HESI A2 Chemistry flashcards
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How to measure like a scientist: units, prefixes, tools and readings
- Where do you read the volume in a graduated cylinder?
- At the bottom of the meniscus, with your eye level with it.
- How do you convert degrees Celsius to kelvins?
- Add 273.15: K = °C + 273.15. The 9/5 factor is only for Fahrenheit.
- A sample warms from 20 °C to 45 °C. By how many kelvins?
- 25 K. A Celsius degree and a kelvin are the same size.
- How do accuracy and precision differ?
- Accuracy is closeness to the true value. Precision is closeness of repeated measurements to each other. Results can be precise and still wrong.
- An object goes from Earth to the moon. What happens to its mass and weight?
- Mass stays the same. Weight changes because the pull of gravity changes.
- Which is smaller, 1 mg or 1 µg?
- 1 µg. A microgram is 0.000001 g and a milligram is 0.001 g.
How matter changes state, and what stays the same
- What is the difference between sublimation and deposition?
- Sublimation is solid to gas, like dry ice. Deposition is gas to solid, like frost.
- What does a thermometer in a pot of melting ice read, and why?
- It stays at 0 °C until all the ice has melted. The added energy pulls particles apart instead of raising temperature.
- Does cutting a brick in half change its density?
- No. Mass and volume are both halved, so density stays the same.
- Is dissolving sugar in water a physical or a chemical change?
- Physical. The sugar is still sugar, so its chemical identity has not changed.
- A reaction in an open container seems to lose mass. Was matter destroyed?
- No. A gas product likely drifted away. In a closed container the total mass stays the same.
- Why does ice float on liquid water?
- Ice is less dense (about 0.92 g/cm³) than liquid water (about 1.0 g/cm³).
What atoms are made of and how they form ions and bonds
- What does an atom's atomic number count, and what does it identify?
- It counts protons and identifies the element. Every atom with 6 protons is carbon.
- How do you find the number of neutrons in an atom?
- Subtract the atomic number from the mass number. Carbon-14 has 14 − 6 = 8 neutrons.
- How do isotopes of the same element differ?
- They have the same protons but different neutrons. Carbon-12, carbon-13 and carbon-14 are all carbon.
- A sodium atom loses one electron. What does it become, and is it still sodium?
- A positive ion, Na⁺, with 10 electrons. It is still sodium because the protons did not change.
- How does an ionic bond differ from a covalent bond?
- Ionic bonds transfer electrons so opposite ions attract (usually metal + nonmetal). Covalent bonds share electrons (usually nonmetals).
- Why is boron's atomic mass listed as about 10.81 when no single atom weighs that?
- The periodic table value is a weighted average of the element's natural isotopes, not the mass number of one atom.
How atoms bond, and how to tell which bond you are looking at
- What does the octet rule say?
- Main-group atoms tend to bond until they have eight valence electrons. Hydrogen is satisfied with two.
- How do ionic and covalent bonds differ?
- Ionic bonds transfer electrons, usually from a metal to a nonmetal. Covalent bonds share electrons, usually between nonmetals.
- How does electronegativity change across a period and down a group?
- It rises from left to right across a period and falls down a group. Fluorine is highest, at 4.0.
- What electronegativity differences mark nonpolar, polar and ionic bonds?
- Below 0.4 is nonpolar covalent, 0.4 to 1.8 is polar covalent, and above 1.8 is ionic. These cutoffs are rough guides.
- Is a hydrogen bond a bond inside a molecule?
- No. It is an attraction between molecules, formed when H is bonded to F, O or N. It is only 5 to 10% as strong as a covalent bond.
- Why does solid table salt not conduct electricity, but melted salt does?
- Conduction needs moving ions. In a solid crystal the ions are locked in place; melting or dissolving frees them.
How to balance a chemical equation without guessing
- Which law explains why chemical equations must balance?
- The law of conservation of mass. Atoms are only rearranged, so total mass of reactants equals total mass of products.
- When balancing, what may you change: coefficients or subscripts?
