Balancing Chemical Equations Quiz

A balanced equation has the same number of atoms of each element on both sides of the reaction. In the equation 2H2 + O2 → 2H2O, there are four hydrogen atoms and two oxygen atoms on both sides, satisfying the law of conservation of mass. The other options either have an unequal number of atoms or do not properly represent the reactants and products, making them unbalanced. Thus, 2H2 + O2 → 2H2O is the only equation that accurately reflects this principle.

In the reaction 2H2 + O2 → 2H2O, the reactants consist of 2 molecules of hydrogen gas (H2) and 1 molecule of oxygen gas (O2). When counting the total number of molecules on the reactants side, you simply add these together: 2 (from H2) + 1 (from O2) equals 3. However, the question specifically asks for the number of distinct types of molecules, which includes only H2 and O2, totaling 2. Thus, the answer reflects the count of different species rather than the total number of individual molecules.

In balancing chemical equations, the law of conservation of mass requires that the number of atoms of each element remains the same on both sides of the equation. This can only be achieved by adjusting the coefficients, which represent the number of molecules or moles of each substance. Changing the subscripts alters the identity of the compounds and can lead to incorrect representations of the reaction. Therefore, only coefficients may be modified to balance the equation correctly while preserving the chemical identities of the reactants and products.

In this reaction, aluminum (Al) reacts with oxygen (O2) to form aluminum oxide (Al2O3). The balanced equation shows that four aluminum atoms combine with three molecules of oxygen to produce two formula units of aluminum oxide. This equation maintains the law of conservation of mass, ensuring that the number of each type of atom is the same on both sides of the equation. The coefficients indicate the stoichiometric relationships between the reactants and products, reflecting the ratios in which they combine and form.

The law of conservation of mass states that in a closed system, the total mass remains constant over time, regardless of the processes occurring within that system. This principle implies that mass cannot be created or destroyed; it can only change forms, such as during physical or chemical reactions. Therefore, the mass before a reaction must equal the mass after the reaction, highlighting the fundamental idea that while substances may transform, the overall quantity of matter remains unchanged.

In the reaction 2Na + Cl2 → 2NaCl, the coefficients indicate the number of atoms or molecules involved. The coefficient "2" before NaCl means that two sodium chloride (NaCl) molecules are produced. Since each NaCl molecule contains one sodium atom, the total number of sodium atoms in the products is 2 (one from each NaCl). Thus, there are 2 sodium atoms present in the products.

In the balanced chemical equation 4Al + 3O2 → 2Al2O3, the coefficient for O2 is determined by the need to balance the number of oxygen atoms with those in the aluminum oxide product. Each molecule of Al2O3 contains three oxygen atoms, and since there are two Al2O3 molecules produced, a total of six oxygen atoms is required. Therefore, to provide these six oxygen atoms, three O2 molecules are needed, leading to the coefficient of 3 for O2 in the balanced equation.

The balanced equation for the combustion of methane shows that one molecule of methane (CH4) reacts with two molecules of oxygen (O2) to produce one molecule of carbon dioxide (CO2) and two molecules of water (H2O). This equation is balanced in terms of the number of atoms of each element on both sides, adhering to the law of conservation of mass. The coefficients indicate the stoichiometric ratios required for complete combustion, ensuring that all reactants are used and products are formed without any excess reactants.

When magnesium reacts with hydrochloric acid, it produces magnesium chloride (MgCl2) and hydrogen gas (H2). The reaction can be represented by the equation: Mg + 2HCl → MgCl2 + H2. Therefore, both products, magnesium chloride and hydrogen gas, are formed in the reaction, making "both a and b" the correct answer. This demonstrates a typical acid-metal reaction, where the metal displaces hydrogen from the acid.

A decomposition reaction involves breaking down a compound into simpler substances. In the reaction CaCO3 → CaO + CO2, calcium carbonate (CaCO3) is decomposed into calcium oxide (CaO) and carbon dioxide (CO2). This process typically requires energy, such as heat, to facilitate the breakdown of the compound. The other options represent different types of reactions, such as synthesis or combustion, rather than decomposition.