When the temperature is decreased How would you describe a chemical reaction that produces heat?

You are absolutely right. And also wrong. You are right, when something becomes cooler, heat is released from it. Your mistake is you are looking for your answer from the wrong perspective, I believe.

When you are dealing with calorimetry and heat of reaction, it can seem contradictory at first. You learn that an exothermic reaction is one in which heat leaves, and an endothermic reaction is one in which heat enters the system. Then you go to determine if a process is endo or exo, and it seems backwards. When measuring, say a chemical reaction in water, you are determining whether the change was exothermic or endothermic. It can seem like the answer is exothermic when heat leaves the water. And it is- for the water. You want to determine the change for the chemical reaction. So, if water temperature decreases, then heat has left the water. For the water, this would be exothermic. But for the reaction, heat has entered that system. So the reaction is then, of course, endothermic.

Remember, you are determining heat of the system. Not heat of surroundings. System = chemicals Surroundings = Water

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

Other examples of endothermic processes include:

• Dissolving ammonium chloride in water
• Cracking alkanes
• Nucleosynthesis of elements heavier than nickel in stars
• Evaporating liquid water
• Melting ice

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)

Other examples of exothermic processes include:

• The thermite reaction
• A neutralization reaction (e.g., mixing an acid and a base to form a salt and water)
• Most polymerization reactions
• Combustion of a fuel
• Respiration
• Nuclear fission
• Corrosion of metal (an oxidation reaction)
• Dissolving an acid in water

Many exothermic and endothermic reactions involve toxic chemicals, extreme heat or cold, or messy disposal methods. An example of a quick exothermic reaction is dissolving powdered laundry detergent in your hand with a bit of water. An example of an easy endothermic reaction is dissolving potassium chloride (sold as a salt substitute) in your hand with water.

These endothermic and exothermic demonstrations are safe and easy:

Here's a quick summary of the differences between endothermic and exothermic reactions:

Endothermic and exothermic reactions refer to the absorption or release of heat. There are other types of energy which may be produced or absorbed by a chemical reaction. Examples include light and sound. In general, reactions involving energy may be classified as endergonic or exergonic, An endothermic reaction is an example of an endergonic reaction. An exothermic reaction is an example of an exergonic reaction.

• Endothermic and exothermic reactions are chemical reactions that absorb and release heat, respectively.
• A good example of an endothermic reaction is photosynthesis. Combustion is an example of an exothermic reaction.
• The categorization of a reaction as endo- or exothermic depends on the net heat transfer. In any given reaction, heat is both absorbed and released. For example, energy must be input into a combustion reaction to start it (lighting a fire with a match), but then more heat is released than was required.

• Qian, Y.‐Z., et al. “Diverse Supernova Sources for the r‐Process.” The Astrophysical Journal, vol. 494, no. 1, 10 Feb. 1998, pp. 285-296, doi:10.1086/305198.
• Yin, Xi, et al. “Self-Heating Approach to the Fast Production of Uniform Metal Nanostructures.” Chemistry of Nanomaterials for Energy, Biology and More, vol. 2, no. 1, 26 Aug. 2015, pp. 37-41, doi:10.1002/cnma.201500123.

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In this investigation, students classify chemical reactions as exothermic or endothermic. Next, students explore the relationship between an observed change in temperature and the classification of a change as chemical or physical.

Objective

Students will explore energy changes during chemical reactions, heat of reaction (ΔH), and the connection between energy changes and chemical changes.

Safety

• Be sure you and the students wear properly fitting goggles.
• Acetic acid (vinegar) vapors can be irritating. Work in a well-ventilated area. In the event of eye contact, flush with water. The concentration of acetic acid in this experiment does not present any significant hazards.
• Calcium chloride can be an irritant to body tissues. In the event of contact, wash affected areas with water. Dispose of calcium chloride solutions according to local regulations.

Materials for Each Group

• Vinegar
• Baking soda
• Calcium chloride
• Water
• Thermometer
• 4 small clear plastic cups
• 1 cup measuring cup
• Measuring spoons (1 tablespoon, ½ teaspoon)

Time Required

One class period, approximately 45–50 minutes.

Lab Tips

After students explore one example of an endothermic change and one example of an exothermic change, they are then asked to explore the connection between energy changes and chemical reactions. To do this, students may need some guidance to arrive at the idea that temperature changes may also accompany dissolving.

Students will have an easier time devising a fair test if they are well versed in the definitions of physical changes and chemical changes. Students should propose an experiment to you before they test their hypothesis. To observe a temperature change during a physical change, students should devise a procedure such as:

• Add 10 mL of water to a small plastic cup and place a thermometer in the water. Record the initial temperature (Ti).
• Add ½ teaspoon of calcium chloride to the water and swirl the cup. After it has stopped changing, record the final temperature (Tf).

Pre-Lab Discussion

This investigation introduces the concepts of enthalpy (heat) of ΔH in the context of exothermic and endothermic reactions. To give students a deeper grounding in the basics and reinforce basic concepts covered previously, you may wish to review the mechanics of chemical changes, how to write balanced chemical equations, and the law of conservation of energy.

Incorporating into the Curriculum

This investigation could be incorporated into a unit on chemical changes or thermochemistry.