- Only coefficients. Subscripts define the substance: changing H₂O to H₂O₂ gives hydrogen peroxide, not water.
- How many oxygen atoms are in 3H₂SO₄?
- 12 oxygen atoms. Multiply the coefficient by the subscript: 3 × 4 = 12.
- Balance: Al + O₂ → Al₂O₃
- 4Al + 3O₂ → 2Al₂O₃. Each side has 4 aluminum and 6 oxygen atoms.
- Balance H₂ + Cl₂ → HCl and name the reaction type.
- H₂ + Cl₂ → 2HCl. Two reactants join into one product, so it is a synthesis reaction.
- Why does a burning sample in an open dish seem to lose mass?
- The missing mass left as gas, such as CO₂ and water vapor. In a sealed container the total mass stays the same.
What makes a chemical reaction faster or slower?
- What two conditions must a collision meet to cause a reaction?
- The particles must hit in the right orientation and with at least the activation energy.
- How does a catalyst speed up a reaction?
- It gives a different pathway with a lower activation energy. It is not used up and gives the same products.
- Why does iron powder react with acid faster than one iron nail?
- Smaller pieces have more surface area, so more of the iron is exposed to the acid.
- Does equilibrium mean reactants and products are present in equal amounts?
- No. It means the forward and reverse rates are equal. Concentrations stay constant but are usually not equal.
- A product is removed from a system at equilibrium. Which way does it shift?
- Toward the products, by Le Châtelier's principle. The system shifts to counteract the stress.
How solutions work: what dissolves, how much, and why cells swell or shrink
- Why can a warm soda hold less carbon dioxide than a cold one?
- Raising temperature decreases the solubility of gases, so the gas escapes from the warmer liquid.
- What does diluting a solution change, and what stays the same?
- Adding solvent lowers the concentration. The amount of solute stays the same.
- What does "like dissolves like" mean?
- Polar solvents such as water dissolve polar and ionic solutes. Nonpolar substances such as oil do not mix with water.
- How does a supersaturated solution differ from a saturated one?
- It holds more solute than the solubility limit and is not at equilibrium. A seed crystal can make the extra crystallize.
- Why is isotonic saline used to prepare IV fluids?
- An isotonic solution causes no net water movement, so cells keep their size. Physiological saline is 0.9% m/v.
How to read, balance, and classify a chemical reaction
- Why can't you balance an equation by changing a subscript?
- Subscripts define the substance. Changing H₂O to H₂O₂ turns water into a different substance. Change coefficients instead.
- Is 2H₂ + O₂ → 2H₂O balanced even though molecules go from 3 to 2?
- Yes. Atoms must balance, not molecules. Each side has the same number of each kind of atom.
- How do single replacement and double replacement differ?
- Single: a lone element replaces another in a compound. Double: two compounds swap partners.
- What does a negative ΔH tell you about a reaction?
- The reaction is exothermic. It releases heat to its surroundings.
- Why does an instant cold pack feel cold on your skin?
- The process inside is endothermic and absorbs heat. Your skin loses heat to the pack; no "cold" is added.
- What forms when methane burns completely, and what type of reaction is it?
- Carbon dioxide and water, with heat released. It is combustion: a fuel reacting rapidly with oxygen.
How to count atoms by weighing them: moles and stoichiometry
- How many particles are in one mole?
- 6.022 × 10²³ particles, a value called Avogadro's number.
- How do you find a substance's molar mass?
- Add the atomic masses of every atom in the formula, multiplying each by its subscript. CO₂ is 12 + 2 × 16 = 44 g/mol.
- How many moles are in 88 g of CO₂?
- 2 mol. Divide grams by molar mass: 88 g ÷ 44 g/mol = 2 mol.
- What do the coefficients in a balanced equation tell you?
- Mole ratios, never gram ratios. In N₂ + 3H₂ → 2NH₃, 1 mol N₂ reacts with 3 mol H₂.
- What are the three steps for a grams-to-grams problem?
- Grams of A to moles of A, then use the mole ratio to get moles of B, then convert moles of B to grams.
