Pendahuluan:

Bau makanan yang dimasak atau asap api adalah bukti adanya reaksi kimia. Meskipun beberapa tanda reaksi kimia mungkin tidak selalu jelas, ada indikator yang mengkonfirmasi terjadinya reaksi tersebut. Fenomena kimia adalah bagian integral dari kehidupan sehari-hari kita, dengan reaksi kimia terus-menerus terjadi di sekitar kita. Kemampuan untuk mengenali reaksi ini penting untuk memahami dunia tempat kita tinggal. Dalam artikel ini, kita akan membahas Hukum Kelestarian Massa.

Perubahan Fisik dan Kimia:

Hukum Kelestarian Massa berlaku untuk dua jenis perubahan: perubahan fisik dan perubahan kimia. Perubahan fisik hanya memengaruhi sifat fisik materi saja, seperti volume, bentuk, dan keadaan (padat, cair, atau gas). Sebagai contoh, ketika air membeku, keadaan fisiknya berubah dari cair menjadi padat, tetapi tetap air. Di sisi lain, perubahan kimia menghasilkan pembentukan suatu zat dengan sifat yang berbeda dari zat asal. Karat pada produk besi adalah contoh perubahan kimia, seperti endapan padat yang terbentuk dari pencampuran dua zat cair seperti perak nitrat dan natrium klorida. Proses yang menghasilkan perubahan kimia disebut reaksi kimia.

Distinguishing between physical and chemical changes can be illustrated using a folded sheet of paper. Folding the paper changes its size and shape, a physical change, while burning the paper results in a chemical change because it produces a new substance. The Figure 2 in the article illustrates this concept.

Persamaan Kimia:

Ekspresi persamaan kimia melibatkan mengidentifikasi reaktan, yaitu zat-zat yang mengalami reaksi, dan produk, yaitu zat-zat yang terbentuk sebagai hasilnya. Reaktan ditulis di sebelah kanan panah, dipisahkan oleh tanda tambah (+), sementara produk ditulis di sebelah kiri panah, juga dipisahkan oleh tanda tambah. Panah di antara keduanya mewakili perubahan yang terjadi selama reaksi kimia. Pembacaan persamaan ini ditandai dengan kata "menghasilkan."

Menggunakan Kata:

Persamaan kimia dapat ditulis menggunakan nama zat yang bereaksi dan produknya. Persamaan verbal untuk reaksi antara cuka dan soda kue dapat diungkapkan sebagai asam asetat + natrium hidrogen karbonat menghasilkan natrium asetat + air + karbon dioksida.

Menggunakan Nama Kimia:

Bahan kimia rumah tangga umum, seperti cuka dan soda kue, memiliki nama kimia seperti asam asetat dan natrium hidrogen karbonat. Nama kimia sering digunakan dalam persamaan kimia daripada nama umum. Persamaan kimia untuk reaksi ini dapat ditulis sebagai berikut:

Menggunakan Rumus Kimia:

Persamaan kimia dapat ditulis lebih ringkas menggunakan rumus kimia. Rumus kimia untuk asam asetat adalah dan untuk natrium hidrogen karbonat, adalah Persamaan kimia menjadi:

Subskrip:

Subskrip, ditulis di kanan bawah atom, menunjukkan jumlah atom dari setiap unsur dalam suatu senyawa. Misalnya, "2" dalam CO₂ menunjukkan bahwa satu molekul karbon dioksida mengandung dua atom oksigen. Jika tidak ada angka yang ditulis di sebelah unsur, itu berarti hanya ada satu atom unsur tersebut dalam senyawa.

This translation covers the main concepts discussed in the original Arabic text about the Law of Conservation of Mass, physical and chemical changes, chemical equations, and the use of words, chemical names, and formulas in expressing chemical reactions.

Mass Conservation:

What happens to the atoms of the reacting substances when they transform into other substances (products)? According to the law of mass conservation, the mass of the resulting substances must be equal to the mass of the reacting (or entering) substances in the chemical reaction. This law was formulated by the French chemist Antoine Lavoisier (1743-1794), one of the first modern chemists who used logic and scientific methods to study chemical reactions. Lavoisier demonstrated through his experiments that nothing is created or destroyed in chemical reactions except by the will of God.

He explained that chemical reactions closely resemble mathematical equations in which the right side is equal to the left side. Similarly, in a chemical equation, the number and type of atoms on both sides of the equation are equal. Each atom in the reactants also appears in the products, as illustrated in the figure. Atoms are neither created nor destroyed in chemical reactions; instead, they are rearranged.

