Acid Base Reactions Help (1 Viewer)

[csi]

Hi guys,

Here are some questions that I need a hand on:

1. Which of the following can react as a B/L base when mixed with water?
(A) HCO3-
(B) Cl-
(C) CH3COOH
(D) NaNO3

2. Describe how phosphine (PH3), oxide (O2-) and the carbonate ion (CO3 2-) act as Arrhenius' bases using reaction equations.

3. Using examples, distinguish between a diprotic and amphiprotic substance using equations.

4. 40mL of a solution of HClO4 was added to neutralise 40mL of a 2 mol L^-1 solution of NaOH. The temperature of the mixture rose from 22.3 to 35.2 degrees celsius. Calculate the enthalpy of neutralisation.

I got really odd numbers for this question, so could you guys please show your working so I know where I've gone wrong? Many thanks!

 

[Eagle Mum]

CH3COOH is an acid as it donates a H+ (CH3COOH <-> CH3COO- + H+).

NaNO3 is a salt.

HCO3- can be both a Brønsted-Lowry base (HCO3- H+ <-> H2CO3 <-> H2O + CO2) as well as a B-L acid (HCO3- <-> CO2 2- + H+).

Cl- is the conjugate base to HCl, but I think the key word in this MCQ is how it reacts in water. The reaction HCl + H2O -> H3O+ + Cl- very much favours the right direction, so unless there is more than one correct answer, the answer would be HCO3- (A)



Phosphine (PH3) is similar in this respect to NH3. In water, it accepts a H+ to become a phosphonium ion so it is an Arrhenius base (PH3 + H2O <-> PH4+ + OH-).

The superoxide O2- also accepts H+ from water molecules so it is an Arrhenius base, but the chemical reaction has to be balanced so oxygen gas is produced as well as the hydroxide ion (4 O2- + 2 H2O → 3 O2 + 4 OH−).

The carbonate ion (CO3 2-) also accepts H+ from the water molecule so it is also an Arrhenius base, noting that it is a two step reversible chemical reaction with bicarbonate as an intermediate species (CO3 2- + 2H+ <-> HCO3- + H+ <-> H2CO3 <-> CO2 + H2O).



In the above example, HCO3- is amphiprotic as it can behave both as an acid, giving up H+ to form CO3 2-, and as a base, accepting H+ to form H2CO3. H2CO3 is a diprotic acid as it can give up two H+ ions (compare with HCl which is a monoprotic acid as it only has one H+ to give).



That’s all the time I have for now (I haven’t checked for typos). I think people could respond more effectively if you post your own responses and request checks rather than post questions which take a while to answer.

 

[csi]

CH3COOH is an acid as it donates a H+ (CH3COOH <-> CH3COO- + H+).

NaNO3 is a salt.

HCO3- can be both a Brønsted-Lowry base (HCO3- H+ <-> H2CO3 <-> H2O + CO2) as well as a B-L acid (HCO3- <-> CO2 2- + H+).

Cl- is the conjugate base to HCl, but I think the key word in this MCQ is how it reacts in water. The reaction HCl + H2O -> H3O+ + Cl- very much favours the right direction, so unless there is more than one correct answer, the answer would be HCO3- (A)



Phosphine (PH3) is similar in this respect to NH3. In water, it accepts a H+ to become a phosphonium ion so it is an Arrhenius base (PH3 + H2O <-> PH4+ + OH-).

The superoxide O2- also accepts H+ from water molecules so it is an Arrhenius base, but the chemical reaction has to be balanced so oxygen gas is produced as well as the hydroxide ion (4 O2- + 2 H2O → 3 O2 + 4 OH−).

The carbonate ion (CO3 2-) also accepts H+ from the water molecule so it is also an Arrhenius base, noting that it is a two step reversible chemical reaction with bicarbonate as an intermediate species (CO3 2- + 2H+ <-> HCO3- + H+ <-> H2CO3 <-> CO2 + H2O).



In the above example, HCO3- is amphiprotic as it can behave both as an acid, giving up H+ to form CO3 2-, and as a base, accepting H+ to form H2CO3. H2CO3 is a diprotic acid (compare with HCl which is a mono pro tic acid) as it can give up two H+ ions.



That’s all the time I have for now (I haven’t checked for typos). I think people could respond more effectively if you post your own responses and request checks rather than post questions which take a while to answer.

