Chemical Reactions and Energy: Key Concepts in Chemistry
Limiting and Excess Reactants
Limiting Reactant: The reactant in a chemical reaction that limits the amount of product that can be formed. The reaction will stop when all of the limiting reactant is consumed.
Excess Reactant: The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant is completely consumed. The excess reactant remains because there is nothing with which it can react.
Molecular and Empirical Formulas
Molecular Formula: The molecular formula is the formula derived from molecules and is representative of the total number of individual atoms present in a molecule of a compound.
Empirical Formula: The empirical formula is the simplest formula for a compound, which is defined as the ratio of subscripts of the smallest possible whole number of the elements present in the formula. It is also known as the simplest formula.
Enthalpy
Enthalpy: A property of a thermodynamic system, is equal to the system’s internal energy plus the product of its pressure and volume. In a system enclosed to prevent matter transfer, for processes at constant pressure, the heat absorbed or released equals the change in enthalpy.
The standard enthalpy change of formation of a compound is the enthalpy change that occurs when one mole of the compound is formed from its elements under standard conditions, with everything in its standard state.
The standard enthalpy change of a reaction is the enthalpy change that occurs when equation quantities of materials react under standard conditions, with everything in its standard state.
Hess’s Law
Hess’s Law states that the change of enthalpy in a chemical reaction (i.e., the heat of reaction at constant pressure) is independent of the pathway between the initial and final states. Hess’s Law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly.
Balanced Equations and Ionic Equations
A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge are the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.
Molecular Formula: A chemical formula that gives the total number of atoms of each element in each molecule of a substance.
Complete Ionic Equation: It is used to describe the chemical reaction while also indicating which reactants/products exist as ions in an aqueous solution.
Net Ionic Equations: Equations that show only the soluble, strong electrolytes reacting (these are represented as ions) and omit the spectator ions, which go through the reaction unchanged.
Types of Chemical Reactions
Acid-Base Reactions
When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H+ cation of the acid combines with the OH– anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt.
Precipitate Reactions
Precipitates form when two soluble salts react in solution to form one or more insoluble products. The insoluble product separates from the liquid and is called a precipitate.
Redox Reactions
Redox (short for reduction–oxidation reaction) is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes.
Light as a Wave: Young’s Interference Experiment
Physicist Thomas Young asserted that light has the properties of a wave in an experiment called Young’s Interference Experiment. This experiment showed that light (waves) passing through two slits (double-slit) add together or cancel each other out, and then interference fringes appear. This phenomenon cannot be explained unless light is considered as a wave.
Light as a Particle: The Photoelectric Effect
The photoelectric effect, proposed by Einstein, is a phenomenon where irradiating a blue light on metal emits electrons from it. However, red light does not cause electron emission from metal no matter how long or how intense the light is applied. So, light in this way came to be called “photons” since it has the properties not only of a wave but also of a particle.
Quantization of Energy
The presence of stable energy levels for anything introduced us to the quantization of energy and brought the Planck constant (h) into the equation of physics for the first time. Rather than continuous waves, light was coming in packets of energy that can be defined with its frequency and Planck’s constant ‘h’. Thus, energy is quantized.