Exothermic and Endothermic Reactions

1. The reaction between zinc and copper sulfate solution causes the temperature of the solution to rise. Is the reaction exothermic or endothermic?

Answer: exothermic

2. Give an example of an application of an endothermic reaction.

Answer: Cold packs for sports injuries.

3. Give a variable that would affect the temperature change when hydrochloric acid reacts with magnesium metal.

Answer: the concentration of the acid, the volume of the acid, the amount of magnesium added, the surface area of the magnesium, the starting temperature of the acid

4. Define the term "activation energy".

Answer: The minimum energy needed for particles to react.

5. State which quantity is greater for an endothermic reaction: the amount of energy absorbed during bond breaking, or the amount of energy released during bond making.

Answer: The amount of (heat) energy absorbed during bond breaking is greater.

6. When ammonium nitrate (NH4NO3 ) is dissolved in water the temperature of the solution formed decreases. The reaction is NH4NO3(s) + H2O  →  NH4NO3(aq). A student investigated how the mass of ammonium nitrate dissolved affected the temperature change of the solution.

a) Write down whether the dissolving of ammonium nitrate is exothermic or endothermic.

Answer: The process is endothermic because the temperature decreases.

b) Draw a fully labelled reaction profile for the dissolving of ammonium nitrate to form a solution.


c) Describe a method the student could use to investigate how the mass of ammonium nitrate dissolved affects the change in temperature of the water. Your method should give valid results.

Answer: Measure out a given volume of water (10−50 cm3). Measure the temperature of the water with a thermometer. Measure out a given mass of ammonium nitrate using a mass balance. Add the ammonium nitrate to the water and stir (with a glass rod). Measure the lowest or final temperature of the water. Repeat using a different mass of ammonium nitrate.

7. Alcohols are compounds made from carbon, hydrogen and oxygen atoms. The complete combustion of alcohols forms carbon dioxide and water. During the reaction, chemical bonds are broken and new chemical bonds form. The equation for the combustion of the alcohol ethanol is CH3CH2OH + 3O2  →  2CO2 + 3H2O

a) Give the type of chemical bonds that occur in alcohol molecules.

Answer: Covalent bonds because the elements bonded are all non-metals.

b) All combustion reactions are exothermic. Draw a fully labelled reaction profile for the combustion of ethanol to form carbon dioxide and water.


c) The figure below shows the displayed formulae for the complete combustion of ethanol, while the table shows the bond energies involved in the reaction. Use this infromation to calculate the overall energy change for this reaction.

Answer: bonds broken = 5(412) + 348 + 360 + 463 + 3(496) = 4719 ; bonds formed = 4(743) + 6(463) = 5750 ; energy change = bonds broken - bonds formed = 4719 - 5750 = -1031 kJ/mol

d) Explain why this reaction is exothermic overall. Give your answer in terms of bond breaking and bond making.

Answer: More energy is released during bond formation than is absorbed during bond breaking, so there is an overall release of energy into the surroundings.

8. Hydrogen peroxide (H2O2 ) is a very useful compound. It is very reactive due to the single covalent bond between the two oxygen atoms. Hydrogen peroxide decomposes to form water and oxygen. The figure below shows the displayed formulae for the decomposition of hydrogen peroxide.

a) The table below shows the bond energies and the overall energy change for the decomposition of hydrogen peroxide. Calculate the bond energy X for the O–O bond in hydrogen peroxide.

Answer: bonds broken = 4(463) + 2X = 1852 + 2X ; bonds formed = 4(463) + 496 = 2348 ;  -208 = 1852 + 2X - 2348 ; so, X = 144 kJ/mol

b) A solution of hydrogen peroxide has a concentration of 0.125 mol/dm3. Calculate the number of moles of oxygen produced when 50cm 3 of this solution fully decomposes.

Answer: mol H2O2 = concentration × volume = 0.125 × (50 ÷ 1000) = 0.00625 mol ;
mol O2 = 12 × mol H2O2 (2:1 ratio from balanced equation) = 0.003125 mol

c) Calculate the volume, in cm3, of this amount of oxygen gas. The volume of 1 mole of any gas at room temperature and pressure = 24.0 dm3.

Answer: volume = mol × 24.0 = 0.003125 × 24.0 = 0.075 dm3 = 75 cm3