OpenStem

@openstem · Joined Jul 2026

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Quiz6 questions

What fundamentally distinguishes chemiluminescence from photoluminescence (fluorescence/phosphorescence)?

AChemiluminescence never involves an excited electronic state at allBIn chemiluminescence, the excited-state product is created directly by a chemical reaction's energy release, with no photon ever absorbed; in photoluminescence, the excited state is created by absorbing a photonCChemiluminescence only occurs in living organismsDChemiluminescence releases energy exclusively as heat, never as light

Chemistry

Chemistry · L5 · Chemiluminescence

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Quiz6 questions

What causes a C–D bond to have a lower zero-point energy than the corresponding C–H bond?

ADeuterium has an extra electron compared to hydrogen, changing the electronic structureBDeuterium is heavier than hydrogen, so the bond vibrates more slowly, giving a lower zero-point energyCDeuterium forms a fundamentally different type of chemical bond to carbonDThe C-D bond dissociation potential-energy curve is a different shape from the C-H curve

Chemistry

Chemistry · L5 · The Kinetic Isotope Effect — 6 Q

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Quiz6 questions

What distinguishes CuAAC from the uncatalyzed thermal Huisgen azide–alkyne cycloaddition?

ACuAAC requires UV light to proceed; the Huisgen reaction does notBCuAAC is fast and highly regioselective for the 1,4-triazole; the uncatalyzed reaction is slow and gives a roughly 1:1 mixture of 1,4- and 1,5-regioisomersCCuAAC forms a six-membered ring, while the Huisgen reaction forms a five-membered ringDCuAAC only works with internal (disubstituted) alkynes, never terminal ones

Chemistry

Chemistry · L5 · Click Chemistry and Bioorthogonal Labeling

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Quiz5 questions

Which loss regime dominates a fuel cell's polarization curve at very low current density?

AMass-transport (concentration) lossesBActivation (kinetic) losses, reflecting sluggish electrode reaction kineticsCOhmic (resistive) lossesDNo losses occur at low current density

Chemistry

Chemistry · L5 · Electrocatalysis & Fuel Cells

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Quiz5 questions

Why do microreactors handle strongly exothermic reactions more safely than batch flasks?

ATheir high surface-area-to-volume ratio dissipates heat far faster, preventing thermal runawayBThey operate at lower pressure than batch reactorsCThey use less concentrated reagent solutions by designDThey cannot support exothermic reactions at all

Chemistry

Chemistry · L5 · Flow Chemistry & Continuous Processing

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Quiz5 questions

What core property of isotopes makes isotopic labeling useful as a mechanistic and tracing tool?

AIsotopes of an element have nearly identical chemistry but a distinguishable physical signal (mass, radioactivity, or spectral shift)BIsotopes of an element have completely different chemical reactivityCOnly radioactive isotopes can ever be used as tracersDIsotopic labeling always doubles the reaction rate

Chemistry

Chemistry · L5 · Isotope Labeling & Tracer Studies

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Quiz5 questions

What causes the lanthanide contraction?

APoor mutual shielding by 4f electrons lets effective nuclear charge on outer electrons rise steadily across the series, pulling the ionic radius inwardBThe lanthanides lose electrons progressively across the seriesCIncreasing atomic mass directly shrinks the ionic radiusDThe 4f orbitals expand outward across the series

Chemistry

Chemistry · L5 · Lanthanide & Actinide Chemistry

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Quiz5 questions

What role does mechanical impact play in a ball-milling reaction?

AIt fractures particles and continuously exposes fresh, unreacted crystal surfaces to each otherBIt dissolves the reactants into a homogeneous solutionCIt only removes moisture from the reaction mixtureDIt cools the reaction to suppress unwanted side reactions

Chemistry

Chemistry · L5 · Mechanochemistry

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Quiz5 questions

What is a secondary building unit (SBU) in MOF chemistry?

