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A positive ΔH indicates that energy is absorbed by the system from the surroundings. This defines an endothermic process. Examples include photosynthesis and melting of ice. Exothermic processes, like combustion, have negative ΔH. ACS objectives stress distinguishing between endothermic and exothermic processes, performing calorimetric calculations, and interpreting enthalpy diagrams. Understanding enthalpy is critical for thermodynamic predictions and practical applications such as energy-efficient industrial reactions.

The more positive the reduction potential, the greater the resistance to oxidation. Ag⁺/Ag has the highest E° (+0.80 V), making silver least likely to corrode. Zn and Fe corrode more readily. ACS objectives stress interpreting reduction potentials and applying them to corrosion and galvanic cell design.

M = mol ÷ L = 0.500 ÷ 2.00 = 0.250 M. Molarity is a central concept in solution chemistry and is used extensively in titrations, dilutions, and concentration calculations. ACS objectives emphasize preparing solutions accurately and applying concentration units in quantitative analysis.

From the balanced equation: 1 mol N₂ → 2 mol NH₃.

Thus, 2.00 mol N₂ → 4.00 mol NH₃.

ACS objectives stress using balanced equations for mole relationships and theoretical yield calculations. This skill is critical in laboratory synthesis and industrial chemical production.

Catalysts lower the activation energy by providing an alternative reaction pathway. They do not affect ΔH, ΔG, or the equilibrium constant; they only increase the rate at which equilibrium is reached. ACS objectives emphasize distinguishing between thermodynamics (spontaneity, energy changes) and kinetics (reaction rates). Catalysis is fundamental in industrial chemistry (e.g., Haber process), environmental applications (catalytic converters), and biology (enzymes).