Energy Changes and Rates of Reactions - Intro to Thermochemistry: Changes in Matter and Energy

let’s do some thermochemistry - the study of energy changes that accompany physical/chemical changes in matter!
in this post, we will discuss... 
  1. changes in matter
  2. heat & energy changes
  3. types of chemical systems
these are just the very basics but it's always good to review!

1. Changes in Matter

before we can talk about energy changes, we need to discuss changes in matter. these changes can be classified as:
  • physical - a change in the arrangement of molecules; does NOT produce any new substances
    • e.g. melting ice, boiling water - it’s still H2O, just in different forms!
    Image result for boiling water
    boiling water... yummm
  • chemical - a change in the arrangement of atoms and their electronic structures; produces new compounds
    • e.g. the combustion of methane - produces carbon dioxide and water
    Image result for burning fossil fuels
    burning fossil fuels... controversial
  • nuclear - a change in the nucleus of an atom; produces new atoms!
      • Image result for nuclear fusion in a star
      a star burning in space... wow!
    • e.g. nuclear fusion in stars - two hydrogen nuclei combine to form a helium nucleus
for the purposes of thermochemistry, we’ll mostly be dealing with chemical changes (i.e. reactions)

2. Heat & Energy Changes

when a chemical reaction occurs, a certain amount or quantity of heat (q) is transferred between substances.
an example of this is when a fire is burning!
something is definitely burning!

when you're standing near a fire, you might feel hot! why might this be?
it's the fire - it's transferring heat to YOUR BODY and causing the temperature to rise!
when we discuss this type of heat transfer, you might hear the words "system" and "surroundings" used a lot. 
"system" refers to the specific thing or group of things that a scientist wants to examine. "surroundings" usually refers to the bit of the universe that interacts with the system. pretty simple! 
in the previous example, whatever's burning would be the system and YOU would be part of the surroundings! pretty cool! 
glass, ice, water
if we're interested in what the ice cubes are doing, we could label them as the "system" and the glass of water as their "surroundings"!
when heat transfers between a system and its surroundings, we measure any changes in temperature to classify the change as "exothermic" or "endothermic":
exothermic - releases heat energy from the system and normally causes a rise in the temperature of the surroundings 
endothermic - absorbs heat energy into the system and normally causes a decrease in the temperature of the surroundings
if it helps, remember that “exo-” means “outer” and “endo-” means “inner”!
(NOTE: be careful not to conflate heat energy and temperature! they're similar because they both measure the kinetic energy (movement) of particles in a substance, but they're different concepts. 
think about a bathtub vs a teacup full of water - even if they're at the same temperature, the bathtub contains more total heat energy than the teacup.
heat energy is the total kinetic energy of all the particles in a system and directly relates to the amount of mass in the system. 
temperature is a measure of the average kinetic energy of the particles in a system.)

3. Types of Chemical Systems

open systems allow matter and energy to freely move in and out
Image result for campfire
a campfire is a common example of an open system!
closed systems allow energy to move in and out, but not matter!
a closed starbucks cup! if you touch it, your hand may get burnt from the energy transfer - but hey! at least scalding hot coffee won't get on your hands cause a closed system wouldn't let coffee leave
isolated systems allow neither energy nor matter to move in or out, making it an ideal system for measuring energy changes
a bomb calorimeter! these are used in professional settings cause they're the closest thing we've got to a truly isolated system - i found this nifty contraption online for about $6500! neat!
anyway i spent really long on this post. i hope it helps :D

Comments

Post a Comment

Popular posts from this blog

~Fixing Up Korra's Face~

Image
So probably almost everyone who has watched The Legend of Korra knows about the Pierrot incident. If you don't, here's a quick rundown of what happened: Bryan and Mike didn't originally know that there was going to be more than one season of Korra, so Studio Mir (the animation studio for Korra) only signed a contract for one season of the show. When they got an offer to animate Book 2, they also got another offer from Sony Pictures to animate a show called "The Boondocks." Since the animation process of Korra was so... (How do I word this? Crazy? I'll just use "crazy.") [EDIT: I think the word I was looking for was "tiring".] Since the animation process of Korra was so tiring, they chose to animate Boondocks instead, and Pierrot filled in with animating Korra. Although Pierrot got the job done, there were some... minor issues.  Now that that's out of the way, let's move on to the reason I posted this in the first place. I decide
Read more

Molar Enthalpies

Image
remember all those pesky chemical equations and how to balance them out?  let's take a look at how they're used in thermochemistry! hot 1 H₂(g) + ½ O ₂ (g)  →  1 H ₂O(g) + 241.8 kJ this is an example of a "thermochemical equation"! it's a little different from what u might be used to for 2 reasons: what is this "241.8" kJ nonsense!?! that's not any chemical I recognize! there's a fractional term (½)!! what's up with that!?!? addressing the "241.8 kJ":  this is a measure of the enthalpy change  ( ΔH) , or the change in the system's total energy  that happened during the reaction!  remember that chemical reactions always involve the making and breaking of bonds - this requires a change in the system's energy. for our purposes, energy change will always be measured in joules or kilojoules (1000 J = 1 kJ)! the fact that the ΔH is listed on the "products" side of the equation means that th
Read more

SCH4U - Energy Changes and Rates of Reactions Sample Test Qs

Image
hi guyz it is exam season! i dont feel like typing a real post so im just gonna do some sample problems hehehe 1. Sehee used a coffee cup calorimeter to dissolve 11.86 g of NH4NO3 in 100 mL of water. The initial temperature of the water was 20.5 degC. At the end of the reaction, the temperature recorded was 11.6 degC. Calculate the enthalpy change in kJ/mol of NH4NO3. Indicate if the reaction is exothermic or endothermic. (Heat capacity of water = 4.18 J/g degC) (5 marks) Answer: NH 4 NO 3 Water ΔH sol'n = ? kJ/mol m = 11.86 g M = 2M N + 4M H + 3M O M = [2(14) + 4(1) + 3(16)] g/mol M = 80 g/mol n = m/M n = (11.86 g) / (80 g/mol) n = 0.1483 mol m = 100 g T 1 = 20.5 degC T 2 = 11.6 degC ΔT = (20.5 - 11.6) degC ΔT = 8.9 degC nΔH sol'n = mcΔT n ΔH sol'n = mcΔT / n nΔH sol'n = (100 g)(4.18 J/g degC)(8.9 degC) / 0.1483 nΔH sol'n = 25086 J/mol (extra digits carried) nΔH sol'n = 25.1 kJ/mol the t
Read more