Question

a student investigated temperature changes during a reaction between citric acid and sodium hydrogencarbonate solution. HOW do the results in table 4 show that the reaction is endothermic?

Answer 1

Answer:

<<Why is the reaction between sodium hydrogen carbonate and hydrochloric acid endothermic?>>

The question is undefined!

You don't specifiy if  NaHCO3  is solid or acqueous (dissolved in water) but, mostly, you don't specifiy if hydrochloric acid is gaseous or in water solution.

Furthermore, if the reaction happens in a container with excess water, the  NaCl  formed could be considered solid or acqueous (it depends on how much water there is and how much you wait...)

So:

ΔHr=−411−286−394−(−951−166)=  

=26 kJ mol−1  

It's a positive value so it's endothermic.

The reason for this is that in order for the reaction to happen, you have to give energy to break:

The ionic lattice  Na++HCO−3  

One  C−O  bond in  HCO−3  

One  H−Cl  bond in  HCl  

but, on the other end, you get energy only by forming of:

The ionic lattice  Na++Cl−  

One  H−O  bond in  H2O  (the other  H−O  bond was already present in  HCO−3 )

so the net result is a loss of energy.

Now let's see what would be with gaseous  HCl ; since its  ΔH  of formation is  −92 kJ mol−1:  

ΔHr=−411−286−394−(−951−92)=  

=−48 kJ mol−1  

Since it's negative, it's exothermic could be exothermic instead of endothermic!

I imagine you refer to solid  NaHCO3,   NaCl  and acqueous  HCl,  so as follows.