Why can t too much alkali be added during the neutralization heat determination 5

Too much alkali loses the meaning of neutralization.

An excess of a substance is intended to ensure that the base (or acid) can be completely neutralized. If the same amount of acid and alkali is used, as the reaction progresses, the opportunity for H+ and OH- to collide with each other is less and less, and it is more and more difficult, and there may be a part of H+ and OH- that cannot react, and in the case of an excess of particles, it greatly increases the chance of complete reaction of another particle.

In this sense, it is also possible to let the acid be slightly excessive and the alkali to be appropriate.

Upstairs is a bit misleading, the so-called neutralization means that the amount of substances of acids and bases happens to be the same as the stoichiometry of the reaction, regardless of whether the reaction is complete or not. It is not considered that neutralization, for example, is to completely react all the hydrogen ions in the acid solution (which is impossible), or to reach a certain artificially specified pH standard.

Adding too much alkali means that the reaction is not strictly neutralized, and there may be an "excessive" reaction (for weak acids, it is obvious that the measurement results are high, and strong acids are not obvious). Another important reason is that the addition of the base itself is usually exothermic (although we have used a dilute solution in the reaction to avoid this hydration exothermic, too much can still cause unnecessary errors).

If you have any questions, please feel free to ask.

Too much alkali loses the meaning of neutralization, and an excess of a substance is stupid in order to ensure that the alkali (or acid) can be completely neutralized. If the same amount of acid and alkali is used, as the reaction progresses, the opportunity for H+ and OH- to collide with each other is less and less, and it is more and more difficult, and there may be a part of H+ and OH- that can not react, and in the case of excessive exposure of one particle, it greatly increases the chance of complete reaction of another particle. In this sense, it is also possible to let the acid be slightly excessive and the alkali to be appropriate.

In order to ensure that the hydrochloric acid is completely neutralized by NaOH, a solution is used to slightly overdose the alkali. It is not advisable to overdose hydrochloric acid, the reason is that dilute hydrochloric acid is relatively stable, this silver sock takes 50ml, and the amount of its substance is.

The NaOH solution is very easy to absorb carbon dioxide in the air, and if you take exactly 50ml, it is difficult to ensure that there is HCl involved in the reaction to neutralize.

Because sodium hydroxide is often mixed with sodium carbonate.

In order to ensure that the amount of acid and base reacted in the determination process is the set amount, a slight excess of alkali is required.

It can avoid the reaction of acid and sodium carbonate to affect the accuracy of the experiment.

Heat of neutralization: Acid and base neutralize to generate heat emitted by 1 mol of water.

Such as: NAOH

hclh2o

naclnaoh

Easy to work with air.

CO2 reaction is generated.

na2co3

or NaHCO3, while the base reacts with the acid, NACO3 or NAHCO3 with H+

Reaction generation. h2co3。In this case, if the acid-base is the same, the acid cannot be completely neutralized by the alkali. Then we use 1molNaOH according to the original data

If you do this, the neutralizing heat calculated by the measured temperature is not accurate. The alkali is slightly oversupplied so that the acid can be completely neutralized to reduce errors. However, if there is too much acid, then we have to change the formula we use, so the laboratory advocates an excess of alkali.

Because the acid is dissolved in water and exothermic, the neutralization heat needs to be completely neutralized, so the reagent needs to be excessive, but because the exothermic effect is calculated, the alkali is used, which has a better impact on the experiment.

In a dilute solution, the heat of reaction when the acid and base are neutralized to form 1 mol of water is called the heat of neutralization.

1. It must be a dilute solution of acid and alkali, because the concentrated acid solution and the concentrated alkali solution will be exothermic when they are diluted with each other.

2. The reaction of dilute solution of strong acid and strong alkali can ensure that the neutralizing heat of H+(AQ)+OH (AQ)====H2O(L) is 57 3 kJ·mol 1, and weak acid or weak.

After being dissolved in water, they are ionized into freely moving anions and cations in water.

For example, HCl is ionized into hydrogen ions (H+) and chloride ions (Cl-), while NaOH is ionized into sodium ions (Na+) and hydroxide ions (OH-). Hydrogen ions and hydroxide ions combine to form water that is extremely difficult to ionize, so what is left in the solution are sodium ions and chloride ions.

Sodium and chloride ions are still ionized in solution and do not combine. But the product is NaCl.

This is to ensure that the base (or acid) can be completely neutralized.

If the same amount of acid and alkali is used, as the reaction progresses, the opportunity of H+ and OH- collision contact is becoming less and less, and it is more and more difficult, and there may be a part of H+ and OH- that can not react, and in the case of an excess of particles, it greatly increases the chance of complete reaction of another particle.

Note: Do not use excessive acid, because the alkali contains impurities sodium carbonate, and excessive acid will cause acid to react with carbonate and cause inaccurate neutralization heat determination.

Summary. Hello dear, if the water sample only needs to determine the total alkalinity, there is no need to use phenolphthalein as an indicator. Phenolphthalein is a widely used indicator for the determination of pH, which is capable of color change in different pH ranges.

If only the total alkalinity of the water sample needs to be determined, is phenolphthalein needed as an indicator?

Hello, if the water sample only needs to measure the total alkalinity, there is no need to block or use phenolphthalein as an indicator. Phenolphthalein is a widely used indicator for the determination of pH, and it is able to undergo color changes in different pH ranges.

Hello dear, in the process of determining total alkalinity, the use of phenolphthalein indicators often does not provide better accuracy or precision. Conversely, the use of phenolphthalein indicators can also lead to measurement errors or unwanted experimental variability. For this reason, if only the total alkalinity of a water sample needs to be determined, an indicator-free method can usually be used, such as using acid-base titration or potentiometric titration.

Why does phenolphthalein instillation produce measurement errors?

Hello, when using phenolphthalein indicator for titration, because phenolphthalein rolls know that the pH value of the acidic and alkaline color transition range of the indicator is, so in the process of titration, when the phenolphthalein solution changes from pink to red, the pH value in the solution does not change significantly, and the length of the ruler interval of phenolphthalein color transformation is about pH units, so this color change is easily disturbed by other factors, resulting in an increase in the error of the titration results.