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/actually 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 ratio
- simplistically, this is thought of as where the moles of each reactant 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 reactions 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
- for determining empirical and molecular formulas of fuels
C (subscript #) = #CO2
H (subscript #) = #/2 H2O
Reactions of acids and bases
- many characteristic properties
- their chemical properties are mainly related to what types of substances they will react with
*ALWAYS produce a salt (an ionic compound) as one of the products; helpful for chemical equations
–> the acid provides the salt’s ANION
–> the other reactant (base) provides the CATION
Most common product: salt water (salt and water); CO2 and H2 are others - if the salt’s formula is determined and balanced (criss-cross) first, it makes figuring out the other products of the equation easier
Strong and weak chemical reactions
- when showing dissociation reactions, it is generally assumed the reaction goes to completion and is not reversible. Therefore, forward arrows are used in this type of chemical equation
Chemical properties of acids and bases
1) Neutralization reactions:
- exothermic: so, observable by measuring temperature change. The products are salt and water.
**e.g.: H2SO4 (aq) + 2NaOH (aq) –> Na2SO4 (aq) + 2H2O (aq)
2) Reactions of acids and carbonates (s)
- NOT exothermic: this reaction produces CO2 (g), H20 (l) & salt (aq).
- Fizzing/bubbles can be observed
**e.g.: 2HCl (aq) + NaHCO3 (s) –> NaCl (aq) + CO2 (g) + H2O (l)
3) Reactions of acids & metal oxides (s)
- No noticeable observations except that oxide appears to dissolve into aqueous salt and water
**e.g.: 2HNO3 (aq) + CaO (s) –> Ca(NO3)2 (aq) + H2O (l)
4) Reactions of acids & relatively reactive metal (s)
- Exothermic: this reaction produces H2 (g), so fizzing/bubbles can be observed. Temperature changes can be measured.
**e.g.: H2SO4 (aq) + Zn (s) –> ZnSO4 (aq) + H2 (g)
Acid-base neutralization reactions
one of the classic exothermic reactions in chemistry
**expected to know for the IB exam
- this allows a 3rd way to determine the endpoint (after the use of indicators and pH meters)
Thermometric Titrations
- involve the monitoring of temperature with additions of acid to base (or base to acid)
- the aim is to determine the concentration of one of the solutions
- a specific small volume of acid is added to a solution of base while monitoring the temperature
- this process is repeated as quickly as possible, stirring the reaction mixture and recording the new temperature of the mixture after each addition; the additions occur after the new temperature stabilizes
Titration
An analytical technique involving solutions and stoichiometry. It is used to determine the concentration of the solution (to determine the amount of solute in a given amount of the solution).
- most commonly standard/stock solution (of known concentration) is added slowly and carefully to a solution of unknown concentration until a clear endpoint (or equivalence/stoichiometric point) of the reaction is reached
**This is the point at which the reactants have exactly reacted with each other based on their stoichiometric mole ratios so that all the reactants are used up and there is no excess of any reactant
- the experiment is setup so that an endpoint can be determined by a color change (often with the help of an indicator), or by means of monitoring temperature, pH or conductivity. Determining the endpoint with high accuracy takes great care and patience!
Parts of titration
- burette: used to accurately measure the amount of standard solution that is being dispensed into the solution of unknown concentration
- Standard solution: the solution of known concentration
- Tap: used to accurately dispense the correct amount of standard solution in order to neutralize the solution of unknown concentration
- Solution being titrated: the solution of unknown concentration, but of known volume
When there is a complete combustion…
- hydrocarbons produce CO2 and H2O
- metals produce metal oxides (e.g. MgO)
On a temp-volume graph…
- The point at which 2 lines meet is the volume needed to reach the equivalence point, and therefore, we can determine the concentration of a solution
Why does temperature eventually stop increasing in a thermometric titration?
Temperature stops increasing when the limiting reactant is completely reacted, meaning no more heat is produced from the reaction.
Why does the temperature decrease with continued additions of other solution?
As more solution is added, the limiting reactant may be used up, causing the reaction to slow or stop. With less heat being produced, the temperature decreases.
Titration with an indicator
- far more common (compared to thermometric)
- a color change in titration to determine the endpoint of a reaction
- usually indicators required; e.g. acid-base indicators which change color at a specific pH value
- starch can also be added if the titration involves I2 as they form a very dark blue complex together
- if the reactants and products have different colors, then it is possible that NO indicator will be needed
Trials in a titration
**The rough trial: usually done somewhat quickly, “overshoots” the endpoint, gets a rough estimate, NOT used in final calculations, but still included in the table (sometimes as trial 1)
**Accurate trials: requires patience and care as one drop can be enough to change the color at the equivalence point; 2 accurate trials should be carried out at a minimum, ideally it would be repeated until 2 identical endpoints have been achieved
What is the impact on the calculated concentration of overshooting the endpoint?
Overshooting the endpoint increases the titre value, leading to an overestimation of the analyte’s concentration in stoichiometric calculations.
This is a systematic error affecting the accuracy of stoichiometric calculations.
