Energy of Reactions Introduction

In this activity you will be introduced to energy changes in chemical reactions. You will be measuring the temperature changes in four reactions, and classifying the reaction based on whether energy is added or released. At the end you will use the classification to decide if the reactions are endothermic

or exothermic and write an appropriate balanced equation.

Procedures:

Reaction of potassium hydroxide solution with dilute hydrochloric acid

  1. Stand the polystyrene cup in a beaker.
  2. Measure out 10 mL of potassium hydroxide solution and pour it into the cup then record its temperature.
  3. Measure out 10mL of hydrochloric acid and carefully add it to the cup with the potassium hydroxide. Gently stir with the thermometer and record the maximum or minimum temperature.
  4. Discard the mixture and rinse the cup.

Reaction of sodium hydrogen carbonate with acetic acid

  1. Repeat steps 1 and 2 from the previous experiment, using acetic acid solution in place of potassium hydroxide.
  2. Add half a spoonful of sodium hydrogen carbonate to the acetic acid. Gently stir with the thermometer and record the maximum or minimum temperature.
  3. Discard the mixture and rinse the cup.

Reaction of copper(II) sulfate solution and magnesium powder

  1. Repeat steps 1 and 2 from the first experiment, using copper(II) sulfate solution in place of potassium hydroxide.
  2. Add a pea sized amount of magnesium powder to the copper(II) sulfate solution. Gently stir with the thermometer and record the maximum or minimum temperature.
  3. Discard the mixture and rinse the cup.

Reaction of sulfuric acid and magnesium ribbon

  1. Repeat steps 1 and 2 from the first experiment, using sulfuric acid solution in place of potassium hydroxide.
  2. Add a 3 cm piece of magnesium ribbon to the sulfuric acid. Gently stir with the thermometer and record the maximum or minimum temperature.
  3. Discard the mixture and rinse the cup.

Mixing water at different temperatures

  1. Obtain between 40 and 50 mL of cold water and measure its temperature.
  2. Obtain an equal amount of hot water and measure its temperature.
  3. Mix the hot and cold water in a polystyrene cup and record the temperature after 15 seconds.

Teacher demonstration: dissolving ammonium nitrate

  1. Measure about 10 mL of water in a test tube and measure the temperature.
  2. Add half a spoonful of solid ammonium nitrate to the water.
  3. Stir and record the temperature after the ammonium nitrate has dissolved.

Follow up Questions:

  1. For each reaction list if energy was released by or absorbed by the chemicals and describe how you tell. Put this in your data table.
  2. Which reactions were endothermic and which were exothermic.
  3. Write a balanced chemical equation for each reaction. Make sure you also put the word energy on the appropriate side of the equation. The teacher demonstration is just breaking up the compound into its ions, water is not a reactant.
  4. Using the observations from the hot and cold water being mixed answer the questions below.
  1. Imagine that you mix 25 g of water at 25oC with 25 g of water at 65oC. Predict the final temperature of the sample.
  2. Imagine that you mix 25 g of water at 25oC with a larger sample of water at 65oC. Will the final temperature be the same as in question 4a? Why or why not.
  3. Suppose you mix 75 g of water at 15oC with 25 g of water at 75oC. Predict the final temperature from the choices below. Explain your choice.

(1) 30OC(2) 45OC(3) 60OC

Set your data table up:

Procedures / Observations / nrg absorbed or released? / How can you tell? / Endothermic / Exothermic? / Balanced chemical equation

Prelab:

1. Determine if the change was endothermic or exothermic.

a. The temperature of a beaker of water goes from 22oC to 50oC after a chemical was added.

b. Freezing water into ice.

c. Burning a piece of wood.

d. Frost forms on a test tube after a chemical is added to water.

2. Which diagram is endothermic?

a) b)