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Unit 4: Chemical Reactions flashcards

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4.1 Introduction for Reactions

What is the difference between a physical and a chemical change?
A physical change alters properties such as state, shape or mixing, but every substance keeps its composition (melting, boiling, dissolving, filtering, distilling). A chemical change forms new substances: atoms are kept but rearranged into new combinations.
What are the four common signs of a chemical change?
Heat or light given out, a gas produced, a precipitate forming, and a colour change. Each is a clue, not proof; the deciding question is whether a substance with a different composition has formed.
How can each sign of a chemical change be faked by a physical change?
Temperature change: dissolving NaOH (hot) or NH₄NO₃ (cold). Gas: boiling, or dissolved gas escaping. Solid: crystals forming as a saturated solution cools. Colour: dilution making a colour paler.
Why is a precipitate from mixing two solutions good evidence of a reaction?
The solid is a substance that was not present in either solution, so a new substance has formed. Explain the reasoning: link the evidence to a new substance forming.
Can a reaction occur with no visible sign?
Yes. A strong acid and a strong base react with no gas, precipitate or colour change; only the temperature rises. Reversibility is not the test either; composition decides.
Is mass conserved in a reaction that gives off a gas?
Yes. In a sealed container the mass is unchanged (baking soda and vinegar in a sealed bag: 64.30 g before and after). An open container loses mass only because the gas leaves; no atoms are lost.

4.2 Net Ionic Equations

What must a balanced chemical equation conserve, and how do you balance it?
Both atoms (so mass) and charge. Balance with coefficients, never by changing subscripts. Check the charges add up as well as the atoms: Zn + Cu²⁺ → Zn²⁺ + Cu has +2 on each side.
What is the difference between molecular, complete ionic and net ionic equations?
Molecular: every substance as a full formula (useful for planning amounts). Complete ionic: strong electrolytes split into separate ions (shows all species, reveals spectators). Net ionic: only the species that change (shows what actually reacts).
Which species are split into ions in a complete ionic equation?
Split: soluble ionic compounds (aq), strong acids (HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄) and soluble strong bases (NaOH, KOH). Keep whole: solids and precipitates, liquids including H₂O(l), gases, weak acids and molecular substances.
How do you write a net ionic equation?
Write the complete ionic equation, then cancel spectator ions, which appear unchanged on both sides. Mg(s) + 2HCl(aq): Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g); Cl⁻ is the spectator.
How do you write an equation for an ionic solid dissolving?
Only the state changes and the solid separates into its ions, keeping coefficients: CaCl₂(s) → Ca²⁺(aq) + 2Cl⁻(aq); AlCl₃(s) → Al³⁺(aq) + 3Cl⁻(aq). This is a physical change with an equation.
What are common errors when splitting compounds into ions?
Splitting a solid or a gas; breaking polyatomic ions such as SO₄²⁻ into atoms; losing coefficients (3Ca(NO₃)₂ → 3Ca²⁺ + 6NO₃⁻); calling an ion a spectator when it changes state (Ca²⁺ coming from solid CaCO₃ is not a spectator).
Which salts are always soluble?
All sodium, potassium, ammonium and nitrate salts. Other solubility data will be given, or shown by a precipitate forming.

4.3 Representations of Reactions

What must a particle diagram of a reaction show?
Atoms conserved (each element's count matches before and after), charge balanced in any solution, particles used and formed in the coefficient ratio, and any excess reactant shown unchanged after. It needs a key using letters or labels as well as shading.
Must the number of molecules stay the same before and after a reaction?
No. Atoms are conserved, not molecules. 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O has 9 molecules before and 10 after, but 4 C, 12 H and 14 O atoms on each side.
How do you turn a particle diagram into a balanced equation?
Count particles, then count atoms to check. Subtract leftover (excess) particles, which are not in the equation. Reduce the used-and-formed numbers to the smallest whole-number ratio, and add state symbols.
How should ionic compounds in solution and precipitates be drawn?
Soluble ionic compounds as separate ions in the formula ratio, polyatomic ions whole: two Al₂(SO₄)₃(aq) formula units = 4 Al³⁺ and 6 SO₄²⁻. A precipitate is a packed cluster of ions, usually at the bottom. Never draw NaCl(aq) as NaCl pairs.
How does a particle diagram distinguish a physical from a chemical change?
Chemical: atoms rearranged into new combinations. Physical: the same particles with only spacing or arrangement changed. 5 H₂O(l) → 5 H₂O(g) is physical; boiling water must not be drawn as H₂ and O₂.
How are solids, liquids and gases shown in a reaction diagram?
Solid: packed and regular. Liquid: close but disordered. Gas: far apart. Sizes and distances are not to scale, and water molecules in solutions are often left out (say so in a label).

