Unit 5: Stoichiometry Flashcards
What do chemical reactions involve?
- The rearrangement of atoms
- Undergoing a chemical reaction is considered a chemical change: the identity and properties of the substances change
Reactants
- The starting materials; the bonds between the atoms in the reactants are broken
Products
- New bonds are formed between the atoms forming products
Chemical property
- describes something about how the substance reacts
- e.g. things like whether a substance will burn (combust), whether it tends to undergo oxidation, and whether a substance is considered chemically reactive or very stable
- the only way to observe it is to observe a substance reacting and changing chemically
Chemical equations
how chemists communicate about chemical reactions
Reactants: lefthand side and Products: righthand side of an arrow (or double arrows)
The arrow pointing from left to right represents the process of change/atom rearrangement
State of reactants/products
- can be in solid, liquid, aqueous or gaseous state
- state symbols: (aq), (l), (s), and (g)
The law of conservation of mass
- states that matter cannot be created or destroyed
- means that the total # of atoms (and the types of atoms), must be the same before and after the reaction takes place
- That is why chemical reactions must be balanced; the # of atoms of each element must be the same before and after the reaction
Theoretical yield
The amount of product (usually in grams) you should produce based on a given starting amount of reactants
Experimental/actual yield
The amount of product usually made in a chemical reaction
Percent yield
(Actual/Theoretical) x 100%
- a measure as a percent of how much of the theoretical, perfect amount product you obtained in an experiment
- Values over 100% are not possible, so they imply the presence of impurities, water, incomplete decomposition, etc.
Atom economy
A means of quantifying how wasteful a reaction/industrial process is
Higher atom economies mean fewer atoms are “wasted” as byproducts, so the atom economy is inversely related to the wastefulness of a reaction
% a. economy = (M of desired product/M of all reactants) x 100%
Limiting reactant
The reactant that gets used up first, thereby limiting the amount of product you can produce
- the theoretical yield depends on limiting reactant
Excess reactant
The reactant that does not run out. Therefore you will have extra leftovers.
- the theoretical yield does not depend on excess reactant
Endpoint/equivalence point
The point at which the amount (moles) of each reactant has exactly canceled each other out according to the mole ration
- simplistically, this is thought of as where the moles of each react equal one another, though that only holds true for 1:1 mole ratios
Spectator ion
The ion that does not react; remains unchanged as a dissolved ion
- can be identified from being aqueous on both sides of the chemical equation
Ionic compounds in aqueous solutions
many chemical reactions take place between reactants in an aqueous solution and often times the reactants are ionic compounds
However, usually, only one of the dissociated ions from a given compound takes part in the reaction, while the other remains unchanged as a dissolved ion.
- example of a single-replacement reaction:
Mg(s) + CuSO4 (aq) –> MgSO4 (aq) + Cu (s)
Spectator ion: SO4 2-(aq)
Ionic equations (net-ionic equations)
very commonly used as they remove the spectator ions from the chemical equation and show only the species that are taking part in the reaction
They must:
- remove spectator ions
- show charges
- be balanced for atoms + charge
Precipitate
An insoluble ionic compound
- when two solutions of ionic compounds are mixed, it is possible that certain anions and cations will bond together, resulting in the near instant formation of a solid ionic compound throughout the mixture. The solid product formed is called a precipitate, and it can be separated from the rest of the mixture via filtration.
Stoichiometry with gases: Avogadro’s Law
- Volumes of gas are proportional to moles of gas at the same temperature and pressure (the identity of the gas does not matter). Hence, mole ratios are also volume ratios (FOR GASES)
- do NOT need to convert in/out of moles when given problems where only volumes of reactants and/or products are given; simply apply the mole ratio in the volumes
Silver halide precipitates
Silver ions form insoluble salts with many ions, including three of the halogens (halides in their ion form): chloride, bromide and iodide ions
- each silver halide has a distinct color
- Hence, adding solution of silver nitrate to an unknown halide solution can be a quick test for the presence of these different halide ions
Cl- : white
Br- : cream
I- : yellow
Combustion
a very exothermic, and specific oxidation reaction where the heat released can be used directly (to warm buildings) or indirectly (to produce electricity or motion, by spinning turbines or crank-shafts)
- arguably one of the most important chemical reaction on the planet
Complete combustion
In complete combustion all of the carbon atoms in the fuel end up being fully oxidized to carbon dioxide molecules - they gain the maximum number of bonds to oxygen
Incomplete combustion
occurs when there is not enough oxygen, or when the oxygen is not sufficiently mixed with the fuel, the carbon atoms do not get fully oxidized
- Instead, some carbon monoxide, CO (g), and soot/smoke, C (s), are produced along with the carbon dioxide
- produces less heat energy as less new bonds to oxygen are formed
- it is dangerous as carbon monoxide is a colorless, odorless, toxic gas and breathing tiny particulates of carbon (smoke) can cause respiratory issues
What happens with the hydrogen atoms in the fuel in combustion?
In both complete and incomplete combustion, the hydrogen atoms in the fuel are converted to water
Standard enthalpy of combustion
ΔHc°: The enthalpy change when 1 mole of a substance is completely combusted in oxygen at 298K and 100kPA
- the ° symbol tells you that this is a standard value (based on SATP)
**the values listed in section 13 of the data booklet; can be understood as literature values for the combustion of these compounds, which means they are the most accurate values available for these enthalpy changes
Combustion mole ratios
C (subscript #) = #CO2
H (subscript #) = #/2 H2O
