Unit 4 Beta

Chemical Reactions: the one-page sheet

4.1 Introduction for Reactions

A chemical reaction rearranges atoms into new substances. Because atoms are conserved, a balanced equation has the same atoms on both sides and the total mass does not change; balance with coefficients, which count particles and moles, never with subscripts.

In a chemical reaction, bonds break and form so the atoms of the reactants end up in new substances, the products.
Evidence (a gas, a new solid, a color or temperature change) suggests a reaction; only a new substance proves one.
A balanced chemical equation has the same number of each kind of atom on both sides. Change coefficients, never subscripts.
Coefficients count particles and moles, not grams. Conservation of mass: total mass is unchanged when nothing can escape.

the same atoms end up arranged into new substances each kind of atom has the same count on both sides of the equation the total mass before a reaction equals the total mass after, if nothing escapes you balance with coefficients only

chemical reaction: A process in which bonds break and form, so the atoms of the starting substances end up in new substances with different properties.
reactant: A starting substance in a chemical reaction, written on the left of the arrow; the new substances formed, on the right, are the products.
chemical equation: A statement of a reaction in formulas: reactants, an arrow, then products. Coefficients in front of each formula make the atom counts equal on both sides.
conservation of mass: In a chemical reaction atoms are rearranged, not created or destroyed, so the total mass of the products equals the total mass of the reactants.

4.2 Net Ionic Equations

Dissolved ionic compounds exist as ions. Remove the spectator ions, which are dissolved and unchanged on both sides, and what is left is the net ionic equation: only the species that change, balanced for atoms and charge, with solids, liquids, gases and weak electrolytes written as formulas.

Molecular equation: whole formulas. Complete ionic equation: every soluble ionic compound split into its ions. Net ionic equation: spectators removed.
Write as ions only what is mostly ions in water. Solids, liquids, gases and weak electrolytes stay as formulas.
A spectator ion is dissolved and unchanged on both sides.
A precipitate is an insoluble solid that forms from solutions. A net ionic equation balances atoms and charge.

the complete ionic equation writes them as ions they are spectator ions and take no part in the change leaves the net ionic equation: only the species that change the net ionic equation must balance both

net ionic equation: An equation for a reaction in solution that shows only the species that change. The molecular equation writes whole formulas; the complete ionic equation splits every dissolved ionic compound into ions; removing the spectator ions gives the net ionic equation.
spectator ion: An ion that is dissolved and unchanged on both sides of a reaction in solution, so it is left out of the net ionic equation.
precipitate: An insoluble solid that forms when solutions are mixed; precipitation is the forming of that solid.

4.3 Representations of Reactions

A reaction particle diagram and a balanced equation say the same thing. Atoms are conserved between the boxes, the changes in particle counts give the coefficients, a leftover reactant was in excess, and state symbols say whether each species is a solid, liquid, gas or dissolved in water.

A reaction particle diagram draws the particles before and after. Every atom drawn before is drawn after.
Find coefficients from what was used and made (After minus Before), reduced to the smallest whole numbers.
The limiting reactant runs out first. A reactant still present After was in excess; leave it out of the equation.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Ionic solids are drawn as packed arrays, never molecules.

a correct diagram has the same count of each kind of atom before and after the change in each count, not its starting value, gives the coefficients the other reactant still appears in the After box a diagram shows gases spread out, solids packed, and dissolved ions apart in water

state symbol: A label after a formula that says how the species exists: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
reaction particle diagram: A drawing of the particles before and after a reaction. It conserves every atom, and the change in particle counts shows the ratio given by the coefficients.
limiting reactant: The limiting reactant is the one that runs out first and so sets how much product can form; any reactant left over is in excess (the excess reactant).

4.4 Physical and Chemical Changes

A physical change rearranges particles without changing them; a chemical change breaks and forms bonds and makes new substances. Phase changes overcome attractions between molecules, which are much weaker than covalent bonds; dissolving an ionic solid breaks strong attractions but makes no new substance.