AA single isolated metal ion with no defined geometryBA rigid, geometrically predictable polynuclear metal-oxo cluster that acts as a network nodeCThe organic linker molecule aloneDA solvent molecule trapped in the framework pores

Chemistry

Chemistry · L5 · Metal-Organic Frameworks

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Quiz5 questions

Why does MLCT excitation make a photocatalyst both a stronger oxidant and a stronger reductant simultaneously?

AIt increases the total electron count on the metalBThe metal-centered electron hole created makes it a better oxidant, while the promoted ligand pi* electron makes it a better reductantCExcitation always favors oxidation only, never reductionDThe metal center is reduced to a lower oxidation state upon excitation

Chemistry

Chemistry · L5 · Photoredox Catalysis

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Quiz5 questions

What four thermodynamic quantities can a single, well-designed ITC titration determine simultaneously?

AOnly the association constant KaBStoichiometry (n), association constant (Ka, hence delta G), binding enthalpy (delta H), and by subtraction, binding entropy (delta S)COnly the melting temperature of the proteinDOnly the crystal structure resolution

Chemistry

Chemistry · L5 · Protein-Ligand Binding Thermodynamics

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Quiz5 questions

What is the key principle behind using a radical clock to measure an ultrafast bimolecular rate constant?

AIt directly times the fast reaction using a high-speed detectorBIt sets the unknown bimolecular reaction in competition with a unimolecular rearrangement of known rate, then calculates the unknown rate from the product ratioCIt only works for reactions slower than one secondDIt measures the radical's mass spectrum instead of its rate

Chemistry

Chemistry · L5 · Advanced Radical Chemistry

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Quiz5 questions

What is the fundamental advantage single-molecule spectroscopy has over ensemble spectroscopy?

AIt always gives a stronger overall signalBIt reveals the underlying distribution of behaviors and real-time dynamics that an ensemble average washes outCIt eliminates the need for any fluorescent labelDIt can only be performed on solid samples

Chemistry

Chemistry · L5 · Single-Molecule Spectroscopy

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Quiz5 questions

What defining feature of an OTTLE cell lets it reach true bulk redox equilibrium quickly?

AA very thin solution layer (~100–200 μm), so the whole optical path electrolyzes on a convenient timescaleBAn unusually high applied potential that forces the reaction to completionCA rotating disk electrode that stirs the solution mechanicallyDThe use of a reference electrode instead of a working electrode

Chemistry

Chemistry · L5 · Spectroelectrochemistry

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Quiz5 questions

What is a 'strategic bond' in retrosynthetic analysis?

AAny bond that can theoretically be disconnected using a known reactionBA disconnection chosen because it maximally simplifies the target, e.g. by revealing symmetry or enabling a convergent couplingCThe first bond formed in the forward synthesisDA bond that must never be broken during synthesis planning

Chemistry

Chemistry · L5 · Total Synthesis Strategy

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Quiz5 questions

Why is femtosecond time resolution necessary to directly observe bond-breaking dynamics?

AFemtosecond pulses are simply more intense than any other laser pulseBA single molecular vibration completes in roughly 10-100 femtoseconds, so only femtosecond-resolution measurements can resolve the actual atomic motion during bond breakingCFemtosecond lasers are the only lasers that can excite electronic transitionsDBond breaking always takes exactly one femtosecond regardless of the bond

Chemistry

Chemistry · L5 · Ultrafast Femtochemistry

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Quiz5 questions

Where does a zeolite's Bronsted acidity structurally come from?

