ENEM Natural Sciences Syllabus

Write the photon energy relation and state the photoelectric effect.

Photon energy: $E = hf = \frac{hc}{\lambda}$, with $h \approx 6.63\times10^{-34}\ \mathrm{J\,s}$. In the photoelectric effect, light above a threshold frequency ejects electrons from a metal, evidence that light is quantized.

State Einstein's mass-energy equivalence and De Broglie's wavelength.

Mass-energy: $E = mc^{2}$. De Broglie wavelength of matter: $\lambda = \frac{h}{p} = \frac{h}{mv}$, showing particles have wave properties.

What distinguishes alpha, beta, and gamma radioactive decay?

Alpha: emission of a helium nucleus $\ce{^4_2He}$ (mass number $-4$, atomic number $-2$). Beta: emission of an electron, converting a neutron to a proton (atomic number $+1$). Gamma: emission of high-energy photons with no change in mass or atomic number.

Define a mole and give Avogadro's number.

A mole is the amount of substance containing as many entities as there are atoms in $12\ \mathrm{g}$ of carbon-12, namely Avogadro's number $N_A \approx 6.02\times10^{23}$ particles per mole.

How are number of moles, mass, and molar mass related?

$$n = \frac{m}{M}$$ where $n$ is moles, $m$ is mass in grams, and $M$ is molar mass in $\mathrm{g/mol}$.

Balance the combustion of methane and state what a balanced equation conserves.

$$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$ Balancing conserves the number of atoms of each element (conservation of mass).

Compare ionic, covalent, and metallic bonding.

Ionic: electron transfer between a metal and nonmetal forming ions (e.g., $\ce{NaCl}$). Covalent: electron sharing between nonmetals. Metallic: a lattice of cations in a 'sea' of delocalized electrons giving conductivity and malleability.

Define acids and bases by the Arrhenius and Bronsted-Lowry concepts.

Arrhenius: acids release $\ce{H+}$ and bases release $\ce{OH-}$ in water. Bronsted-Lowry: an acid is a proton ($\ce{H+}$) donor and a base is a proton acceptor.

Define pH and give the pH of a neutral solution at 25 °C.

$$\mathrm{pH} = -\log[\ce{H+}]$$ At 25 °C neutral is $\mathrm{pH}=7$; acidic $<7$, basic $>7$. Also $\mathrm{pH} + \mathrm{pOH} = 14$.

What is the general formula and defining functional group of alkanes, alkenes, and alkynes?

Alkanes $C_nH_{2n+2}$ (single C–C bonds); alkenes $C_nH_{2n}$ (one C=C double bond); alkynes $C_nH_{2n-2}$ (one C≡C triple bond).

Identify the functional groups of alcohols, carboxylic acids, and ketones.

Alcohol: hydroxyl $\ce{-OH}$ on a carbon. Carboxylic acid: carboxyl $\ce{-COOH}$. Ketone: carbonyl $\ce{C=O}$ bonded to two carbons.

What is an isomer, and how do structural and geometric (cis-trans) isomers differ?

Isomers share a molecular formula but differ in structure. Structural isomers differ in atom connectivity; geometric (cis-trans) isomers have the same connectivity but different spatial arrangement around a rigid double bond.

Describe an esterification reaction.

A carboxylic acid reacts with an alcohol to form an ester plus water: $$\ce{R-COOH + R'-OH <=> R-COO-R' + H2O}$$ It is acid-catalyzed and reversible.

How are elements arranged in the periodic table, and what are groups and periods?

Elements are ordered by increasing atomic number. Periods are horizontal rows (same number of electron shells); groups are vertical columns with similar chemical properties (same number of valence electrons).

State the periodic trends for atomic radius and electronegativity.

Atomic radius decreases left-to-right across a period and increases down a group. Electronegativity increases left-to-right and decreases down a group (fluorine is the most electronegative).

