Chemical Changes November 10, th grade Chapter 3.4

Meme Moment

Symbols for Molecules HBrO 4 Each atom has a symbol from the periodic table Symbols can be 1 or 2 letters. First = capital, second = lower case or none Numbers say how many No number = 1

Law of Conservation of Mass “Matter is never created or destroyed.” Technically, you can destroy matter under very rare circumstances (E=mc 2 ), but the law of conservation of mass is still a scientific law because: o It always works in a narrow case o It’s a fact o It says we’ll always see this during a chemical change without saying why

Chemical Changes & Counting Atoms When you have a chemical change, the number and type of atoms stays the same The molecules change

Chemical Changes & Counting Atoms Sometimes it’s not a 1-to-1 reaction. If you only reacted 1 O 2 molecule, there wouldn’t be anywhere to put the leftover hydrogens!

Chemical Changes & Energy A new molecule isn’t the only thing you get Sometimes there’s noticeable heat or other energy changes too o Explosions o Combustion Even if you don’t notice it, every physical or chemical change has a change in energy too o Heat/thermal energy most common

Temperature vs Thermal Energy Very specific science vocabulary! In science, it’s not good enough to just say that temperature is how hot or cold something is o Have to know that it’s the energy of molecules moving Temperature and thermal energy are not the same thing, but they’re closely related

Thermal Energy Thermal energy: the total energy of all the moving particles in an object. Thermal energy is the process, temperature is the result Flows from high energy to low energy (warm to cold) You can have high thermal energy with a low temperature Thermal energy = Joules (J) Temperature = Kelvins (K)

Combining It Sometimes thermal energy or other energy (light, etc) is produced by or added to a reaction

How do Molecules Store Energy? Molecules can store energy in their chemical bonds Chemical energy: energy stored in the chemical bonds of a molecule It takes energy to make or break chemical bonds If a molecule has a lot of chemical energy, it’s probably very reactive. E.g. TNT

Converting Types of Energy Thermal Energy Added Chemical Bonds Changed = Chemical Energy Thermal Energy Released

Endo vs Exo Sometimes you add more thermal energy than you get out (or the other way around) Endothermic change: extra energy goes in (absorbed) o Ice melts in the sun (sun adds thermal energy) o Boil a handwarmer so you can use it later Exothermic change: extra energy comes out (released because it was stored up until then) o Snap a handwarmer to use it o TNT creates a fiery explosion