A physical change (melting, boiling, dissolving, mixing) changes how particles are arranged, not what they are.
A chemical change breaks and forms bonds within particles, so new substances with new properties form.
Decide at the particle level: are the particles after the same as the particles before?
Dissolving salt breaks strong ionic attractions yet makes no new substance, so it sits between the two.

the particles themselves stay the same, so no new substance forms new particles, and so new substances, form phase changes need far less energy than breaking molecules apart it has features of both kinds of change

physical change: A change in the arrangement, spacing or state of particles that leaves the particles themselves unchanged, so no new substance forms; melting, boiling and dissolving are examples.
chemical change: A change in which bonds within particles break and new bonds form, producing new substances with new properties.

4.5 Stoichiometry

Coefficients relate moles, so every stoichiometry problem converts to moles, uses a mole ratio from the balanced equation, and converts back. The limiting reactant is the one that makes the least product; it sets the theoretical yield, and percent yield compares what was collected with that maximum.

Stoichiometry: grams → moles (÷ molar mass) → mole ratio from coefficients → moles → grams (× molar mass).
For a solution, moles = molarity × volume in liters.
Limiting reactant: find how much product each reactant could make; the smaller amount wins. Never compare grams.
Percent yield = actual ÷ theoretical × 100. Gravimetric analysis weighs a dried precipitate to find how much of an ion was present.

a mole ratio from the equation converts moles of one substance into moles of another you convert to moles first with molar mass or molarity it alone sets the theoretical yield; the other is left in excess actual yield differs from theoretical, measured as percent yield

stoichiometry: Using the mole ratios given by the coefficients of a balanced equation to find the amounts of reactants and products; a mole ratio such as 2 mol H₂O / 1 mol O₂ is the conversion factor.
theoretical yield: The theoretical yield is the most product the limiting reactant can make; the actual yield is the amount collected; percent yield = actual yield ÷ theoretical yield × 100.
gravimetric analysis: Finding how much of a substance or ion a sample holds by turning it into a precipitate of known formula, then filtering, drying to constant mass and weighing the solid.

4.6 Introduction to Titration

In a titration, a titrant of known concentration is added from a buret until it has reacted with the analyte in the mole ratio, the equivalence point, signaled by an indicator's end point. Moles of titrant (M × V) and the mole ratio give the moles, then the concentration, of the analyte.

In a titration, the titrant (known concentration) is added from a buret to the analyte (unknown amount).
At the equivalence point, moles of titrant match the analyte by the mole ratio. The indicator's color change, the end point, signals it.
Calculation: M × V (in L) of titrant → mole ratio → moles of analyte → ÷ volume of analyte.
Rinse the buret with titrant, clear the tip, read the bottom of the meniscus at eye level to 0.01 mL, and repeat until trials agree.

the moles added are known from molarity × volume moles of analyte follow from moles of titrant and the balanced equation the end point signals the equivalence point the result is precise, and outlying trials are spotted

titration: A method for finding the amount of a substance (the analyte) by adding a solution of known concentration (the titrant) from a buret until the two have reacted in the mole ratio of the balanced equation.
equivalence point: The point in a titration where the moles of titrant added have reacted exactly, by the mole ratio, with the analyte. The end point is where the indicator changes color, which should be very close to it.
buret: A long graduated glass tube with a stopcock, used to deliver a measured volume of titrant. It is read at the bottom of the meniscus (the curved liquid surface) to 0.01 mL.
indicator: A substance added in small amounts to the analyte that changes color when the first slight excess of titrant is present, marking the end point of a titration.

4.7 Types of Chemical Reactions

Reactions can be sorted by what changes between particles: ions combining into an insoluble solid (precipitation), an H⁺ moving (acid-base), electrons moving (electron transfer, including every combustion). A hydrocarbon burning in excess oxygen gives carbon dioxide and water.

