0.7 Representing chemical change  (Page 6/6)

The following video explains some of the concepts of balancing chemical equations.

Balancing more advanced chemical equations

Write balanced equations for each of the following reactions:

1. ${\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{Al}}_2\left({\mathrm{SO}}_4\right){}_3\left(\mathrm{aq}\right)+3{{H}}_2{O}\left(\mathrm{l}\right)$
2. ${\mathrm{Mg(OH)}}_2\left(\mathrm{aq}\right)+{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to \mathrm{Mg}\left({\mathrm{NO}}_3\right){}_2\left(\mathrm{aq}\right)+2{{H}}_2{O}\left(\mathrm{l}\right)$
3. Lead (II) nitrate solution reacts with potassium iodide solution.
4. When heated, aluminium reacts with solid copper oxide to produce copper metal and aluminium oxide ( ${\mathrm{Al}}_2{\mathrm{O}}_3$ ).
5. When calcium chloride solution is mixed with silver nitrate solution, a white precipitate (solid) of silver chloride appears. Calcium nitrate ( $\mathrm{Ca}\left({\mathrm{NO}}_3\right){}_2$ ) is also produced in the solution.

Balanced equations are very important in chemistry. It is only by working with the balanced equations that chemists can perform many different calculations that tell them what quantity of something reacts. In a later chapter we will learn how to work with some of these calculations. We can interpret balanced chemical equations in terms of the conservation of matter, the conservation of mass or the conservation of energy.

Summary

• A chemical equation uses symbols to describe a chemical reaction.
• In a chemical equation, reactants are written on the left hand side of the equation and the products on the right. The arrow is used to show the direction of the reaction.
• When representing chemical change, it is important to be able to write the chemical formula of a compound.
• In any chemical reaction, the law of conservation of mass applies. This means that the total atomic mass of the reactants must be the same as the total atomic mass of the products. This also means that the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the product.
• If the number of atoms of each element in the reactants is the same as the number of atoms of each element in the product, then the equation is balanced .
• If the number of atoms of each element in the reactants is not the same as the number of atoms of each element in the product, then the equation is not balanced .
• In order to balance an equation, coefficients can be placed in front of the reactants and products until the number of atoms of each element is the same on both sides of the equation.

End of chapter exercises

1. Propane is a fuel that is commonly used as a heat source for engines and homes. Balance the following equation for the combustion of propane: $${\mathrm{C}}_3{\mathrm{H}}_8\left(\mathrm{l}\right)+\mathrm{O}{}_2\left(\mathrm{g}\right)\to \mathrm{CO}{}_2\left(\mathrm{g}\right)+\mathrm{H}{}_2\mathrm{O}\left(\mathrm{l}\right)$$
2. Aspartame, an artificial sweetener, has the formula ${\mathrm{C}}_{14}{\mathrm{H}}_{18}{\mathrm{N}}_2{\mathrm{O}}_2$ . Write the balanced equation for its combustion (reaction with ${\mathrm{O}}_2$ ) to form ${\mathrm{CO}}_2$ gas, liquid $\mathrm{H}{}_2\mathrm{O}$ , and ${\mathrm{N}}_2$ gas.
3. ${\mathrm{Fe}}_2\left({\mathrm{SO}}_4\right){}_3+\mathrm{K(SCN)}\to {\mathrm{K}}_3{\mathrm{Fe(SCN)}}_6+{\mathrm{K}}_2{\mathrm{SO}}_4$
4. Chemical weapons were banned by the Geneva Protocol in 1925. According to this protocol, all chemicals that release suffocating and poisonous gases are not to be used as weapons. White phosphorus, a very reactive allotrope of phosphorus, was recently used during a military attack. Phosphorus burns vigorously in oxygen. Many people got severe burns and some died as a result. The equation for this spontaneous reaction is: $${\mathrm{P}}_4\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{P}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)$$
   1. Balance the chemical equation.
   2. Prove that the law of conservation of mass is obeyed during this chemical reaction.
   3. Name the product formed during this reaction.
   4. Classify the reaction as endothermic or exothermic. Give a reason for your answer.
   5. Classify the reaction as a synthesis or decomposition reaction. Give a reason for your answer.
   (DoE Exemplar Paper 2 2007)
5. Mixing bleach ( $\mathrm{NaOCl}$ ) and ammonia (two common household cleaners) is very dangerous. When these two substances are mixed they produce toxic chloaramine ( ${\mathrm{NH}}_2\mathrm{Cl}$ ) fumes. Balance the following equations that occur when bleach and ammonia are mixed:
   1. $\mathrm{NaOCl}\left(\mathrm{aq}\right)+{\mathrm{NH}}_3\left(\mathrm{aq}\right)\to {\mathrm{NaONH}}_3\left(\mathrm{aq}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$
   2. If there is more bleach than ammonia the following may occur: $\mathrm{NaOCl}+{\mathrm{NH}}_3\to \mathrm{NaOH}+{\mathrm{NCl}}_3$
      Nitrogen trichloride ( ${\mathrm{NCl}}_3$ ) is highly explosive.
   3. If there is more ammonia than bleach the following may occur: ${\mathrm{NH}}_3+\mathrm{NaOCl}\to \mathrm{NaOH}+{\mathrm{NH}}_2\mathrm{Cl}$
      These two products then react with ammonia as follows:
      ${\mathrm{NH}}_3+{\mathrm{NH}}_2\mathrm{Cl}+\mathrm{NaOH}\to {\mathrm{N}}_2{\mathrm{H}}_4+\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$
      One last reaction occurs to stabilise the hydrazine and chloramine: ${\mathrm{NH}}_2\mathrm{Cl}+{\mathrm{N}}_2{\mathrm{H}}_4\to {\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{N}}_2$
      This reaction is highly exothermic and will explode.
6. Balance the following chemical equation: ${\mathrm{N}}_2{\mathrm{O}}_5\to {\mathrm{NO}}_2+{\mathrm{O}}_2$
   
7. Sulphur can be produced by the Claus process. This two-step process involves reacting hydrogen sulphide with oxygen and then reacting the sulphur dioxide that is produced with more hydrogen sulphide. The equations for these two reactions are:
   $\begin{array}{c}{\mathrm{H}}_2\mathrm{S}+{\mathrm{O}}_2\to {\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\\ {\mathrm{H}}_2\mathrm{S}+{\mathrm{SO}}_2\to \mathrm{S}+{\mathrm{H}}_2\mathrm{O}\end{array}$
   Balance these two equations.