4.4 Physical and Chemical Changes

What is the particle-level difference between physical and chemical processes?
A chemical process breaks and/or forms chemical bonds (covalent, ionic, metallic), so new substances appear. A physical process changes only intermolecular attractions (dispersion, dipole–dipole, hydrogen bonding, ion–dipole), which are much weaker.
What is the quick formula test for a physical or chemical change?
Compare the formulas before and after. A new formula means atoms were rearranged: chemical. The same formula in a new state or spacing means physical.
How much larger is the energy to break bonds than to boil water?
About 41 kJ mol⁻¹ boils water, but about 920 kJ mol⁻¹ breaks both O–H bonds: more than 20 times as much. Melting or boiling a molecular substance breaks no covalent bonds.
Is dissolving physical or chemical?
Dissolving sugar is physical: sucrose–sucrose hydrogen bonds are replaced by sucrose–water ones. Dissolving HCl gas is usually chemical: the H–Cl bond breaks and H₃O⁺ forms. Dissolving NaCl sits on the boundary (ionic bonds break, ion–dipole attractions form), so either answer needs reasoning.
Liquid nitrogen boils away in an open flask. Physical or chemical, and why?
Physical. N₂ molecules stay intact; only the London dispersion forces between them are overcome.
A heated solid loses mass and gives a gas that turns limewater cloudy. What kind of change?
Chemical. The gas is CO₂, a new substance made by breaking and forming bonds. Give a bond-level reason, not just a label.

4.5 Stoichiometry

What do the coefficients in a balanced equation represent?
Mole ratios: the proportions in which particles, and therefore moles, react and form. They are not mass ratios. In C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, 0.20 mol C₃H₈ reacts with 1.0 mol O₂ and forms 0.60 mol CO₂.
What is the general method for a stoichiometry calculation?
Convert the given quantity to moles (n = m / M, n = PV / RT, or n = M × V). Cross the equation with the mole ratio: n(B) = n(A) × (coefficient B ÷ coefficient A). Convert moles of B to the quantity asked for.
How do you identify the limiting reactant?
Divide each reactant's moles by its coefficient; the smallest value is limiting. Compare moles ÷ coefficient, never grams. For A + 2B → AB₂ with 3.0 mol A and 4.0 mol B: B gives 4.0 ÷ 2 = 2.0, less than 3.0, so B is limiting and 2.0 mol AB₂ forms.
What is theoretical yield, and how is percent yield calculated?
Theoretical yield is the product calculated from the limiting reactant, assuming complete reaction. Percent yield = actual ÷ theoretical × 100, with the same units top and bottom.
What happens to the product if you add more of the excess reactant?
Nothing. The product is fixed by the limiting reactant. Adding more limiting reactant gives more product, in proportion, until it stops being limiting.
What unit errors are common in stoichiometry?
mL instead of L (25.0 mL = 0.0250 L) and °C instead of K (25 °C = 298.15 K). Use R = 0.08206 L atm mol⁻¹ K⁻¹ with atm. Keep full values until the last line.
What assumptions does a stoichiometry calculation make?
The equation is balanced and is the only reaction; the reaction goes to completion so the limiting reactant is fully used; reactants are pure and measured accurately; gases behave ideally.

4.6 Introduction to Titration

What is a titration, and what must the titration reaction be like?
A method to find the amount of an analyte by reacting it with a titrant of known concentration added from a burette. The reaction must be specific (reacts only with the analyte) and quantitative (goes to completion with a known equation).
What is the difference between the equivalence point and the endpoint?
The equivalence point is when the titrant has exactly used up the analyte in the ratio of the balanced equation; it is calculated. The endpoint is the observed change (indicator colour, permanganate pink, pH jump, conductivity change); it should match the equivalence point closely.
How do you calculate an analyte concentration from titration data?
Balanced equation → n(titrant) = M × V (in L) → n(analyte) = n(titrant) × (coeff. analyte ÷ coeff. titrant) → c(analyte) = n(analyte) ÷ V(original sample), not the total volume in the flask.
Why can M₁V₁ = M₂V₂ fail in a titration?
It assumes a 1:1 mole ratio. For 10.00 mL of 0.0500 M Ca(OH)₂ with 0.100 M HCl, the ratio is 2 HCl : 1 Ca(OH)₂, so equivalence needs 10.0 mL, not 5.00 mL. Go through moles.
How do you locate the equivalence point on a titration curve?
It is where the measured property changes most sharply, or where two straight-line sections meet (for example in a conductivity titration).
How do common titration errors affect the result?
Overshooting the endpoint (more titrant recorded than needed) makes the calculated analyte too high. Adding water to the flask does not change the titre, because the moles of analyte are unchanged.