AIsomorphic substitution of Al3+ for Si4+ in the framework leaves an extra negative charge, balanced by a proton on a bridging Si-OH-Al oxygenBFree sulfuric acid trapped within the poresCSurface hydroxyl groups unrelated to the aluminosilicate frameworkDA metal ion exchanged onto the external crystal surface only

Chemistry

Chemistry · L5 · Zeolite Catalysis

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Note~94 words · 1 min

Materials Around Us Everything is made of a material. A material is what a thing is made from. We choose materials because of what they can do. Think about a cup. It could be plastic, glass, or metal. Each one feels different and works diff

Chemistry

Chemistry · L1 · Materials and What They Are Good For

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Note~232 words · 1 min

Something New! A chemical change makes a whole new substance. Once it happens, you can't turn it back into what it was before. How do you know a chemical change happened? Look for these clues: Cooking an egg: the clear, runny egg white turn

Chemistry

Chemistry · L1 · Spotting a Chemical Change

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Note~191 words · 1 min

Metals and Non-metals Look around you. Some things are metals, like a spoon or a nail. Other things are non-metals, like a rubber band or a wooden pencil. Here is how to tell them apart. Not every metal is magnetic! A magnet sticks to iron

Chemistry

Chemistry · L1 · Metals and Non-metals

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Note~330 words · 2 min

Why Do Things Float or Sink? Drop something into water and it will do one of two things: float on top, or sink to the bottom. What decides which one happens? It Is Not Just About Weight A huge metal ship floats, but a tiny pebble sinks. Tha

Chemistry

Chemistry · L1 · Why Do Things Float or Sink?

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Note~208 words · 1 min

Hot and Cold in the Kitchen The kitchen is full of heating and cooling. A stove makes things hot. A fridge or freezer makes things cold. When water boils on the stove, it gets so hot that it turns into steam — a gas you can see rising above

Chemistry

Chemistry · L1 · Heating and Cooling in the Kitchen

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Note~265 words · 1 min

Mixing in the Kitchen Your kitchen is full of mixtures! A mixture is two or more things mixed together, where each part stays itself. Mixtures You Can Still Take Apart Some kitchen mixtures are easy to take apart again, because you can stil

Chemistry

Chemistry · L1 · Mixing in the Kitchen

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Note~284 words · 1 min

Water, a Special Liquid Water is the most common liquid on Earth. It fills oceans, rivers, and rain clouds. It is also inside you! But water does a few things that make it very unusual. Every drop of water is made of tiny water molecules, f

Chemistry

Chemistry · L1 · Water, a Special Liquid

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Note~287 words · 1 min

Where Do Materials Come From? Every material comes from somewhere. Some materials grow or are found in nature. Others are made by people. We can sort materials by asking: where did this come from? Natural Materials A natural material grows

Chemistry

Chemistry · L1 · Where Do Materials Come From?

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Note~238 words · 1 min

Dissolving vs. Melting Stir sugar into warm tea and it seems to vanish. Leave an ice cube on the counter and it also seems to vanish. But these two things are not the same at all. Melting Is About Heat Melting happens when a solid gets warm

Chemistry

Chemistry · L1 · Dissolving vs. Melting

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Note~348 words · 2 min

Crystals and Crystallization A crystal is a solid whose particles are arranged in a regular, repeating pattern. That pattern repeats over and over in every direction, like a tiny, perfectly ordered stack of building blocks. Because the patt

Chemistry

Chemistry · L2 · Crystals and Crystallization

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Note~269 words · 1 min

What Is Air Made Of? Air feels like nothing, but it is not a single substance — it is a mixture of several gases, each doing its own job. The proportions barely change from place to place, which is why scientists can give air a fairly fixed

Chemistry

Chemistry · L2 · Gases in the Air

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Note~290 words · 1 min

Polymers and Plastics A polymer is a very long chain made of many small, repeating units called monomers, joined together end to end. Picture a paper-clip chain made of thousands of identical clips — each clip is a monomer, and the whole ch

Chemistry

Chemistry · L2 · Polymers and Plastics

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Note~254 words · 1 min

Staying Safe with Chemicals at Home Your home has many bottles and containers. Some hold food. Some hold cleaning chemicals or medicine. Chemicals can help us clean and stay healthy, but some of them can hurt us if we don't use them the rig

Chemistry

Chemistry · L1 · Staying Safe with Chemicals at Home

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