Question

(a) When the metallic element sodium combines with the nonmetallic element bromine, \(\mathrm{Br}_{2}(l)\), how can you determine the chemical formula of the product? How do you know whether the product is a solid, liquid, or gas at room temperature? Write the balanced chemical equation for the reaction. (b) When a hydrocarbon burns in air, what reactant besides the hydrocarbon is involved in the reaction? What products are formed? Write a balanced chemical equation for the combustion of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}(l)\), in air.

Short answer

(a) The chemical formula of the product when sodium combines with bromine is NaBr. NaBr is a solid at room temperature. The balanced chemical equation for the reaction is: \( 2\mathrm{Na}(s) + \mathrm{Br}_{2}(l) \to 2\mathrm{NaBr}(s) \). (b) In the combustion of a hydrocarbon, oxygen (O₂) is the other reactant involved. The products formed are carbon dioxide (CO₂) and water (H₂O). The balanced chemical equation for the combustion of benzene is: \( 2 \mathrm{C}_{6} \mathrm{H}_{6}(l) + 15 \mathrm{O}_{2}(g) \to 12 \mathrm{CO}_{2}(g) + 6 \mathrm{H}_{2} \mathrm{O}(g) \).

Step-by-step solution

Find the charges of the elements

First, we need to determine the charges of the elements when sodium (a metallic element) combines with bromine (a non-metallic element). Sodium is in group 1 on the periodic table, so it has a +1 charge when it forms an ion. Bromine is in group 17, so it has a -1 charge when it forms an ion.

Determine the chemical formula

Since sodium has a +1 charge and bromine has a -1 charge, we can determine the chemical formula of the compound by combining the ions in a 1:1 ratio: Na(+1) + Br(-1) → NaBr

Determine the state of the product at room temperature

Sodium bromide (NaBr) is an ionic compound, and ionic compounds typically form solid crystals at room temperature. Therefore, NaBr is a solid at room temperature.

Write the balanced chemical equation

To write a balanced chemical equation, we must first make sure that the number of atoms of each element is the same on both sides. The reaction is: 2Na(s) + Br₂(l) → 2NaBr(s) (b)

Find the reactant involved in the combustion reaction

In the combustion of a hydrocarbon, oxygen (O₂) from the air is the other reactant involved besides the hydrocarbon itself.

Determine the products formed

The products formed in the combustion of any hydrocarbon are carbon dioxide (CO₂) and water (H₂O).

Write the balanced chemical equation for the combustion of benzene

To write a balanced chemical equation for the combustion of benzene (C₆H₆), we must first balance the number of carbon, hydrogen, and oxygen atoms on both sides. The unbalanced equation is: C₆H₆(l) + O₂(g) → CO₂(g) + H₂O(l) Balancing the equation, we get: \( 2 \mathrm{C}_{6} \mathrm{H}_{6}(l) + 15 \mathrm{O}_{2}(g) \to 12 \mathrm{CO}_{2}(g) + 6 \mathrm{H}_{2} \mathrm{O}(g) \)

Key concepts

Balancing Chemical Equations

Understanding how to balance chemical equations is a cornerstone of studying chemistry. It ensures that the Law of Conservation of Mass is upheld, meaning the mass of reactants equals the mass of products. In simpler terms, what goes into a chemical reaction must come out, just in a different form.

Suppose you have a chemical reaction that initially looks like this:

Unbalanced Reaction Example

Na + Br₂ → NaBr.
On the left, we have one sodium (Na) atom and two bromine (Br) atoms. On the right, we end up with one compound, NaBr, which only accounts for one bromine atom. To balance it, we ensure the number of atoms of each element is equal on both sides of the equation. Balancing leads to this equation:
2Na + Br₂ → 2NaBr.
Now, the equation is balanced, with two sodium atoms and two bromine atoms on each side. Balancing equations often includes adding coefficients (numbers in front of molecules), and sometimes requires adjusting the states of matter (solid, liquid, gas) for accuracy and context.

Combustion of Hydrocarbons

The combustion of hydrocarbons is an exothermic reaction wherein hydrocarbon molecules react with oxygen to produce carbon dioxide and water while releasing heat. This type of reaction is commonly observed in everyday life, from gasoline engines to natural gas heating.

In the case of burning hydrocarbons, also referred to as burning fuels, we require sufficient oxygen. The general formula for the combustion of a hydrocarbon like benzene (C₆H₆) can be quite complex due to the large number of carbon and hydrogen atoms:

Example Combustion of Benzene

2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O.
This equation shows how benzene reacts with oxygen to produce twelve molecules of carbon dioxide and six molecules of water. To understand this process, one must grasp the stoichiometry involved, which includes determining the proper coefficients to balance the equation so that the atoms of each element are conserved.

Formation of Ionic Compounds

Ionic compounds are formed when metals transfer electrons to non-metals, creating ions that attract each other due to opposite charges. The metal loses electrons to become a positively charged ion, while the non-metal gains electrons to become a negatively charged ion.

Consider sodium (Na), a metal that loses one electron to form a Na⁺ ion, and bromine (Br₂), a non-metal that gains one electron per atom to form two Br⁻ ions. They combine in a one-to-one ratio to create sodium bromide (NaBr), a neutral compound:

Formation of Sodium Bromide

Na⁺ + Br⁻ → NaBr.
Ionic compounds typically form solid crystals at room temperature because the strong electrostatic forces, known as ionic bonds, hold the ions in a rigid structure. The ability to predict the formula of ionic compounds comes from understanding the charges of the ions and how they combine to form neutral compounds.