As part of your GCSE chemistry studies, you’ll need to have a good understanding of neutralisation and how it works. One of the most effective ways to achieve this is to conduct your own neutralisation reaction – after all, there’s no substitute for practical, hands-on experience.
In this article we explain what neutralisation is and provide step-by-step instructions on how to conduct a simple neutralisation experiment using hydrochloric acid and calcium hydroxide powder.
In this post:
What is neutralisation?
Neutralisation is the chemical reaction between an acid and a base. It’s generally an exothermic reaction that produces salt and sometimes water (the latter is generally produced if the reactants both have a strong dissociation constant).
The generalised equation for a neutralisation reaction is:
As the name implies, the products of a neutralisation reaction have a neutral pH. This is especially true for strong acids and strong bases that are completely balanced. Of course, there are exceptions when it comes to reactions that involve either a weak acid or a weak base, or both. Depending on which is the stronger reactant, the resulting salt can either be slightly acidic or basic when dissolved in water.
Neutralisation reactions happen all around us every day, often without us even realising. Human blood, for instance, maintains its slightly basic pH balance thanks to the neutralisation reaction of a buffer of carbonic acid and bicarbonate anion.
Why do we use neutralisation?
As well as playing a vital role in many biological functions, neutralisation has several industrial and commercial applications. A key part of the water treatment process, neutralisation reactions help to remove potentially harmful contaminants from wastewater.
The agricultural industry also uses neutralisation to maintain the ideal soil pH (lime is often added to neutralise acidic soil). Even everyday human activities like brushing your teeth, washing your hair and doing the laundry rely on neutralisation.
How to write neutralisation equations in chemistry
When writing neutralisation equations in chemistry, it’s important to make sure the equation is balanced. This means the total number of elements on the reactant side should equal the total number of elements on the product side. The two sides of the equation must be separated by a chemical reaction arrow, which indicates the flow of the reaction.
Here is an example of a neutralisation reaction written as a balanced equation:
H2SO4+2KOH → K2SO4+2H2O
In this balanced chemical equation, the reactants are sulphuric acid and potassium hydroxide. Meanwhile, the products are potassium sulphate and water. As you can see, the elements on either side of the equation are balanced – there are four hydrogen atoms, one sulphur atom, five oxygen atoms, and two potassium atoms.
What’s the objective of this experiment?
The main objective of the below experiment is to demonstrate a neutralisation reaction. Hydrochloric acid (the acid) and calcium dioxide (the base) are used as reactants.
Slowly adding the base to the acid solution enables you to monitor the gradual change in pH and the completion of the neutralisation reaction. You can then record and plot the results in a graph.
Equipment & tools needed
To perform this experiment, you’ll need to use the following equipment and tools:
- A 500ml beaker
- Universal indicator paper
- A timer
- A pH scale (pH metre)
- Smaller beaker for the powder
- Glass stirring rod
- Triple beam balance or a digital gram scale
Step 1: Dilute hydrochloric acid into a beaker
Typically, the laboratory-grade hydrochloric acid solution that’s used as a reagent is 37% concentration by mass (equivalent to 12 mol/L). It doesn’t matter how much you dilute the acid, as long as you know the precise concentration.
What matters here is the change in pH that you can measure. For example, if you have 50 mL laboratory-grade hydrochloric acid, you can reduce the concentration to one-tenth of the original by adding 450 mL pure water.
Step 2: Add the liquid to universal indicator paper
Use the paper indicator to measure the pH of the liquid solution you’ve diluted. Although this measurement won’t be precise, it will indicate whether the solution is highly acidic.
Step 3: Set a timer for 30 seconds
You’ll need to set the timer for 30 seconds to precisely and accurately measure the pH level of the solution. This applies to both the indicator paper and the pH scale. This should be ample time to get a good stable reading.
Step 4: Use a pH scale to indicate the pH level
To measure the starting pH, immerse the probe of the pH scale into the acid solution. Note the pH as the baseline initial level. It’s important to check the instrument is properly calibrated beforehand.
Step 5: Add calcium hydroxide powder
Before adding calcium hydroxide to the acidic solution, make sure you’re wearing protective equipment such as rubber gloves, goggles, and a lab gown or apron.
It’s also important to calculate how much calcium hydroxide you’ll need to completely neutralise the acidic solution. To do this, you first need to calculate the molarity based on the balanced equation and the concentration of the acid.
2HCl(aq) + Ca(OH)2(s) → CaCl2(aq) + 2H2O(l)
Step 6: Use the pH scale to record the pH level
Gradually add the calcium hydroxide powder a few grams at a time. After adding each portion, stir the solution and measure the pH level. Record your observations and measurements.
Step 7: Continue these steps until there is no change in pH
Continue adding the powder until there’s no change in the pH level of the solution. Record your results in tabulated form. Putting the data into a graph may also help you to visualise the trend more easily.
Results & analysis
Your results and analysis will largely depend on the initial concentration of the acid and the way you added the powder. Apply what you’ve learned in stoichiometry in your analysis. Generally speaking, there should be a gradual increase in the pH of the acidic solution until it reaches the neutralisation point of pH 7.
Recap
In this activity, you’ll learn first-hand how a neutralisation reaction occurs. As you gradually add the calcium hydroxide powder to the hydrochloric acid solution, you’ll be able to measure the change in the pH level.
How you set up and perform your experiment will significantly affect the results and analysis. It’s also important to do some stoichiometric calculations.
