Basic concepts and the law of conservation of mass
The record of a chemical transformation includes initial substances (reactants) located to the left of the arrow, and products located to the right. The general term uniting all participants in the process is reactants.
The physical meaning of any equation is that beneath each chemical formula lies an equal number $N$ of structural units. During a reaction, atoms do not disappear or appear out of nowhere; only their regrouping takes place. This axiom is based on the law of conservation of mass.
Historically, this law was proven by M.V. Lomonosov in 1756. At that time, it was known that calcining metals in the open air paradoxically increases their weight. Lomonosov conducted an experiment by calcining metals in a sealed glass retort. Weighing showed that the mass of the retort with its contents remained absolutely constant. The increase in the weight of the metal itself was exclusively due to the absorption of part of the air inside the closed vessel.
Signs of chemical transformations
Chemical processes are always accompanied by changes that can be detected visually or organoleptically (using the senses).
- Energy effects. Processes can occur with the release of a huge amount of energy. This manifests as heating of the mixture, flashes of light, or explosions. A striking example is the exothermic interaction of aluminum shavings with the heavy red-brown liquid bromine ($Br_2$), which ends in an intense flash.
- Color change. New substances possess different optical properties. Thus, when a colorless ammonia solution is added to copper sulfate, a complex compound is formed, producing an intense purple color.
- Gas evolution. Gases can leave solutions, melts, and even solid mixtures. This looks like the formation of foam or the rise of numerous bubbles. The most important sign is also the odor (for example, the suffocating odor of sulfur dioxide or the pungent odor of ammonia).
- Formation of precipitates. The interaction of certain soluble salts forms insoluble compounds. Mixing lead nitrate and potassium chromate produces a bright yellow precipitate of lead chromate.
When working with reagents, it is necessary to strictly follow safety rules: protect your eyes, keep test tubes away from your face, and conduct experiments with toxic and volatile compounds exclusively in a fume hood.
Algorithm for balancing coefficients
Stoichiometric coefficients ($u$) are necessary to equate the number of atoms of each element before and after the transformation. A coefficient equal to one is never written.
Basic balancing algorithm:
- Counting should begin with those elements whose number of atoms is different from one.
- The mathematical least common multiple (LCM) is used for balancing.
- Elements that are part of most substances in the scheme (most often oxygen) are balanced last.
- Coefficients cannot be fractional. If a fraction is required during counting (e.g., 3.5 or 5.5 in front of a diatomic gas molecule), simply double all the coefficients found in the equation.
After a complete balance of atoms on the left and right is achieved, the transformation sign in the scheme (arrow) is replaced with an equals sign.
Features of multi-step processes and mixtures
Many chemical reactions occur stepwise. The main principle of sequential reactions: the product of the previous stage becomes the reactant for the next. For such processes, algebraic laws are applicable — the equations of individual stages can be added and subtracted to derive the final summary equation.
However, this rule is strictly forbidden to be applied to mixtures of substances that react independently of each other. If, for example, a mixture of magnesium and aluminum shavings burns in oxygen simultaneously, two parallel independent reactions occur. Attempting to add their equations mathematically will lead to false results devoid of any chemical meaning.
Briefly: answers to topical questions
What types of chemical reactions are characteristic of bioorganic compounds?
Bioorganic compounds are characterized by electrophilic and nucleophilic substitution reactions, as well as nucleophilic addition.
- Electrophilic substitution ($S_E$) — occurs in aromatic compounds for which addition reactions are difficult.
- Nucleophilic substitution ($S_N$) — typical for carboxylic acids. In these reactions, a carbon atom loses or accepts an electron pair from a reagent (nucleophile).
- Nucleophilic addition ($A_N$) — characteristic of compounds with a $C=O$ double bond; however, the presence of a hydroxyl group can block this process, as subsequent water elimination is energetically unfavorable.
How do catalysts and enzymes affect the rate of a chemical reaction?
Catalysts and enzymes significantly increase the rate of a chemical reaction by lowering the energy barrier.
- Catalysts — interact with the reactant to form an intermediate compound. The reaction proceeds via a new pathway with a reduced activation energy, after which the catalyst is regenerated.
- Enzymes — biological catalysts that are not consumed in the process. Their action depends on the probability of optimal molecule orientation (steric factor) during the formation of the activated complex. The rate of the enzymatic reaction is proportional to the product of the concentrations of the reacting substances, while the enzyme concentration is many orders of magnitude lower than the substrate concentration.
What thermodynamic parameters determine the spontaneous course of a chemical reaction?
The spontaneous course of a chemical reaction is determined by the balance of enthalpy and entropy factors.
- Enthalpy factor — the tendency of the system to release energy. Under normal conditions, reactions accompanied by the release of heat (exothermic) predominantly proceed spontaneously.
- Entropy factor — the tendency to increase disorder. The direction of all processes is ultimately determined by an increase in entropy, which is often realized through temperature equalization.
The state in which these two factors balance each other is called chemical equilibrium. If a reaction is thermodynamically possible, but kinetically proceeds at an immeasurably low rate, the system is in a retarded state.
Universal balancing rule: if oxygen (or another element) is "scattered" across all formulas of the right and left parts, leave its balancing for last. It will balance out automatically.
Class and exam questions
What is the difference between reactants and products?
Reactants are the starting substances that enter into interaction (on the left side of the equation). Products are the new substances formed as a result of the reaction (on the right side of the equation).
What to do if balancing oxygen results in a fraction?
Stoichiometric coefficients must be integers. If a half-coefficient is needed for balance, multiply all equation coefficients by two.
Why is it forbidden to tightly seal a test tube during gas evolution?
Accumulating gas creates excess pressure, which can lead to vessel explosion, splashing of aggressive reagents, and serious injuries.
What else to study on this topic
- Trial-and-error method for balancing complex reactions (oxidative roasting of chromite)
- Stepwise interaction of alkalis with acidic oxides using carbon dioxide as an example
- Methods for laboratory collection and identification of evolved gases
- Thermal decomposition of complex oxygen-containing salts
- Historical significance of the experiments of A. Lavoisier and M.V. Lomonosov
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