show cause and effect in chemical reaction

 Buffer

Why do we care about the pH of a solution? Most of the body’s cells only function within a very narrow range near neutral pH. Enzymes that help us grow and reproduce, break down the food we eat, and assist in other vital functions each work within a specific, narrow pH range. To help maintain this pH, buffers are present in nearly all living solutions.

A buffer is any substance that minimizes change in the pH of a solution. Most buffers consist of a combination of a weak acid and the weak base, where the base is the anion remaining after the weak acid dissociates (separates) to release H⁺ ions. This may sound confusing, but it is actually a fairly simple back and forth reaction where the buffer acts as a “friend” to H⁺ ions when necessary, but also to OH^- ions when necessary, maintaining a constant balance in the pH value.

Imagine a family with three children. If the oldest child and the youngest child tend to fight a lot, the middle child often acts as a “buffer” between the two fighting children. When we say buffer in this situation, we mean that the middle child will play older kid games at times when the oldest child needs attention and play simpler, younger kid games at times when the youngest child needs attention. By shifting to meet the needs of each child, the middle child buffers the situation, resulting in less angry children. This may not be optimal for the middle child buffer, but it makes the parents happier!     

if we add a strong acid or strong base to water, the pH will change dramatically. For instance, adding a strong acid such as HCl to water results in the reaction HCl + H₂O → H₃O⁺ + Cl^-. In other words, the proton (H⁺) from the acid binds to neutral water molecules to form H₃O⁺ raising the concentration of H⁺. The resulting large concentration of (H⁺) makes the solution more acidic and leads to a dramatic drop in the pH.

                                    

Solid NaOH consists of Na⁺ and OH^- ions packed into a crystalline lattice. When this solid is added to water, the ions float apart leading to extra OH^- ions in the water: NaOH → OH^- + Na⁺. The resulting large concentration of OH^- makes the solution more basic and leads to a dramatic increase in the pH. (Remember that since the product of concentrations, [OH^-][H⁺], remains fixed at K_w=10^-14, as the concentration of OH^- ions goes up, the concentration of H⁺ ions goes down.)     

                               

If we mix a weak acid (HA) with its conjugate base (A^-), both the acid and base components remain present in the solution. This is because they do not undergo any reactions that significantly alter their concentrations. The acid and conjugate base may react with one another, HA + A^- → A^- + HA, but when they do so, they simply trade places and the concentrations [HA] and [A^-] do not change. In addition, HA and A^- only rarely react with water. By definition, a weak acid is one that only rarely dissociates in water (that is, only rarely will the acid lose its proton H⁺ to water). Likewise, since the conjugate base A^- is a weak base, it rarely steals a proton H⁺ from water.

So, the weak acid and weak base remain in the solution with high concentrations since they only rarely react with the water. However, they are very likely to react with any added strong base or strong acid.

                               

If a strong base is added to a buffer, the weak acid will give up its H⁺ in order to transform the base (OH^-) into water (H₂O) and the conjugate base: HA + OH^- → A^- + H₂O. Since the added OH^- is consumed by this reaction, the pH will change only slightly.

                        

If a strong acid is added to a buffer, the weak base will react with the H⁺ from the strong acid to form the weak acid HA: H⁺ + A^- → HA. The H⁺ gets absorbed by the A^- instead of reacting with water to form H₃O⁺ (H⁺), so the pH changes only slightly.

                                

Buffers are extremely important to living organisms because most biochemical processes proceed normally only when the pH remains within a fairly narrow range. An excess of H⁺ or OH^- can interfere with the structure and activity of many biomolecules, especially proteins. Therefore, buffers are commonly used in living organisms to help maintain a relatively stable pH. In humans, for example, buffers act to maintain blood pH between 7.35 and 7.45 even though acids and bases are continually being added to and removed from the blood as it travels through the body. The 3 main buffer systems in our bodies are the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system.

In the laboratory, molecular and cellular biologists make extensive use of buffers to stabilize the pH of aqueous solutions. When studying biomolecules in a test tube, the biomolecules may be altered or may behave in ways that are uncharacteristic of their natural behavior if they are in a solution with a pH that is significantly different from the pH of their natural environment.

Reference

http://chemcollective.org/activities/tutorials/buffers/buffers3act

https://en.wikipedia.org

https://dlc.dcccd.edu/biology1-2/ph-and-buffers

http://examples.yourdictionary.com/cause-and-effect-examples.html]