Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

# AS Enthalpy Change of a Displacement Reaction

The enthalpy change of a displacement reaction

Part 1 – zinc + Copper Sulfate

word document

Zinc is more reactive than copper, so when zinc is added to copper (II) sulfate solution, copper is displaced. This reaction is associated with an enthalpy change and enthalpy is converted into heat energy. The heat energy warms up the surroundings (which in this case is the waste in which the reaction takes place.

Materials:

• watch glass or weighing boat
• styrofoam (expanded polyestyrene) cup
• thermometer
• timer

Chemicals:

• 6 g zinc powder
• 25 cm3 of 1.0 M Copper sulfate solution

Procedures:

1. Measure out 25 cm3 of 1.0 M copper (II) sulfate solution into an expanded polyestirene cup
2. place the thermometer and start the timer. Measure the temperature every 30 seconds for 3 minutes.
3. At 3 minutes (When the temperature is constant), add all the zinc powder to the cup and stir carefully with the thermometer. You can use a styrofoam lid.
4. continue to stir and record the temperature every 30 seconds until the temperature has passed its maximum and has been falling for at least 2 minutes.
5. Note any changes to the appearance of the solution and the solid and then disposed of the contents of the cup into the container provided.

Results:

Record your results (and any additional observations) in your notebook using a data table showing the time and temperatures measured.

 Time (Min) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 Temp (°C) add  zinc

Temperature rise: (ΔT) = Maximum temperature – Temp before zinc was added.

Analysis:

Answer the following questions after the results. Show your work and calculations (Include units)

1. Calculate the number of moles of copper sulfate in 25 cm3 of 1.0M copper sulfate solution
2. Calculate the number of moles of zinc in the mass you used.
3. Write a balanced equation for the reaction of the zinc and copper sulfate solution.
4. How many moles of zinc would react with one mole of copper sulfate?
5. By considering your answers in 1, 2 and 4. state which is the reagent in excess.
6. How many moles of each substance will actually react?
7. The energy required to raise the temperature of 1 g of water by 1 °C is 4.18 J. (we will use 4.2 J in this lab). Calculate the energy required to raise the temperature of 25 cm3 of your solution by Δt by using the equation:
 Q = m x c x Δt

Note: assume that the density of the solution is 1gcm-3, and so, the mass of the water is 25g.

8. Where did the energy come from?
9. Calculate the enthalpy change for this displacement reaction in kJ mol-1
10. Is the reaction exothermic or endothermic? Which willl be the ΔH sign?
11. The accepted value of ΔH for this reaction is -219 kJ mol-1. Give two reasons for the difference between your answer and this value.

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Part 2 – Citric acid + Sodium Hydrogen Carbonate

In the second part of the lab, you will repeat the procedure by adding 8 g of Sodium bicarbonate to 25 cm3 of a solution 1.0M of Citric acid.

This is an example of a common chemical reaction

 ACID + CARBONATE –> SALT + WATER + CARBON DIOXIDE

CITRIC ACID + SODIUM BICARBONATE —-> SODIUM CITRATE + WATER + CARBON DIOXIDE

H3C6H5O7(aq) + 3NaHCO3(aq) –> Na3C6H5O7(aq) + 3CO2(g) + 3H2O(l)

The procedures are exactly the same. Build a second table and fill out the information in the same way.

Analysis:

Answer the following questions after the results. Show your work and calculations (Include units)

1. Calculate the number of moles of Citric acid in 25 cm3 of 1.0M solution
2. Calculate the number of moles of sodium bicarbonate in the mass you used.
3. Write a balanced equation for the reaction.
4. How many moes of sodium bicarbonate would react with one mole of citric acid?
5. By considering your answers in 1, 2 and 4. State which is the reagent in excess.
6. How many moles of each substance will actually react?
7. The energy required to raise the remperature of 1 g of water by 1 C is 4.18 J. (we will use 4.2 J in this lab). Calculate the energy required to raise the temperature of 25 cm3 of your solution by Δt by using the equation:
 Q = m x c x Δt

Note: assume that the density of the solution is 1g/cm3, and so, the mass of the water is 25g.

8. Where did the energy come from?
9. Calculate the enthalpy change for this displacement reaction in kJ mol-1
10. Is the reaction exothermic or endothermic? Which willl be the ΔH sign?
11. The accepted value of ΔH for this reaction is 70 kJ/mol. Give two reasons for the difference between your answer and this value.