Unit 4 - Chemical Reactions Flashcards
Physical change
Occurs when a substance undergoes a change in properties but not a change in composition
The force of attraction being broken and formed is the intermolecular forces between different molecules. The chemical bonds between the individual atoms remain.
EX: Phase changes and formation/separation of a mixture (filtration, distillation, chromatography)
Chemical change
Occurs when a substance is converted into a new substance with a different composition
The force of attraction being broken and formed is the chemical bonds between the individual atoms.
EX: Chemical reaction (production of heat/light, formation of a gas, formation of a precipitate, color change)
Why are reactions in aqueous solution best represented with net ionic equations?
The net ionic equation shows the formation of the solid precipitate from the reactant aqueous ions. It also eliminates spectator ions from the equation, which are unchanged by the reaction.
Why is it important to dry masses several times before weighing them?
They must be dried several times to make sure all the water is removed. It is important for solid compounds or precipitates to be as dry as possible in order to get an accurate mass of just the solid rather than the solid + water
How do you tell if a given reaction is an acid-base reaction?
It involves the transfer of one or more H+ ions from an acid (proton donor) to the hydroxide OH- group of a base (proton acceptor).
Sometimes H2O is also formed. An ionic compound (salt) is always formed.
How do you tell if a given reaction is a redox (oxidation-reduction) reaction?
It involves the transfer of electrons from one substance to another (from the species that is oxidized to the species that is reduced).
This is indicated by a change in the oxidation number of each element.
How do you tell if a given reaction is a precipitation reaction?
It involves the combination of of two different ionic compounds in aqueous solution. It results in the formation of a compound that is insoluble (or slightly soluble) and solid, known as the precipitate.
Oxidation number rules (6)
Oxidation number for a charges species is its charge (EX: For Li+ it is +1)
Oxidation number for lone elements is zero (EX: Fe, Sn)
Oxidation number of H when bonded to a nonmetal is +1 (HCl, CH4) and to a metal is -1 (NaH)
Oxidation number of F is always -1
Oxidation number of halogens (Cl, I, Br) is usually -1 but when bonded to O or F, it’s positive
(EX: HOCl → Cl is + 1)
Oxidation number of O is usually -2 except in peroxides (O2^2-) where it is -1
What does amphoteric mean in regards to water?
This means water can act as both an acid and a base
Electrolyte
Electrolyte: a compound that can dissociate into ions and conduct electricity
Strong electrolyte – compounds can 100% dissociate, forming lots of ions that are mobile and can carry an electric current → bright light (strong conductors)
- Strong acids
- Strong bases
- Soluble salts (any soluble ionic compound like NaCl)
Weak electrolyte – compound can only partially dissociate, forming few ions → dim light (weak conductors)
- Weak acid
- Weak base
Nonelectrolyte
Nonelectrolytes: a compound that stays as a molecule in solution (no ions), has 0% dissociation, can’t conduct electricity
- Molecular (covalent) compounds like alcohol and glucose (bc they’re not made of ions)
- Pure liquid, gas, or solid
- Insoluble salt/ionic compound
Oxidizing agent
Undergoes reduction - gains electrons
Oxidation # of atom decreases
Reducing agent
Undergoes oxidation - loses electrons
Oxidation # of atom increases
Describe the factors that increase the solubility of gases
- Lowering the temperature
(With higher temperatures, the gas molecules move faster and are able to escape from the liquid. The solubility of the gas then decreases).
- Increase the partial pressure of the gas above the surface of the solution/solvent
(When pressure is applied, the gas will move into the solvent and occupy some of the spaces between the solvent particles).
Why do some reactions need a spark to go through?
The reaction may have high activation energy
A spark increases the proportion of collisions that have enough energy to overcome the barrier of activation energy and form product particles by adding energy to reactant particles