CK-12-Chemistry Intermediate

17.3. Heat and Changes of State http://www.ck12.org

2.∆H 2 = 2 .00 mol×

6 .01 kJ

1 mol

= 12 .0 kJ

3.∆H 3 = 36 .04 g× 4 .18 J/g·◦C× 100 ◦C×

1 kJ

1000 J

= 15 .1 kJ

4.∆H 4 = 2 .00 mol×

40 .7 kJ

1 mol

= 81 .4 kJ

5.∆H 3 = 36 .04 g× 1 .87 J/g·◦C× 40 ◦C×

1 kJ

1000 J

= 2 .70 kJ

∆Htotal=∆H 1 +∆H 2 +∆H 3 +∆H 4 +∆H 5 = 113 .4 kJ

Step 3: Think about your result.

The total heat absorbed as the ice at−30°C is heated to steam at 140°C is 113.4 kJ. By far, the largest absorption of
heat comes during the vaporization of the liquid water.

Practice Problem

4. Calculate the heat released when 100.0 g of water at 35.0°C is converted to ice at−18.0°C.

Heat of Solution

Enthalpy changes also occur when a solute undergoes the physical process of dissolving into a solvent. Hot packs
and cold packs (Figure17.9) use this property. Many hot packs use calcium chloride, which releases heat when it
dissolves according to the equation below.

CaCl 2 (s)→Ca^2 +(aq)+2Cl−(aq)+ 82 .8 kJ

Themolar heat of solution(∆Hsoln) of a substanceis the heat absorbed or released when one mole of the substance
is dissolved in water. For calcium chloride,∆Hsoln=−82.8 kJ/mol.

FIGURE 17.9

Chemical hot packs and cold packs work

because of the heats of solution of the

chemicals inside them. When the bag is

squeezed, an inner pouch bursts, allowing

the chemical to dissolve in water. Heat is

released in a hot pack and absorbed by a

cold pack.

Many cold packs use ammonium nitrate, which absorbs heat from the surroundings when it dissolves.

NH 4 NO 3 (s)+ 25 .7 kJ→NH+ 4 (aq)+NO− 3 (aq)