- With 4 mol H₂ and 3 mol O₂ making water, which reactant is limiting?
- H₂. In 2H₂ + O₂ → 2H₂O, 4 mol H₂ needs only 2 mol O₂. So 4 mol of water forms and 1 mol O₂ is left.
Oxidation and reduction: who loses electrons and who gains them
- What does OIL RIG stand for?
- Oxidation Is Loss, Reduction Is Gain, of electrons.
- How do oxidation and reduction change an oxidation number?
- Oxidation raises the oxidation number. Reduction lowers it.
- What is the oxidation number of carbon in CO₂?
- +4. Two oxygens give 2 × (−2) = −4, and the compound is neutral, so carbon is +4.
- In 2Na + Cl₂ → 2NaCl, which substance is the oxidizing agent?
- Chlorine. It gains electrons (0 to −1) and is reduced. Sodium is the reducing agent because it is oxidized.
- Does oxidation require oxygen?
- No. Sodium is oxidized when it reacts with chlorine. Oxidation means losing electrons.
- What is the final electron acceptor in cellular respiration?
- Oxygen. It is reduced and forms water.
How acids, bases, pH and buffers work
- How much more H+ does a pH 3 solution have than a pH 4 solution?
- Ten times more. Each pH unit is a tenfold change in hydrogen ion concentration, and a lower pH means more H+.
- Does a strong acid mean a concentrated acid?
- No. Strong means the acid ionizes completely in water. It does not describe how much acid is in the solution.
- What is the general pattern of neutralization?
- Acid + base → salt + water. Milk of magnesia reacting with stomach acid is an example.
- What happens to blood pH if a person breathes out less CO2 than usual?
- Less acid is removed, so blood pH falls and blood becomes more acidic. Breathing out more CO2 raises blood pH.
- In the blood's carbonic acid-bicarbonate buffer, which partner picks up extra H+?
- Bicarbonate (HCO3-) picks up extra H+ and becomes carbonic acid. Carbonic acid is the partner that neutralizes extra base.
- What is the normal range of arterial blood pH?
- 7.35 to 7.45. Below 7.35 is acidosis and above 7.45 is alkalosis.
Radioactive decay, half-life, fission and fusion made simple
- Uranium-238 (atomic number 92) gives off an alpha particle. What are the new numbers?
- Mass number 234, atomic number 90. Alpha decay loses 2 protons and 2 neutrons: mass number drops by 4, atomic number by 2.
- What happens to the nucleus in beta decay?
- A neutron becomes a proton and an electron is emitted. Atomic number rises by 1 and mass number stays the same.
- How does gamma radiation change the mass number and atomic number?
- It changes neither. Gamma rays are high-energy electromagnetic radiation with no charge.
- What stops alpha, beta and gamma radiation?
- Alpha is stopped by paper, beta by a thin sheet of metal. Gamma is only reduced, best by thick, dense lead.
- An isotope has a 2-day half-life. How much of 64 g remains after 8 days?
- 4 g. Eight days is 4 half-lives, so 64 → 32 → 16 → 8 → 4 g.
- How do nuclear fission and fusion differ?
- Fission splits a heavy nucleus and powers today's reactors. Fusion joins light nuclei and powers the sun.
The four macromolecules of life and how cells build them
- Which reaction releases water: dehydration synthesis or hydrolysis?
- Dehydration synthesis releases water when monomers join. Hydrolysis uses water to split a polymer into monomers.
- How do plants and animals each store glucose?
- Plants store it as starch. Animals store it as glycogen, mainly in the liver and muscles.
- What is a triglyceride made of?
- One glycerol attached to three fatty acids.
- What happens to an enzyme heated far above its ideal temperature?
- It denatures. Its shape changes but its amino acid sequence stays, and the lost shape stops it from working.
- What type of molecule are almost all enzymes, and what job do they do?
- Proteins. They act as catalysts, speeding up reactions, and come out unchanged so they can be used again.
- Which sugar does DNA contain, and which does RNA contain?
- DNA contains deoxyribose. RNA contains ribose.
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