cal 36 g

water vapor (100° C) à ice (0° C) + heat

36 g 36 g 25 920 cal

2 H₂ + O₂ à 2H₂O + heat energy released

4 g 32 g 36 g 136 600 cal

2H₂O + energy à 2H₂ + O₂

36 g 136 600 cal 4 g 32 g

Chemical Changes in State (Phase)

The molecular make-up (the specific arrangement of atoms) is changed, resulting in new substances being formed and energy changes occurring.

EXOTHERMIC - any chemical change that releases energy is exothermic

	the amount of heat released is greater than the amount of heat used to start the reaction
	bond making is exothermic (energy is released into surroundings)
	example: oxidation à wooden splint burning ( heat, light, gases like CO₂ and H₂O being given off with carbon and ashes left over)
	other examples: burning H₂ in O₂, body reactions, dissolving metals in strong acids, mixing acid and water, homogenization, plaster of Paris in water, sugar dehydration

ENDOTHERMIC - any chemical change that absorbs energy is endothermic

	energy continues to be absorbed as long as the reaction continues
	bond breaking is endothermic (energy is absorbed from surroundings)
	example: electrolysis à splitting some compound (usually water) by running an electric current through it
	other examples: photosynthesis, pasteurization, canning vegetables

Sugar dehydration demo here

The chemical change involving splitting or forming water takes about 5 times as many calories as the physical change of state. The reason is that atoms (or molecules) are bonded together in a compound; the stronger the bond the more energy holding the parts together, thus more energy required to break these bonds. A physical change needs far less energy to overcome intermolecular forces holding groups of molecules together. Much more energy is needed to break bonds within molecules than to overcome the forces between molecules.

physical change -- strength of intermolecular forces increased or decreased

chemical change -- bonds formed or broken

energy absorbed -- bonds broken or intermolecular forces overcome

energy released -- bonds formed or intermolecular forces strengthened

Problems:

Tell whether each of the following is a chemical or physical change and further describe each chemical change as endothermic or exothermic and the physical changes as absorbing or releasing energy.

	dry ice sublimates
	CO₂ + H₂O + sunlight à glucose
	air in heated tire expands
	burning coal
	water frozen into ice
	acid dissolves metal

Solid - Gas Phase Change

This change involves sublimation which is the direct change of a solid to a gas (deposition is the opposite).

Examples include: moth balls (naphthalene), paradichlorobenzene, camphor, iodine crystals, and CO₂ fire extinguishers (advantages: does not conduct electricity, colder than water, replaces O₂ since CO₂ is heavier and settles on ground area and the CO₂ does not combust), will sublime away reducing cleanup - disadvantages include difficulty in keeping container pressurized over time, fact that you cannot use on living things due to extreme cold, and cost).