Chemistry » Physical and Chemical Change » Balancing Chemical Equations

State Symbols and Other Information

State Symbols and Other Information

The state (phase) of compounds can be expressed in the chemical equation. This is done by placing the correct label on the right hand side of the formula. The following four labels can be used:

  1. (g) for gaseous compounds

  2. (l) for liquids

  3. (s) for solid compounds

  4. (aq) for an aqueous (water) solution

To show that heat is needed for a reaction, a Greek delta (Δ) is placed above the arrow. For example: \(\text{NH}_{4}\text{Cl} \stackrel{\Delta}{\rightarrow} \text{NH}_{3} + \text{HCl}\)

Example: Balancing Chemical Equations

Question

Solid zinc metal reacts with aqueous hydrochloric acid to form an aqueous solution of zinc chloride \((\text{ZnCl}_{2})\) and hydrogen gas. Write a balanced equation for this reaction.

Step 1: Identify the reactants and products

The reactants are zinc \((\text{Zn})\) and hydrochloric acid \((\text{HCl})\). The products are zinc chloride \((\text{ZnCl}_{2})\) and hydrogen \((\text{H}_{2})\).

Step 2: Write the equation

\[\text{Zn} + \text{HCl} \rightarrow \text{ZnCl}_{2} + \text{H}_{2}\]

Step 3: Balance the equation

You will notice that the zinc atoms balance but the chlorine and hydrogen atoms do not. Since there are two chlorine atoms on the right and only one on the left, we will give \(\text{HCl}\) a coefficient of \(\text{2}\) so that there will be two chlorine atoms on each side of the equation.

\[\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_{2} + \text{H}_{2}\]

Step 4: Check that all the atoms balance

When you look at the equation again, you will see that all the atoms are now balanced.

Step 5: Ensure all details (e.g. state symbols) are added

In the initial description, you were told that zinc was a metal, hydrochloric acid and zinc chloride were in aqueous solutions and hydrogen was a gas.

\[\text{Zn (s)} + \text{HCl (aq)} \rightarrow \text{ZnCl}_{2}\text{(aq)} + \text{H}_{2}\text{(g)}\]

Optional Video: Balancing Chemical Equations

Optional Experiment: The relationship between product and reactant

Aim

To investigate the relationship between the amount of product and the amount of reactant.

Apparatus

  • flask

  • measuring cylinder

  • water bowl

  • delivery tube

  • funnel with stopcock

  • stopper

  • sodium hydrogen carbonate \((\text{Na}(\text{HCO}_{3})\) powder

  • dilute sulfuric acid \((\text{H}_{2}\text{SO}_{4})\)

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Method

  1. Weigh \(\text{20}\) \(\text{g}\) of \(\text{NaHCO}_{3}\) and place it into a flask.

  2. Set up the above apparatus.

  3. Measure out \(\text{5}\) \(\text{mL}\) of \(\text{H}_{2}\text{SO}_{4}\) and carefully pour this into the funnel (make sure that the stopcock is closed).

  4. Slowly add the \(\text{H}_{2}\text{SO}_{4}\) to the \(\text{NaHCO}_{3}\) by opening the stopcock.

  5. Observe what happens.

  6. Record the volume of gas collected in the measuring cylinder.

  7. Repeat the above steps but this time use \(\text{10}\) \(\text{mL}\) of \(\text{H}_{2}\text{SO}_{4}\).

  8. Write a balanced equation for this reaction. (Hint: carbon dioxide gas is formed, as well as water and sodium sulfate.)

Results and discussion

You should observe that more gas is formed when using more \(\text{H}_{2}\text{SO}_{4}\).

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