2 Sodium carbonate reacts with hydrochloric acid in an exothermic reaction.
The equation for the reaction is:
Na2CO3(s) + 2 HCl(aq) → 2 NaCl(aq) + CO2(g) + H2O(l)
A student investigated the effect of changing the mass of sodium carbonate powder on the highest temperature reached by the reaction mixture.
2-1 Plan a method to investigate the effect of changing the mass of sodium carbonate powder on the highest temperature reached.
[6 marks]
ANSWER
Following steps are to be followed:-
Measure volume of HCl with a measuring cylinder;
Pour HCl into a suitable container eg. Polystyrene cup;
Measure the initial temperature of HCl with a thermometer;
Measured sodium carbonate with a balance and then add it;
Stir the solution well;
Measure the highest temperature reached;
Repeat with different masses of sodium carbonate or add successive masses of sodium carbonate to the same mixture;
Repeat the whole investigation using the same starting temperature;
Use the same volume and concentration of HCl each time.
Figure 3 shows a line of best fit drawn through the student’s results
Figure 3
2-2 Determine the gradient of the line of best fit in Figure 3.
Give the unit.
[5 marks]
ANSWER
2-3 The initial temperature of the reaction mixture is where the line of best fit would meet the y-axis.
Determine the initial temperature of the reaction mixture. Show your working on Figure 3.
[2 marks]
ANSWER
Extrapolates line to the y-axis 20.6 (°C) as shown above.
Another student repeated the investigation but added sodium carbonate until the sodium carbonate was in excess.
Which sketch graph shows the results obtained when sodium carbonate was added until in excess?
ANSWER
C
Figure 4 shows a reaction profile for the reaction of sodium carbonate with hydrochloric acid
Figure 4
What do labels X and Y represent on Figure 4?
[2 marks]
ANSWER
X = Energy;
Y= overall energy change.
2-6 How does the reaction profile show that the reaction is exothermic?
Use Figure 4.
[1 mark]
ANSWER
The level of products is below level of reactants.
