A chemical reaction involves the transformation of some substances into different substances. These reactions can be either spontaneous or nonspontaneous.
Spontaneous reactions occur due to a positive feedback loop called a reaction cycle. A reaction cycle requires the following three components:
The reactants in a chemical reaction are the starting materials that get transformed into the products. The compounds in the above diagram are the reactants for the above reaction cycle.
An oxidation-reduction (or redox) reaction is a specific type of spontaneous chemical reaction where the molecules undergoing this change carry electric charges which are opposite in sign. For example, when hydrogen gas undergoes oxidation, it loses its charge and becomes hydrogen ions (H+).
There are two types of redox reactions: reduction and oxidation. In a reduction reaction, something gains an electron(s) and in an oxidation reaction, something loses an electron(s).
Look at the reaction equations
The reaction between hydrochloric acid and sodium hydroxide is written as follows:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
In this reaction, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. The driving force for this reaction is the formation of a more stable product: sodium chloride.
When molecules or atoms of different compounds interact, they may form a new compound. The new compound may be more stable, or have properties that are more desirable. In this case, the molecules of sodium chloride are more desirable than the products of the reaction between hydrochloric acid and sodium hydroxide.
The molecules of sodium chloride are less reactive and less volatile than the products of the reaction between hydrochloric acid and sodium hydroxide. Therefore, the chemical reactions producing these compounds are more stable.
Understand the reaction mechanisms
The reaction between hydrochloric acid and sodium hydroxide is a neutralization reaction. This means that the reactions both produce an ionic compound and consume an ionic compound.
In this case, the sodium hydroxide reacts with the hydrochloric acid to form sodium chloride and water. The chemical reactions that occur are:
Sodium hydroxide is an electrolyte, which means it dissolves in water. Hydrochloric acid is not an electrolyte, which means it does not dissolve in water. When these two compounds react, the result is a solid material that does not dissolve in water.
The first step in understanding the driving force for this reaction is to recognize that part of the NaOH actually does dissolve in the HCl. This is because part of the NaOH molecule dissociates into Na+ and O− ions when it encounters H+ ions from the HCl.
Predict the direction of the reaction
Once you understand what is driving the reaction, you can predict the direction of the reaction. If the Na+ ions are more reactive, then the sodium hydroxide will be consumed in the reaction.
If HCl is more reactive, then the hydrochloric acid will be consumed in the reaction. If both reagents are equally reactive, then there will be a double displacement reaction where both reagents are consumed.
Knowing whether a chemical compound dissociates or not depends on its electron configuration. Ionic compounds with electronegative atoms such as oxygen and nitrogen tend to dissociate in water, for example. Non-ionic compounds such as benzene do not dissociate in water.
All of these details matter when predicting the direction of a chemical reaction.
List conditions that favor the direct reaction
The direct reaction is favored when the concentrations of the reactants are high, the solutions are pure, and there is an adequate amount of energy to drive the reaction. The reactions that produce hydrogen gas are especially dependent on energy.
The reaction between hydrochloric acid and sodium hydroxide is no different. In order for this reaction to occur, the chemicals must be in a liquid form and at the same concentration.
If one of the chemicals is more concentrated than the other, then a different reaction will take place. For example, if you have very concentrated sodium hydroxide (soap) and very concentrated hydrochloric acid (HCl) then a new product will form: calcium chloride (CaCl2). This is because there is not enough energy to break down both compounds and create a new product.
List conditions that favor the reverse reaction
When the pH of the solution is higher, the concentration of hydronium ions is higher, the reaction will tend to be spontaneous due to the higher concentration of AHA molecules that will disassociate.
When the concentrations of both NaOH and HCl are higher, then again, the reaction will tend to be spontaneous due to the higher concentration of NaOH molecules that will disassociate and HCl molecules that will react.
When the temperature is higher, then again, the reaction will tend to be spontaneous due to the increased energy of particles which can make this reaction occur.
Having a larger surface area for the NaOH and HCl to interact with each other can also favor the reaction occurring. This is because there is more opportunity for collision between particles.
Know how to stop the reaction once it starts
Once the HCl and NaOH are mixed, the reaction begins almost instantly. The reaction can be stopped by adding a liquid that is more concentrated than the NaOH solution.
For example, if the solution was made with water, then ice would be added to stop the reaction. As ice melts, it absorbs some of the NaOH, making it more concentrated. This then reverses the reaction to produce white snow!
The addition of any solid substance will also stop the reaction. Once this substance is added, all traces of sodium chloride are no longer present. The substance itself may look like it was melted or dried up, but it was able to prevent any further reactions from occurring.
These tips can be useful if a chemical emergency occurs and you need to quickly stop the reaction between these two chemicals.