Energy Changes (review)

1. Neutralisation reactions take place between acids and alkalis. A student investigated the temperature change during the reaction between sodium hydroxide and hydrochloric acid. They measured 20.0 cm3 of sodium hydroxide solution and placed it in a polystyrene cup. They recorded the temperature of the solution and then added hydrochloric acid to it. The student’s results are shown in the table below.


a) Give the word equation for the reaction between sodium hydroxide and hydrochloric acid.

Answer: sodium hydroxide + hydrochloric acid  →  sodium chloride + water


b) Use the table above to determine whether the reaction between sodium hydroxide and hydrochloric acid is exothermic or endothermic.

Answer: The reaction is exothermic because the temperature of the solution rises. Therefore, energy is being released into the solution during the reaction.


c) Use the table above to suggest what volume of hydrochloric acid was required to neutralise the sodium hydroxide solution.

Answer: 25.0 cm3 of hydrochloric acid. After this volume the temperature stops rising so the neutralisation reaction is over.


d) Another student wanted to investigate how the concentration of the hydrochloric acid solution affected the temperature change during the reaction between sodium hydroxide and hydrochloric acid. Describe a method that the student could use to carry out their investigation. Your method should give valid results.

Answer: A suitable method described, including at least one control variable for valid results: Use a measuring cylinder to measure out a given volume of hydrochloric acid of a certain concentration. Record the temperature of the hydrochloric acid solution using a thermometer. Measure out and add a given volume of sodium hydroxide solution. Stir the mixture with the thermometer and record the (maximum) temperature rise. Repeat these steps using different concentrations of hydrochloric acid solution.
Control variables: the concentration of the sodium hydroxide solution, the volumes of the hydrochloric acid and sodium hydroxide solutions.


2. One method of producing water is to react hydrogen gas with oxygen gas. The figure below shows the displayed formula for this reaction, while the table shows the bond energies involved in the reaction.


a) Calculate the overall energy change for this reaction.

Answer: bonds broken = 2(436) + 496 = 1368  ;  bonds formed = 4(463) = 1852  ;
energy change = bonds broken - bonds formed = 1368 - 1852 = - 484 kJ/mol


b) Draw a fully labelled reaction profile for the reaction between hydrogen and oxygen to form water.

Answer: 


c) Hydrogen fuel cells produce water by reacting hydrogen with oxygen. The fuel cell uses the chemical reaction to produce a voltage. Give the two half equations that take place during the running of a hydrogen fuel cell.

Answer: 2H2  →  4H+ + 4e  and  O2 + 4H+ + 4e  →  2H2O


d) Explain why rechargeable batteries need to be recharged while fuel cells do not.

Answer: In rechargeable batteries the reactants eventually get used up. Recharging the battery reverses the chemical reactions to reform the reactants. Fuel cells have a continuous supply of reactants so do not need recharging.