Energy Flow

Energy

• the ability to do work• energy has no mass nor volume but is only

detected by its effects• thermochemistry studies energy changes

during chemical reactions and state changes• chemical potential energy is the energy

stored in chemical bonds

Heat (q)• energy that transfers from one object to

another moves from hot to cold no such thing as cold (only the absence of heat)

• the term “universe” is… system- part you focus on surrounding- everything else/outside energy is conserved so that all energy of the

“universe” remains unchanged (law of conservation of energy)

http://search.live.com/images/results.aspx?q=thermochemistry&form=QBIR#focal=f63acfb36eecbdd0428d134cb42a44ea&fu

rl=http%3A%2F%2Fhighered.mcgraw-hill.com%2Fsites%2Fdl%2Ffree%2F0072512644%2F117362%2F06_p239bottom.jpg

Direction of Heat Flow

• Endothermic• system gains heat as the surrounding cools down• positive q

• Exothermic• system loses heat as the surrounding heats up• negative q

Measuring Heat (not temp.)

• use the calorie (cal) or joule (J)• 1 cal is the quantity of heat needed to raise 1 g

of pure water, 1ºC• in foods, one Calorie is equal to 1000 calories• 41 Calories in a candy bar means that when

burned, it will release 41 Calories (41,000 calories)

Heat Capacity and Specific Heat• heat capacity

• depends on object’s mass and chemical compostion

• amount of heat needed in increase an object 1ºC• specific heat

• depends on chemical composition and mass is held constant (1 gram)

• the amount of heat needed to increase 1 gram of a substance 1ºC

qq m · ΔT

q = heat (in joules or calories)m = mass (g)ΔT = change in temperature (ºC)C = specific heat (J/g·ºC) or (cal/g·ºC)

C =

• What is the specific heat of a substance that absorbs 2500 joules of heat when a sample of  10,000 g of the substance increases in temperature from 10.0ºC to 70.0ºC?

2500 J 10,000 g (60ºC)

= 0.0042 J/g·C

C =

• The specific heat of ethanol is 2.46 J/g·oC.  Find the heat energy required to raise the temperature of 193 g of ethanol from 19oC to 35oC

q 193 g (16oC) q = + 7600 J

2.46 J/g·oC =

• Hard problem• A 25 g sample of iron (initially at 800.00oC)

is dropped into 200 g of water (initially at 30.00oC).  The final temperature of the system is 40.22oC.  Find the specific heat of iron. (Specific heat of water is 4.18 J/g·oC)

• = 0.45 J/goC