View attachment 30171

Thank you

 

[CM_Tutor]

Hi guys,

Here are some questions that I need a hand on:

1. Which of the following can react as a B/L base when mixed with water?
(A) HCO3-
(B) Cl-
(C) CH3COOH
(D) NaNO3

2. Describe how phosphine (PH3), oxide (O2-) and the carbonate ion (CO3 2-) act as Arrhenius' bases using reaction equations.

3. Using examples, distinguish between a diprotic and amphiprotic substance using equations.

4. 40mL of a solution of HClO4 was added to neutralise 40mL of a 2 mol L^-1 solution of NaOH. The temperature of the mixture rose from 22.3 to 35.2 degrees celsius. Calculate the enthalpy of neutralisation.

I got really odd numbers for this question, so could you guys please show your working so I know where I've gone wrong? Many thanks!

Building on what @Eagle Mum has said:

1. I really wish that question writers would take more care. The word "can" means that this is asking whether it is possible for the substances / species to act as a B-L base, and all of them can do this. I am able to write equations for it happening in each case, though in three of the four cases the reaction will be an equilibrium that lies so far to the left that it is reasonable to consider that the reaction does not occur to any appreciable extent in water. A better question would be which of these will act as a B-L base in water, in which case only one will - the bicarbonate ion - and so the answer is indeed A.

2. Eagle Mum is correct about the superoxide ion (O₂^-) acting as an Arrhenius base, but I think the question was actually meaning the oxide ion (O^2-), which also reacts as an Arrhenius base:

O^2- + H₂O -----> 2 OH^-

3. I think this is seeking you to observe that the conjugate base of a diprotic weak acid will be amphiprotic, such as is HCO₃^- an amphiprotic species from the weak diprotic acid H₂CO₃.

I also advise care on the use of "act as" and "react with". An amphiprotic substance is one that is capable of acting as either a L-B acid or base (depending on circumstances). An amphoteric substance is one that is capable of reacting with either an acid or a base (depending on circumstances). Since the reactions of an amphiprotic substance when acting as a L-B acid or base are necessarily with a L-B base or acid (respectively), an amphiprotic substance must be amphoteric. The reverse is not true, however; there are amphoteric substances that can react with both acids and bases but do not do so as L-B acids or bases and so are not themselves amphiprotic.

4. HClO₄ + NaOH -----> NaClO₄ + H₂O

Assuming the density of the solution is 1 g mL^-1, so that m(solution) = 80 mL x 1 g mL^-1 = 80 g and that the mixture is close enough to water to have c_sol'n = 4.184 J K^-1 mol^-1:

 

[csi]

Building on what @Eagle Mum has said:

1. I really wish that question writers would take more care. The word "can" means that this is asking whether it is possible for the substances / species to act as a B-L base, and all of them can do this. I am able to write equations for it happening in each case, though in three of the four cases the reaction will be an equilibrium that lies so far to the left that it is reasonable to consider that the reaction does not occur to any appreciable extent in water. A better question would be which of these will act as a B-L base in water, in which case only one will - the bicarbonate ion - and so the answer is indeed A.

2. Eagle Mum is correct about the superoxide ion (O₂^-) acting as an Arrhenius base, but I think the question was actually meaning the oxide ion (O^2-), which also reacts as an Arrhenius base:

O^2- + H₂O -----> 2 OH^-

3. I think this is seeking you to observe that the conjugate base of a diprotic weak acid will be amphiprotic, such as is HCO₃^- an amphiprotic species from the weak diprotic acid H₂CO₃.

I also advise care on the use of "act as" and "react with". An amphiprotic substance is one that is capable of acting as either a L-B acid or base (depending on circumstances). An amphoteric substance is one that is capable of reacting with either an acid or a base (depending on circumstances). Since the reactions of an amphiprotic substance when acting as a L-B acid or base are necessarily with a L-B base or acid (respectively), an amphiprotic substance must be amphoteric. The reverse is not true, however; there are amphoteric substances that can react with both acids and bases but do not do so as L-B acids or bases and so are not themselves amphiprotic.

4. HClO₄ + NaOH -----> NaClO₄ + H₂O

Assuming the density of the solution is 1 g mL^-1, so that m(solution) = 80 mL x 1 g mL^-1 = 80 g and that the mixture is close enough to water to have c_sol'n = 4.184 J K^-1 mol^-1:

Thank you! Helpful as usual