Dans un réacteur dont le volume intérieur est 1,7 litre on...

Dans un réacteur dont le volume intérieur est 1,7 litre on introduit $3,82 \mathrm{~g}$ d'un alliage de zinc et de cuivre (laiton), puis $120 \mathrm{~mL}$ d'une solution d'acide chlorhydrique à 1 mol.L ${ }^{-1}$. Aussitôt après le réacteur est fermé hermétiquement et mis en relation avec un manomètre à mercure ( $c f$. schéma). La température ambiante est $18{ }^{\circ} \mathrm{C}$. L'acide chlorhydrique attaque seulement le zinc ; l'équation stœechiométrique de cette réaction s'écrit : $$ \mathrm{Zn}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})=\mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g}) . $$ On observe que la pression dans le réacteur augmente, puis se stabilise. Après retour à la température initiale (la réaction produit un échauffement) la dénivellation entre les deux branches du manomètre est $20 \mathrm{~cm}$. Peut-on en déduire la composition de l'alliage ( $\%$ en masse de $\mathrm{Zn}$ et $\mathrm{Cu}$ )?

Dans un réacteur dont le volume intérieur est 1,7 litre on introduit $3,82 \mathrm{~g}$ d'un alliage de zinc et de cuivre (laiton), puis $120 \mathrm{~mL}$ d'une solution d'acide chlorhydrique à 1 mol.L ${ }^{-1}$. Aussitôt après le réacteur est fermé hermétiquement et mis en relation avec un manomètre à mercure ( $c f$. schéma). La température ambiante est $18{ }^{\circ} \mathrm{C}$.
L'acide chlorhydrique attaque seulement le zinc ; l'équation stœechiométrique de cette réaction s'écrit :
$$
\mathrm{Zn}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})=\mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g}) .
$$

On observe que la pression dans le réacteur augmente, puis se stabilise. Après retour à la température initiale (la réaction produit un échauffement) la dénivellation entre les deux branches du manomètre est $20 \mathrm{~cm}$.
Peut-on en déduire la composition de l'alliage ( $\%$ en masse de $\mathrm{Zn}$ et $\mathrm{Cu}$ )?

Key Concepts

Stoichiometry

Stoichiometry is the quantitative analysis of reactants and products in a chemical reaction. It involves using the balanced chemical equation to relate the amounts (in moles) of substances involved, determining how much product can be formed based on the given quantities of reactants, and identifying the limiting reagent, which is the reactant entirely consumed during the reaction.

Limiting Reagent Analysis

Limiting reagent analysis is the method used to determine which reactant is totally consumed in a reaction, thereby limiting the extent of the reaction and the maximum yield of products. This concept is critical for calculating the theoretical amount of products formed, based on the initial chemical quantities.

Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. This law is essential for calculating the amount of gaseous product formed or consumed in a reaction when the other conditions (pressure, volume, and temperature) are known.

Manometric Pressure Measurement

Manometric pressure measurement involves the use of a manometer, a device that measures the pressure of gases by balancing the weight of a column of liquid against the pressure exerted by the gas. Analyzing the height difference in the liquid column allows for the quantification of the gas pressure generated in a reaction.

Chemical Composition Analysis in Alloys

Chemical composition analysis in alloys involves determining the proportion of different elements present in a metal mixture. When a reaction specifically targets one component in the alloy, the amount of product formed can be used to back-calculate the percentage by mass of that component, thereby revealing the overall composition of the alloy.

Transcript

00:01 So for brass, it's an alloy between a zinc and copper.

00:04 So we have a 0 .5065 grams of brass alloy.

00:09 And then we're going to read this alloy with excess hydrocar acid.

00:14 So for this alloy contains zinc and copper, only zinc.

00:20 We're going to react with hydrochlor acid, but copper will stay the same.

00:24 We'll stay inert.

00:25 So when the zinc is going to read it and we find that it will give 0 .095 gram of zinc chloride.

00:34 And when we are using an excess hydrochlor acid.

00:38 Okay, so for part a, what would be the percentage composition of brass alloy? and second, how to check the result if they're changing the above procedure.

00:48 So let's start with part a.

00:50 Again, we are going to convert all the mass to lombone modes.

00:56 So we're going to convert the mass of sincoride to number of mole of synchloride.

01:03 So for sing, we have one sync plus two cori, and we should be able to find that is equal to 7 .23 times 10 to the power of four moles.

01:20 All right, so from here, from the reaction equation, we can see that the molecule is in cori and saying is 1 to 1.

01:28 So therefore, the number of mode of synch, so we found a little bit.

01:33 So we're looking at the number of mole of sync, it equals to 7 .23 times 10 to power of left for most.

01:43 And then we can find the corresponding mass...

Please give Ace some feedback