Chemical Equation Balancing:

When you write a chemical equation for a reaction, you must not neglect the law of mass conservation. Look again at Figure 4, which shows that the numbers of carbon, oxygen, hydrogen, and sodium atoms on both sides of the arrow are equal. This means that the equation is balanced, and the law of mass conservation has been applied.

However, not all equations can be balanced as easily. Consider, for example, the black silver formed in the reaction of silver with hydrogen sulfide in the air (hydrogen sulfide). The unbalanced equation is as follows:

Ag+H2S →Ag2S + H2

If you calculate the number of atoms for each element in the reactants and products, you'll find that the number of hydrogen and sulfur atoms is equal on both sides. Still, there is one silver atom in the reactants, while there are two in the products. This cannot be correct because, in a chemical reaction, a silver atom cannot be created from nothing. Therefore, the law of mass conservation is not represented correctly in this equation! Place the number 2 in front of the silver atom in the reactants and check the equation's balance by calculating the number of atoms for each element.

2Ag+H2S→Ag2S+H2

The equation is now balanced, with equal numbers of silver atoms on both sides of the arrow. Remember that when balancing a chemical equation, coefficients are placed before the formulas, as done for the silver atom. These coefficients should not change the subscripts to the right of the atoms in the chemical compound formula; changing them alters the compound type.

Energy in Chemical Reactions:

Chemical reactions often release or absorb energy. In the energy released from a welding flame, for example, when hydrogen and oxygen unite to produce water, the equation is as follows:

2H2 + O2 → 2H2O + energy 

Energy Release:

Where does this energy come from? To answer this question, consider the chemical bonds that are broken or formed

 when atoms gain, lose, or share electrons. In such reactions, bonds in the reactants break to form new bonds in the products. In reactions that release energy, the products are more stable than the reactants, and their bonds have lower energy than the bonds in the reactants. The excess energy is released in various forms, including light, sound, and heat.

Many types of reactions release heat, such as combustion, where a substance combines with oxygen to produce heat, light, carbon dioxide, and water.

Quick Release:

Energy is released rapidly sometimes, as seen in a charcoal lighter, where a liquid combines with the oxygen in the air, producing enough heat to ignite the charcoal in a few minutes.

Slow Release:

There are materials that also combine with oxygen but release heat slowly, making it invisible or imperceptible. For example, when iron reacts with oxygen in the air to form rust, heat is released slowly. This slow release of heat is used in warm compresses that are applied to certain body parts for several hours. Figure 7 illustrates the difference between the quick and slow release of heat.

Energy Absorption:

But what happens when the reaction is reversed? In reactions where energy is absorbed, the reactants are more stable than the products, and the bonds between them have less energy than the bonds in the products.

2H2O+energy →2H2 +O2

The above reaction shows that the additional energy needed to supply the reactants to form the products can be in the form of electricity, as shown in Figure 8. The energy (released or absorbed) accompanying chemical reactions can take various forms, including electrical, light, sound, and thermal energy.

When energy is released or absorbed in chemical reactions, specific terms are used to indicate this. A reaction that absorbs heat is called an endothermic reaction, where heat is absorbed. Conversely, an exothermic reaction releases heat. The word "therm" refers to heat, as seen in the term "thermos," a heat insulator, and the temperature measuring device, thermometer.

Some chemical reactions and physical processes require heat energy before they occur. Cold packs, for example, are examples of heat-absorbing physical processes. Figure 9 shows a cold pack containing water and ammonium nitrate. When this pack is broken, the ammonium nitrate dissolves in water, absorbing heat from the surrounding environment (air or the skin of the affected person) after applying the cold pack to the injured area.

Energy in the Chemical Equation:

The word "energy" is written in the chemical equation with the reactants or the products. If the word energy is written with the reacting substances, it indicates that it is an essential component for the reaction to occur. For example, we need electrical energy to break water molecules into hydrogen and oxygen. It is crucial to know that energy is necessary for this reaction to take place. Similarly, in exothermic reactions that release heat, the word "energy" is added to the products to indicate the liberation of energy. The word "energy" is added, for example, in the reaction between oxygen and methane when the burner flame ignites, as shown in Figure 10.

Conclusion:

1. We recognize the importance of understanding chemical reactions in our daily lives. The ability to distinguish between physical and chemical changes enables us to understand how reactions occur and form new substances.

2. Through the concepts presented about chemical equations and the use of chemical names and formulas, we gain a deeper insight into how reactants are transformed into new products.

3. The law of mass conservation shows us that nothing is destroyed or created in chemical processes, but rather, there is rearrangement and change in the distribution of atoms. This understanding helps us explain ordinary phenomena and changes we observe in our lives.

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