Chemistry · Chemical reactions · Grade 7

Reaction equations

If iron burns in a closed flask, what do the scales show? Where do the atoms go?

The situation

In the chemistry room a glass bell jar stands on a balance, with steel wool inside. The teacher ignites it with an electric coil: the wool glows and turns into a dark powder, yet the balance reading does not change to the milligram. But when the tap of the bell jar is opened, the balance gets heavier. A candle burning on an open dish, by contrast, gets lighter and lighter. Where does the matter come from, where does it go, and if atoms do not disappear, how should the reaction be written?

Topic

Chemical equations and the law of conservation of mass. A sealed flask on a balance: even if iron powder burns inside, the mass does not change — atoms do not vanish, they only regroup

Key ideas

Conservation of mass
Coefficients
Types of reaction

Formulas

\sum m (reactants) = \sum m (products)

The law of conservation of mass: the mass of the substances that reacted equals the mass of the substances formed.

2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O

A reaction equation: the coefficients ensure that the number of atoms of each element is the same on the left and on the right.

3 Fe + 2 O_{2} \overset{}{\to} Fe_{3} O_{4}

When iron burns in oxygen, iron scale (Fe₃O₄) forms — the mass increases.

V (H_{2}) : V (O_{2}) : V (H_{2} O) = 2 : 1 : 2

The law of combining volumes: under the same conditions the volumes of reacting gases are in the ratio of small whole numbers — the ratio of the coefficients in the equation.

\begin{matrix} A + B \to A B & A B \to A + B \\ A + B C \to A C + B & A B + C D \to A D + C B \end{matrix}

Types of reaction: combination, decomposition, displacement, exchange.

Theory

The law of conservation of mass

The mass of the substances that reacted equals the mass of the substances formed. The reason: in a reaction atoms are neither lost nor created anew — they are only regrouped. In 1673 R. Boyle heated lead in a sealed retort and weighed the vessel after opening it; he explained the increase in mass by “particles of fire”. In 1756 M. V. Lomonosov repeated the experiment without opening the vessel: the mass did not change; he wrote that a metal gets heavier only if air gets in (the results were not published at the time). In the 1770s A. Lavoisier burnt tin in a closed vessel and showed that the air in the vessel became lighter by exactly as much as the metal became heavier, and he stated the law clearly in his 1789 textbook. In an open vessel the result looks different: if a metal is burnt the mass increases (oxygen from the air combines with it), and if a candle or wood burns the mass decreases (CO₂ and water vapour escape).

The chemical reaction equation

On the left of an equation are the reactants, on the right the products, with an arrow between. The number in front of a formula is the coefficient (how many molecules or formula units); the subscripts in a formula cannot be changed, otherwise it becomes a different substance (H₂O ≠ H₂O₂). The order of balancing: 1) count the atoms of each element on the left and on the right; 2) begin with the most complex substance; 3) balance oxygen and hydrogen last; 4) if a fraction appears, multiply all the coefficients by 2; 5) the coefficients should be the smallest whole numbers. For example: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O — C: 3 = 3, H: 8 = 4 · 2, O: 5 · 2 = 3 · 2 + 4. In a balanced equation the masses of the left and right sides are also equal — the two-pan balance in the scene comes into balance.

Types of reaction

Combination — one substance is formed from two or more: 2Mg + O₂ → 2MgO. Decomposition — several substances from one: 2H₂O₂ → 2H₂O + O₂↑ (MnO₂ is a catalyst and is not used up). Displacement — the atoms of a simple substance take the place of one of the atoms in a compound: Fe + CuSO₄ → FeSO₄ + Cu. Exchange — two compounds swap their components: CuSO₄ + 2NaOH → Cu(OH)₂↓ + Na₂SO₄. That a reaction has taken place is shown by signs: release of a gas, formation of a precipitate, a change of colour, release of heat and light.

The first laws of chemistry

Constant composition (J. Proust, late 18th century): however a pure substance is obtained, its composition is the same — in any pure water the mass fraction of hydrogen is 11.19 %. Multiple proportions (J. Dalton, 1803): if two elements form several compounds, the masses of one element that combine with the same mass of the other are in the ratio of small whole numbers — in the nitrogen oxides, 8, 16, 24, 32 and 40 g of oxygen per 14 g of nitrogen (1 : 2 : 3 : 4 : 5). Combining volumes (J. Gay-Lussac, 1808): the volumes of reacting and produced gases are in the ratio of small whole numbers: 1 volume of H₂ + 1 volume of Cl₂ → 2 volumes of HCl. This was explained in 1811 by A. Avogadro: equal volumes contain equal numbers of molecules, so the ratio of volumes is the ratio of the coefficients. For details see the “Mole, molar mass and volume” experiment.

Lomonosov and the chemical laboratory

M. V. Lomonosov (1711–1765) had the first scientific chemical laboratory in Russia built at the St Petersburg Academy of Sciences (it began work in 1748): practical experiments with glass, pigments and minerals were carried out there, and accurate weighing was the main method. In a letter to L. Euler in 1748 he set out the general idea of conservation — “as much as is taken away from one body is added to another”. Because he sought to link chemistry with physics and mathematics, he is called one of the first advocates of the idea of physical chemistry.

Safety and the model

Safety: the glowing steel wool and the candle are hot — work on non-flammable mats; do not look straight at a burning magnesium flame — it is very bright; NaOH is an alkali — wear safety goggles and, if it gets in the eyes, rinse with plenty of water; copper salts are poisonous — do not taste them; a mixture of hydrogen and chlorine explodes in light — only the teacher demonstrates such experiments. Model: the bell jar is airtight, the air is at 20 °C and 101.3 kPa; the steel wool turns completely into Fe₃O₄ (in reality a mixture of iron oxides); paraffin is taken as C₂₅H₅₂; the candle under the bell jar goes out when the oxygen falls to ≈ 16 %; the types of reaction are shown speeded up; the synthesis of ammonia is taken to go to completion (in reality it is a reversible reaction); volumes are compared at the same temperature and pressure.

Did you know?

In 1756 M. V. Lomonosov heated a metal in a sealed vessel and showed that its mass did not change until the vessel was opened, but these results were not published at the time.
In his 1789 “Elementary Treatise on Chemistry” A. Lavoisier stated the law of conservation of mass and gave a list of 33 “simple substances” — among them were light and the “substance of heat” (caloric).

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Combustion, flames and oxides — Grade 7
Rate of reaction and catalyst — Grade 8
Reversible reactions and equilibrium — Grade 8
Chemical industry and plants — Grade 9