Precipitation reaction: two solutions give an insoluble solid. Acid-base: an H⁺ moves. Electron transfer: electrons move from one particle to another.
Clue for electron transfer: an element appears uncombined on one side and in a compound on the other.
Combustion of a hydrocarbon in excess O₂ gives CO₂ and H₂O; with too little O₂, toxic CO can form.
Older names describe the pattern: synthesis, decomposition reaction, single replacement, double replacement.

an insoluble solid forms: a precipitation reaction the reaction is an acid-base reaction, such as H⁺ + OH⁻ → H₂O electrons have moved between particles its carbon ends up in CO₂ and its hydrogen in H₂O

precipitation reaction: A precipitation reaction forms an insoluble solid when two solutions are mixed. Reactions are also described by their pattern: synthesis (substances combine into one), decomposition (one substance breaks into several), single replacement (an element replaces another in a compound) and double replacement (two compounds swap partners).
combustion: A reaction of a substance with oxygen gas, usually giving off heat and light. A hydrocarbon (a compound of only carbon and hydrogen) burning in excess oxygen gives carbon dioxide and water.

4.8 Introduction to Acid-Base Reactions

In a Brønsted-Lowry acid-base reaction, a proton (H⁺) moves from the acid to the base. The acid becomes its conjugate base and the base becomes its conjugate acid; each pair differs by one H⁺. Water is amphoteric, and in water a proton is carried as hydronium, H₃O⁺.

A Brønsted-Lowry acid is a proton donor; a Brønsted-Lowry base is a proton acceptor (it needs a lone pair).
A conjugate acid-base pair differs by exactly one H⁺: HF/F⁻, NH₄⁺/NH₃, H₂O/OH⁻, H₃O⁺/H₂O.
Amphoteric species such as H₂O and HCO₃⁻ can donate or accept a proton.
In water, H⁺ is carried as hydronium, H₃O⁺. Neutralization: H⁺ + OH⁻ → H₂O.

what remains is its conjugate base, with one less H and one less + charge it becomes its conjugate acid, with one more H and one more + charge it can act as an acid or a base: it is amphoteric acid in water gives hydronium ions, H₃O⁺

Brønsted-Lowry acid: A Brønsted-Lowry acid is a proton (H⁺) donor; a Brønsted-Lowry base is a proton acceptor, using a lone pair to bond to the H⁺.
conjugate acid: Two species that differ by exactly one H⁺. When an acid gives up a proton it becomes its conjugate base; when a base accepts a proton it becomes its conjugate acid.
hydronium: The ion H₃O⁺, a water molecule bonded to an extra proton. Acids in water give hydronium ions; H⁺(aq) is shorthand for it.
amphoteric: Able to act as either an acid or a base, depending on what it reacts with; water and HCO₃⁻ are examples (also called amphiprotic).
neutralization: A reaction between an acid and a base; for hydroxide bases the net change is H⁺(aq) + OH⁻(aq) → H₂O(l).

4.9 Oxidation-Reduction (Redox) Reactions

In a redox reaction electrons move from the species oxidized (oxidation number rises) to the species reduced (oxidation number falls). The oxidizing agent is reduced and the reducing agent is oxidized. Balance redox reactions with half-reactions so electrons lost equal electrons gained, then check atoms and charge.

Oxidation is loss of electrons (oxidation number goes up); reduction is gain (it goes down).
Assign an oxidation number: elements 0; monatomic ions = charge; O usually −2; H usually +1; the sum equals the charge.
The oxidizing agent is reduced; the reducing agent is oxidized.
Write each half-reaction, multiply so electrons lost = electrons gained, add, and check atoms and charge.

oxidation and reduction always happen together a rise shows oxidation and a fall shows reduction the oxidizing agent is the species reduced, and the reducing agent is the one oxidized half-reactions are multiplied so electrons lost equal electrons gained

oxidation number: A bookkeeping charge given to each atom as if every bond were ionic, with shared electrons assigned to the more electronegative atom (also called oxidation state). A rise means electrons were lost; a fall means electrons were gained.
redox: A reaction in which electrons move from one species to another. Oxidation is loss of electrons (oxidation number rises); reduction is gain of electrons (oxidation number falls). The two always happen together.
half-reaction: An equation for just the oxidation or just the reduction part of a redox reaction, with the electrons shown, such as Zn → Zn²⁺ + 2e⁻.
oxidizing agent: The oxidizing agent takes electrons from another species, so it is itself reduced; the reducing agent gives electrons to another species, so it is itself oxidized.