4.7 Types of Chemical Reactions

What three reaction types does the course use, and what is transferred in each?
Redox: electrons transferred (at least one oxidation number changes). Acid-base: a proton, H⁺, transferred (no oxidation number changes). Precipitation: aqueous ions come together into an ionic solid (no oxidation number changes). Classify from the net ionic equation.
What are oxidation and reduction?
Oxidation is loss of electrons, so the oxidation number goes up. Reduction is gain of electrons, so the oxidation number goes down. Electrons move from the species oxidized to the species reduced; electrons lost = electrons gained.
What are the rules for assigning oxidation numbers?
Element: 0. Monatomic ion: its charge. Sum in a formula = overall charge. F in compounds: −1. Group 1 / 2 metals: +1 / +2. H: +1 (−1 in metal hydrides). O: −2 (−1 in peroxides).
What are the oxidation numbers of Cr in Cr₂O₇²⁻ and S in SO₄²⁻?
Cr: 2x + 7(−2) = −2, so x = +6. S: x + 4(−2) = −2, so x = +6. Oxidation numbers are bookkeeping that assume fully ionic bonds; there is no S⁶⁺ ion.
Why is Mg + 2H⁺ → Mg²⁺ + H₂ classified as redox, not acid-base?
Oxidation numbers change: Mg goes 0 → +2 (oxidized) and H goes +1 → 0 (reduced). The presence of H⁺ alone does not make a reaction acid-base.
What is combustion, and what does complete combustion of a hydrocarbon give?
Combustion is a redox reaction with O₂. Complete combustion of a hydrocarbon gives CO₂ and H₂O only.
Can a precipitate form when NH₄NO₃(aq) is mixed with KCl(aq)?
No. Every possible product is an ammonium, potassium or nitrate salt, and these are always soluble. Never choose such a salt as a precipitate.

4.8 Introduction to Acid-Base Reactions

What are Brønsted-Lowry acids and bases?
An acid is a proton (H⁺) donor; a base is a proton acceptor, using a lone pair. A base does not need OH⁻: NH₃, CO₃²⁻ and H₂O can all accept protons. H⁺(aq) and H₃O⁺(aq) mean the same in equations.
How do you write a conjugate acid or conjugate base?
Conjugate base = acid minus one H⁺, charge down by 1 (HSO₃⁻ → SO₃²⁻; HCO₃⁻ → CO₃²⁻). Conjugate acid = base plus one H⁺, charge up by 1 (NH₃ → NH₄⁺; OH⁻ → H₂O). A pair differs by exactly one H⁺.
How many conjugate pairs are in a proton-transfer reaction?
Always two: acid / conjugate base and base / conjugate acid. In HF + H₂O ⇌ H₃O⁺ + F⁻: HF / F⁻ and H₂O / H₃O⁺ (water acts as the base).
What does amphiprotic mean?
Able to both donate and accept a proton. Water accepts a proton to become H₃O⁺ (with HNO₂) or donates one to become OH⁻ (with CH₃NH₂).
What is the difference between a strong and a weak acid?
A strong acid ionizes essentially completely in water (a model shows only H₃O⁺ and anions): HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄ (first H⁺). A weak acid ionizes partially; mostly HA molecules remain (HF, HNO₂, ethanoic acid). Strong is not the same as concentrated.
How is acid strength linked to conjugate base strength?
The stronger the acid, the weaker its conjugate base. The conjugate base of a strong acid (Cl⁻) is a negligible base. HNO₂ is weak and HNO₃ strong, so NO₂⁻ is the stronger base.

4.9 Oxidation-Reduction (Redox) Reactions

What is a half-reaction?
An equation showing just the oxidation or just the reduction, with electrons written in. Oxidation has electrons on the right (Zn → Zn²⁺ + 2e⁻); reduction has them on the left (Cu²⁺ + 2e⁻ → Cu).
How do you balance a redox half-reaction in acidic solution?
Balance atoms other than O and H; add H₂O to balance O; add H⁺ to balance H; add e⁻ to balance charge. Example: NO₃⁻ + 4H⁺ + 3e⁻ → NO + 2H₂O. Never use O₂ or O²⁻ to balance oxygen.
How do you combine two half-reactions into a balanced redox equation?
Multiply each half-reaction so the electrons equal the lowest common multiple, add them, and cancel the electrons and any H⁺ or H₂O on both sides. 3Mg + 2Au³⁺ → 3Mg²⁺ + 2Au transfers 6 electrons (not 3 + 2 = 5).
How do you check a balanced redox equation?
Check both atoms and total charge on each side; the two sides must have equal charge, which need not be zero. No electrons may remain in the final equation. Electrons = change in oxidation number × number of atoms that change.
How do you balance a redox equation in basic solution?
Balance as if in acid, then add OH⁻ to both sides for each H⁺, combine H⁺ + OH⁻ into H₂O, and cancel water that appears on both sides.
How are balanced redox equations used in calculations?
Their coefficients are mole ratios for stoichiometry and redox titrations. Fe + 2Ag⁺ → Fe²⁺ + 2Ag means 2 mol Ag⁺ react per mol Fe.

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