Energy & Chemical Change

Chapter 16

16.1 ENERGY

• Energy = the ability to do work or produce heat.

– Kinetic energy is energy of motion.– Potential energy is stored energy.

• Potential (highest)

loosing potential

gaining kinetic

• Kinetic (highest)

• Chemical systems (like a roller coaster or pendulum) contain both kinetic and potential energy.

16.1

• Law of conservation of energy = in any chemical reaction or physical process, energy can be converted from one form to another, but is neither created nor destroyed.

Potential to electric

Chem potential to heat

16.1

• Chemical potential energy = energy stored in a substance because of its composition.

• Octane (C8H18), a major component of gasoline, stores potential energy in its hydrogen – carbon bonds.

• -C-C-C-C-C-C-C-C-

• As it oxidizes, the potential energy changes to kinetic energy that moves the pistons, and also to heat.

C8H18 + O2 CO2 + H2O + energy(heat)

16.1 Measuring heat

• Heat = (q) energy that is in the process of flowing from a warmer object to a cooler object.

• calorie= amount of heat required to raise the temperature of one gram of pure water by one degree Celsius.

• Nutritionally, a Calorie (capitalized) = 1000 calories.

16.1

• Joule (j)= SI unit of heat and energy.

• Conversion factors page 491:

• 1 cal = 4.184 j

• 1 kj = 1000 joules

• 1 Calorie = 1 kilocalories = 1000 cal

From 16.1

q = c x m x ∆T

q = heat absorbed or released

c = specific heat of substance (table 16-2)

m = mass

∆ T = temperature change

P 495 problem 4

• If the temperature of 34.4 g of ethanol increases from 25.0°C to 78.8°C, how much heat has been absorbed by the ethanol?

q = c x m x ∆T

• q = 2.44 J/g°C x 34.4 g x 53.8°C

• q = 4,520 J

16.2

• Calorimeter = an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process

Determining specific heat

Use the calorimeter to determine the specific heat of an unknown metal.

1. Put a known amount of water into a styrofoam cup and find initial temperature. m = 125 g

2. Heat the metal to a known temperature and then place it in the water.

Tmetal = 115°C

• 3. Place metal into water until temperature stabilizes. T = 29.3°C

• 4. Calculate the heat gained by the water.

q = c x m x ∆T

• qwater =

4.184 J/g°C x 125 g x (29.3°C - 25.6°C)

• qwater = 1900 J

5. Assume that the water gains the same amount of heat that the metal loses.

q metal = c x m x ∆T

1900 J = c metal 50.0g x 85.7°C

1900 J = c metal

50.0g x 85.7°C

c metal = .44 J/g°C

• System = reaction or process being studied

• Surroundings = everything other than the system

• Universe = system + surroundings

• Enthalpy (H) = the heat content of a system at constant pressure.

• ∆Hrxn = ∆Hproducts - ∆Hreactants

• When reactants have more energy than products, ∆Hrxn is negative.

• This means the system loses energy, is exothermic.

• ∆Hrxn = ∆Hproducts - ∆Hreactants

• When products have more energy than reactants, ∆Hrxn is positive.

• This means the system gains energy, is endothermic.

16.3

• Thermochemical equation = a balanced chemical equation that includes the physical states of all reactants and products and the enthalpy change (∆H).

4 Fe (s) + 3O2 (g) 2Fe2O3 (s) + 1625 kJ

4 Fe (s) + 3O2 (g) 2Fe2O3 (s) ∆H=-1625 kJ

The negative value for ∆H means that the system (reaction) is losing energy, giving off heat, is exothermic.

27 kJ + NH4NO3 (s) NH4+

(aq) + NO3-(aq)

NH4NO3 (s) NH4+

(aq) + NO3-(aq) ∆H = 27 kJ

The positive value for ∆H means that the system (reaction) is gaining energy, taking in heat, is endothermic.

• PreAP also must know sections 3-5,

• unfortunately, nothing exists on powerpoint at this time on these sections…