**titre is the solution of known concentration added from the burette
**analyte is the solution of unknown concentration in the flask that reacts with the titrant.
Dilution (what is it & benefits)
Dilution is a technique for lowering concentration. More solvent is added to a solution, which lowers the ratio of solute-to-solvent
- it is a solution to issues that arise with solutions being TESTED having high concentrations (would take huge amounts of added solution to reach the endpoint) or if the solution BEING ADDED has a high concentration (the endpoint would be reached too quickly, would require very little solution added), and some solutions’ with high conc. have also darker colors making it more difficult to observe.
**Hence, diluting the solution in question by a known factor, the concentration can be adjusted to one that works well for the titration. The dilution can be accounted for with a simple calculation in the data processing.
Dilution (relationships)
- volume and concentration are inversely proportional
example: Volume increased 10x —> 1/10th initial concentration
Dilution formula: C1V1 = C2V2
Acid deposition
broad term referring to both wet and dry processes by which acidic substances leave the atmosphere
- forms of wet deposition: acid rain, snow, fog, mist etc.
- forms of dry deposition: solid acidic particles such as dust or smoke, acidic gases directly reacting with other materials etc.
Normal rain: pH 5.6 app. due to the presence of dissolved carbon dioxide. Acid rain referres to rain with pH well below this, often below pH 5. This only occurs when other stronger acids develop in rain.
*The effects of acid rain (and other forms of acid deposition) are numerous and affect manmade structures, bodies of water, soil composition, as well as animal and human health. While there are many specific reactions, most are simply different examples of acid reactions, such as acids with carbonates or acids with metals.
e.g.
CO2 (g) + H2O (l) —> H2CO3 (aq)
H2CO3 (aq) —> H+ (aq) + HCO3 - (aq)
Redox titrations
based on the same principles as acid-base titrations. The goal is to precisely and accurately identify the endpoint of the reaction.
- involve electron transfers, and when they involve transition metal ions, they can often be carried out without an indicator, since the transition metal ions changes color as it undergoes the reaction.
**can be tricky because the mole ratios can be more difficult and because the reactions often involve acid (H+) as an additional reaction, which can make deciphering information a bit trickier than normal.
Indirect titrations
used to determine the amount of the original substance that reacted.
- Sometimes you might want to determine the amount of a substance, but there might not be a convenient reaction to directly titrate for that substance. In these cases, the substance is reacted with another chemical first. Then, a titration is carried out to determine how much product was created or to determine how much excess reactant is left over.
**These titrations are tricky because they involve multiple chemical equations, so they are inherently more confusing and involve multiple mole ratios.
Back titration
can be used to determine the amount of the original substance that reacted.
- Sometimes you might want to determine the amount of a substance, but there might not be a convenient reaction to directly titrate for that substance. In these cases, the substance is reacted with another chemical first. Then, a titration is carried out to determine how much how much excess reactant is leftover.
**These titrations are tricky because they involve multiple chemical equations, so they are inherently more confusing and involve multiple mole ratios.
The Winkler Method
a chemical approach to determine the concentration of dissolved oxygen, O2 (aq), in a sample of water. It is often used in environmental science to consider the biological activity and/or health of a particular body of water.
- It is difficult to titrate for a dissolved gas directly, because they are very sensitive to changes in temperature or even shaking of the solution. Therefore, it is necessary to react the gases first, and then perform a titration later to indirectly determine their concentration.
**In the Winkler method, the water has a series of chemicals added to it, which “fix” the oxygen amount into solution and ultimately produce iodine, I2. The amount of iodine depends on the amount of oxygen in the water sample, and can therefore be linked to the amount of oxygen in the water sample.
Water of crystallization titration
Previously, we have learned about hydrated ionic compounds. We specifically did an experiment to determine the empirical formula of copper(II)sulfate - 5 hydrate, CuSO4 .5H2O.
While the ratio of water molecules can be determined via heating and measuring mass loss, the amount can also be determined via titration. The titration is used to determine the mass of the main compound. Any remaining mass can be attributed to water, and can therefore be used to determine the ratio of water molecules in the empirical formula.
Acids & bases to REMEMBER
Strong acids:
HCl (hydrochloric), HNO3 (nitric), H2SO4 (sulfuric)
Strong bases (*hydroxide ending in each):
NaOH (sodium), KOH (potassium), LiOH (lithium), Ba(OH)2 (barium)
Weak acids:
CH3COOH (ethanoic), H2CO3 (carbonic), H3PO4 (phosphoric)
Weak bases:
NH3: ammonia
CH3CH2NH2: ethylamine
What categories of substances are considered bases?
- metal oxides and hydroxides
- ammonia and amines
- soluble carbonate and hydrogen carbonates
- alkalis: soluble bases, yielding hydroxide ions in solution
How does acid deposition form?
When certain nonmetals, such as sulfur or nitrogen, are present in fuels/ fuel mixtures, they can become oxidized during the combustion of the fuel. This leads to the production of gaseous sulfur oxides and nitrogen oxides, both of which (as nonmetal oxides) are acidic.
- Sulfur is often an impurity in coal. Coal burning produces a large amount of sulfur dioxide, and through subsequent oxidation, sulfur
- Nitrogen makes up around 78% of the Earth’s atmosphere. During the combustion of petrol/gasoline in internal combustion engines, the temperature can get so high that the nitrogen present in the air is oxidized alongside the fuel. This can produce nitrogen monoxide and nitrogen dioxide.
