Chemical Reactions Flashcards
At the base, what is a chemical change or chemical reaction?
All particles of a substance move randomly or constantly (Kinetic Molecular Theory).
If particles collide with enough energy and the right orientation, there will be a rearrangement of these particles.
What is a physical change vs a chemical change?
Physical change:
- changes in state
- involves 3D arrangement of MOLECULES
Chemical change:
- production of new substances that have different chemical and physical properties than the original substance
What are signs a chemical change has taken place?
- Colour change
- Production of gas
- Production of precipitate
- Energy change (heat/light/sound)
Odor change, too.
Often difficult to reverse, but not a guarantee.
How are reactions communicated?
A chemical equation.
Reactants: substance that go into a chemical reaction
Products: substances that come out of the reaction
What are the states in a reaction?
- ionic (if H20 is present, check solubility table) - solid/aqueous
- molecular - all (we should know it if not given)
- acid - aqueous
- element (see Periodic Table)
What is the wording for a chemical equation?
… reacts with … to produce … and …
Define chemical change.
Involve breaking and making the bonds in the overall process. Usually, you got to break chemicals apart before making a new one.
Chemical changes deal with valence electrons and their bonds. They are not nuclear changes, which involve the rearrangements of particles inside the nucleus of atoms. This creates enormous changes in energy, producing new elements. Some undergo spontaneous changes in their nucleus, are not stable, and emit energy called radiation.
Each reaction is a sum of two steps:
1. Breaking apart reactants
o-o + []-[] + Energy -> o + o + [] + []
2. Forming products
o + o + [] + [] = o-[] + o-[] + Energy
What do all chemical changes have to follow?
The Law of Conservation of Energy: Energy can be converted from one form to another; but not created or destroyed.
Always true. While sometimes it may seem energy was created (burning wood), there is often energy stored in the bonds of the reactants.
What are exothermic reactions?
A reaction is if the energy in the breaking step is less than the energy in the making step.
They produce energy and often feel hot.
Reactants -> Products + Energy
Examples: Burning gasoline, explosions, fireflies, turning food into energy in our bodies
THE ENERGY PRODUCED IS MORE THAN THE ENERGY REQUIRED TO START
What are endothermic reactions?
A reaction is if the energy in the breaking step is greater than the energy in the making step.
They require energy, may feel cold, and you have to keep applying energy “doing something” or else it does not continue.
Reactants + Energy -> Products
Examples: Cold Pack, Photosynthesis, Baking a Cake
THE ENERGY PRODUCED IS LESS THAN THE ENERGY REQUIRED TO START
How is the Law of Conservation of Energy represented in chemical reactions?
The total energy of reactants and products is the same.
Antoine Lavoisier: during a chemical reaction, the total mass of the reactants is always equal to the total mass of the products
Atoms cannot be created or destroyed. The types of atoms in a chemical reaction are conserved, as well as the number of each atom.
Ex: Buring wood, ashes weight less than original log. But the smoke and gases and the oxygen burned would add up to make it equal.
What are the five types of chemical reactions?
Formation, Decomposition, Single Replacement, Double Replacement, Hydrocarbon Combustion
What is a formation reaction?
Also called synthesis and simple composition.
Two or more reactants (often elements) combine to produce one new product. They are usually exothermic. Always check if water is present, as they are ionic compounds, which may or may not be soluble in water.
Element + Element -> Compound
2 H2 (g) + O2 (g) -> 2 H2O (l)
What is a decomposition reaction?
In a simple decomposition reaction, one compound breaks down into two or more simpler compounds or elements. Most decomposition reactions require energy to take place; they are endothermic. May also be molecular compounds.
Compound -> Element + Element
2 H2O (l) -> 2 H2 (g) + O2 (g)
Remember diatomic/polyatomic elements!
What is a single replacement reaction?
One ion takes the place of (replaces) another within a compound. These reactions will often involve alkali metals or halogens because they are so reactive. Replacement reactions usually occur within a solution.
Compound + Element -> (different) Compound + (different) Element
* Free ion kicks out combined ions of the same type
I2 (s) + 2 HCl (aq) -> 2 HI (aq) + Cl2 (g)
Remember to check ionic compounds for solubility!
Use the first listed (most common) ion charge for multivalent compounds. Also, the charge stays consistent if not specified throughout the equation.
