Endoexothermic

All chemical reactions involve bond breaking and bond making.

Bond breaking is endothermic (energy is absorbed from surroundings)

Bond making is exothermic (energy is released into surroundings)

Imagine stretching a rubber band until it breaks. You must do work to stretch the band because the tension in the band opposes your efforts. You lose energy; the band gains it. Something similar happens when bonds break in a chemical reaction. The energy required to break the bonds is absorbed from the surroundings.

Energy is absorbed or released when the heat capacities of the products and reactants differ. Usually this is small. Remember that heat capacity is best thought of with a penny and specific heat best thought of as copper metal.

Neutralization reactions are usually exothermic but when you add baking soda to vinegar it is slightly endothermic. The neutralization reaction actually does release heat:

HC₂H₃O₂ + NaHCO₃ à CO₂ + NaC₂H₃O₂ (aq) + H₂O

This is because there is net bond formation. The products collectively have lower energy than the reactants. But evaporation of the liquid occurs as the carbon dioxide escapes from solution. Evaporation absorbs heat, cooling the liquid. (The expansion of the carbon dioxide gas bubbles as they are released also helps to cool the surroundings by Joule-Thomson cooling). The net result is an endothermic reaction.

Mixing a strong acid with water is exothermic. Breaking a chemical bond requires energy (remember that stretching a spring until it breaks requires energy). Forming a chemical bond will release energy. So in a reaction that releases heat (exothermic) there must be net bond formation. Lets looks at HCl dissolved in water:

HCl à H⁺ (aq) + Cl^1- (aq)

You would think at first this would be a heat absorbing (endothermic) process, because it looks like the bond between H and Cl is broken. But there is another reaction hiding here. The hydrogen ion reacts with water to form a complex of the form: H₃O·(H₂O)⁺_n where n is a number between 1 and 9. It is much easier just to write H⁺(aq). Because the hydrogen ion is so tiny, a large amount of charge is concentrated in a very small area, and the polar water molecules are strongly attracted to it. This "hydration" of the hydrogen ion involves the formation of a covalent bond to one of the waters and a large number of strong hydrogen bonds, so it’s a strongly exothermic process. This causes the mixing of a strong acid with water to be strongly exothermic overall.

Exothermic processes	Endothermic processes
making ice cubes	melting ice cubes
formation of snow in clouds	conversion of frost to water vapor
condensation of rain from water vapor	evaporation of water
a candle flame	forming a cation from an atom in the gas phase
mixing sodium sulfite and bleach	baking bread
rusting iron	cooking an egg
burning sugar	producing sugar by photosynthesis
forming ion pairs	separating ion pairs
combining atoms to make a gas molecule	splitting a gas molecule apart
mixing strong acids and water	mixing water and ammonium nitrate
nuclear fission	melting solid salts

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

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

Endothermic vs Exothermic Reactions