energy transformations

• heat, light, and sound are energy
• chemical reactions involve energy because chemical bonds are being broken and made
• energy transfer maybe in the form of heat or work
• heat and work have very important relationship with the energy of the system and surroundings
• COLLISIONS FORM PRODUCTS BECAUSE OF ENERGY

  A) NO

  B) NO

  
• system is the portion of the universe singled out for study
• Everything else outside the

  system is the surroundings.
• Identify the system and the surroundings in

  the following situation:

  A) system

  B) surroundings

  
• types of systems

  A) open

  B) closed

  C) isolated

  
• The transfer of energy between a

  system and surroundings due to

  temperature difference.
• heat flows from a higher to lower

  temperature.
• assigned convention for heat, Q

  A) absorbs

  B) releases

  
• Q = mcΔT

  where Q = heat absorbed or released, J

  m = mass of substance, g

  c = specific heat of substance, J/g°C

  ΔT = change in temperature, °C

  J=g*J/g*degreecelsius*degree C
• What is the heat in Joules required

  to raise the temperature of 25

  grams of water from 0°C to 100°C?

  c of water = 4.18 J/g·°C

  Answer: 10450 J
• The force applied over a given distance. It

  is the energy transfer between a system

  and the surroundings due to a force acting

  through a distance.
• assigned convention for work W

  when the system goes to the surroundings it is expansion because it spreads.

  when a surroundings goes to the system it is compression because it will be compressed.

  A) expansion

  B) compression

  
• w = -pΔV

  where w = work, J

  p = pressure applied, atm

  ΔV = change in volume, L

  1 L·atm = 101.3 joules
• w = -pΔV

  w and pΔV have opposite signs

  because when a gas expands (ΔV is

  positive), work flows into the

  surroundings (w is negative)
• How much work is needed in a

  system to expand from 25 to 100 L

  against a pressure of 5 atm? Is

  work done by the system or on the

  system?

  Answer: -37,987.5 J or -38 kJ

  Energy flows out of the gas, so the value of w is negative.
• Is heat absorbed or given off in the

  following processes?

  A) released

  B) absorbed

  
• In an endothermic

  reaction

  • Heat (absorbed) flows

  into the system from

  the surroundings.

  • Heat is a reactant

  (added).
• Which of these is an endothermic?

  a. Liquid water changes to solid ice

  b. Solid ice changes to liquid water

  Why?

  Heat is absorbed so the particles move faster.

  answer b
• In an exothermic

  reaction

  • Heat (released) flows

  out of the system into

  the surroundings.

  • Heat is a product.
• Which of these is an exothermic change?

  Instant cold packs

  Instant hand warmer pack

  
  

  answer instant hand warmer pack
• endothermic or exothermic

  Photosynthesis.

  (CO2 + H2O + Energy → O2 + Glucose)

  Endothermic. Energy is a reactant.

  Fireworks exploding?

  Exothermic.

  Powder + O2 → Heat + Light + Gas

  Baking a cake?

  Endothermic. Batter + Heat → Cake
• reaction type : endothermic and exothermic
• energy change: heat absorbed and heat released
• heat in reaction: product and reactant
• Conversion of pressure units

  1 atm = 101,325 Pa
• Energy flow during gas expansion

  If energy flows out of the gas, the value of w is negative