Write the reaction quotient/equilibrium expression for $\ce{aA + bB <=> cC + dD}$.

$$K = \frac{[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}$$ Only gases and aqueous species appear; pure solids and liquids are omitted.

State Le Chatelier's principle.

If a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts to partially counteract the disturbance and restore equilibrium.

Define oxidation and reduction in a redox reaction.

Oxidation is loss of electrons (oxidation number increases); reduction is gain of electrons (oxidation number decreases). The oxidizing agent is reduced; the reducing agent is oxidized (mnemonic: OIL RIG).

Distinguish an endothermic from an exothermic reaction using enthalpy.

Exothermic: releases heat, $\Delta H < 0$. Endothermic: absorbs heat, $\Delta H > 0$. $\Delta H$ is the change in enthalpy between products and reactants.

What are the greenhouse effect and the main greenhouse gases?

Gases in the atmosphere absorb and re-emit infrared radiation, warming Earth's surface. Main gases include carbon dioxide $\ce{CO2}$, methane $\ce{CH4}$, water vapor, and nitrous oxide $\ce{N2O}$.

What causes acid rain and the destruction of the ozone layer?

Acid rain forms when $\ce{SO2}$ and $\ce{NO_x}$ react with water to give sulfuric and nitric acids. Ozone depletion is driven mainly by CFCs (chlorofluorocarbons) that release chlorine, which catalytically breaks down $\ce{O3}$.

List the main organelles of the eukaryotic cell and their functions.

Nucleus stores DNA; mitochondria perform cellular respiration; ribosomes synthesize proteins; endoplasmic reticulum processes/transports molecules; Golgi apparatus packages secretions; lysosomes digest material; chloroplasts (plants) do photosynthesis.

Write the overall equations for photosynthesis and aerobic cellular respiration.

Photosynthesis: $$\ce{6CO2 + 6H2O ->[light] C6H12O6 + 6O2}$$ Aerobic respiration: $$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy}$$

Contrast prokaryotic and eukaryotic cells.

Prokaryotes (bacteria, archaea) lack a membrane-bound nucleus and organelles, with DNA free in the cytoplasm. Eukaryotes (protists, fungi, plants, animals) have a true nucleus and membrane-bound organelles.

State Mendel's laws of segregation and independent assortment.

Law of segregation: each individual has two alleles per gene that separate during gamete formation, one per gamete. Law of independent assortment: alleles of different genes segregate independently of one another.

For a monohybrid cross $Aa \times Aa$, give the genotypic and phenotypic ratios.

Genotypic ratio $1\,AA : 2\,Aa : 1\,aa$; phenotypic ratio $3$ dominant $: 1$ recessive.

How is biological sex determined in humans, and what is a sex-linked trait?

Females are $XX$ and males are $XY$; the sperm's sex chromosome ($X$ or $Y$) determines sex. A sex-linked trait is carried on a sex chromosome (usually X), e.g., hemophilia and color blindness, appearing more often in males.

State the main mechanism of Darwin's theory of evolution.

Natural selection: individuals with heritable traits better suited to the environment survive and reproduce more, so advantageous alleles increase in frequency over generations, driving adaptation.

Distinguish homologous from analogous structures.

Homologous structures share a common evolutionary origin but may differ in function (e.g., human arm and bat wing) — evidence of divergent evolution. Analogous structures share function but not origin (e.g., insect and bird wings) — convergent evolution.

Define a food chain and the flow of energy through trophic levels.

A food chain shows feeding order: producers → primary consumers → secondary consumers → decomposers. Energy flows one way and only about 10% transfers to the next trophic level, the rest lost as heat.

Define the components of the human circulatory and respiratory gas exchange.

The heart pumps blood through arteries, veins, and capillaries; hemoglobin in red blood cells carries oxygen. Gas exchange occurs in the lungs' alveoli: $\ce{O2}$ diffuses into blood and $\ce{CO2}$ diffuses out to be exhaled.