In this activity, you will explore the energy changes that accompany chemical reactions. To understand the energy implications of chemical reactions, it’s important to keep in mind two key ideas:

1. It takes energy to break bonds.
2. Energy is released when bonds are formed.
   

To understand this, consider the chemical reaction between vinegar (also known as acetic acid to chemists) and baking soda (known as sodium bicarbonate). Before the atoms of acetic acid and sodium bicarbonate can be rearranged to form the products, the bonds between the atoms in those molecules must be broken, and because the atoms are attracted to one another, it takes energy to pull them apart.

Then, when the products are formed (sodium acetate, water, and carbon dioxide) energy is released because atoms that have an attraction for one another are brought back together. Not every bond between atoms in the reactants is necessarily broken during a chemical reaction, but some bonds are.

By comparing the energy used when bonds in the reactants are broken with the energy released when bonds in the products are formed, you can determine whether a chemical reaction releases energy or absorbs energy overall.

Chemical reactions that release energy are called exothermic. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Chemical reactions that absorb (or use) energy are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products. If a chemical reaction absorbs as much energy as it releases, it is called isothermic—there is no net energy change.

But because we can’t observe bonds breaking or being formed, how can we distinguish between exothermic and endothermic chemical reactions?

Identifying Exothermic & Endothermic Reactions

There are two methods for distinguishing between exothermic and endothermic reactions.

1. Monitor temperature change When energy is released in an exothermic reaction, the temperature of the reaction mixture increases. When energy is absorbed in an endothermic reaction, the temperature decreases. You can monitor changes in temperature by placing a thermometer in the reaction mixture.
2. Calculate the enthalpy of reaction (ΔH) To classify the net energy output or input of chemical reactions, you can calculate something called the enthalpy change (ΔH) or heat of reaction, which compares the energy of the reactants with the energy of the products. Enthalpy is a measure of internal energy. So, when you calculate the difference between the enthalpy of the products and the enthalpy of the reactants, you find the enthalpy change (ΔH), which can be represented mathematically as:

   ΔH = energy used in reactant bond breaking + energy released in product bond making

   Wait, how can you find a difference by adding? The enthalpy values are added in the equation above because, by definition, energy used in reactant bond breaking is always positive (+) and energy released in product bond making is always negative (−). If ΔH is negative (−) then the chemical reaction is exothermic, because more energy is released when the products are formed than energy is used to break up the reactants. If ΔH is positive (+) then the chemical reaction is endothermic, because less energy is released when the products are formed than the energy is used to break up the reactants. You can also use energy level diagrams to visualize the energy change during a chemical reaction as a result of the energies used and released according to the above equation for ΔH. To understand these diagrams, compare the energy level of the reactants on the lefthand side with that of the products on the right-hand side.

   The graph below charts the energy change when a candle burns. The wax (C34H70) combusts in the presence of oxygen (O2) to yield carbon dioxide (CO2) and water (H2O). Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic, and ΔH for the reaction is negative.

In this investigation, you will observe whether energy is absorbed or released in two different chemical reactions and categorize them as exothermic and endothermic. You will also explore the relationship between energy changes and chemical reactions.

Baking Soda and Vinegar

1. Pour about 10 mL of vinegar into a small plastic cup. Then, place a thermometer into the vinegar. Record the initial temperature (Ti) in the table below.
2. While the thermometer is in the cup, add about ½ teaspoon of baking soda to the cup.
3. Watch the thermometer for any change in temperature. After it has stopped changing, record the final temperature (Tf) and any other observations you made in the table below.

Baking Soda and Calcium Chloride

1. Make a baking soda solution by dissolving about 2 tablespoons of baking soda in 1 cup of water. Stir until no more baking soda will dissolve.
2. Place about 10 mL of baking soda solution in a small plastic cup. Then, place a thermometer into the baking soda solution. Record the initial temperature (Ti) in the table below.
3. While the thermometer is in the cup, add ½ teaspoon of calcium chloride to the cup.
4. Watch the thermometer for any change in temperature. After it has stopped changing, record the final temperature (Tf) and any other observations you made in the table below.

1. Calculate the temperature change for both chemical reactions. To do this, subtract the initial temperature (Ti) from the final temperature (Tf), and record the difference in the column labeled ΔT. You may see this calculation expressed elsewhere as ΔT = Tf−Ti. 2.
2. Based on your observations of the baking soda and vinegar reaction, is the reaction exothermic or endothermic? Apply your knowledge of energy changes in chemical reactions to complete the table above.
3. Based on your observations of the baking soda solution and calcium chloride reaction, is this chemical reaction exothermic or endothermic? Apply your knowledge of energy changes in chemical reactions to complete the table above.

1. In the chemical reaction between baking soda and vinegar, what did you observe other than a temperature change? What might this tell you about one of the products of this chemical change?
2. In the chemical reaction between baking soda solution and calcium chloride, what did you observe other than a temperature change? What might this tell you about one of the products of this chemical change?
3. Use your answers from questions 1 and 2 to help you write the chemical equation for:
   1. the chemical reaction between baking soda and vinegar
   2. the chemical reaction between baking soda and calcium chloride
4. Using the language of breaking and making bonds, explain the net energy change for the chemical reaction between baking soda and calcium chloride.
5. Draw energy profiles for both chemical reactions. Refer to the exothermic energy profile shown previously as an example. Are they the same or different?
6. What is the sign of the heat of reaction (ΔH) for an exothermic reaction? Why?

Reflecting on the Investigation

1. Based on your investigation so far, do you think that energy changes only accompany chemical reactions? Using only the materials from the first two reactions, design an experiment that would test this idea. Propose a procedure and have it approved by your teacher before you continue experimenting.
2. Is dissolving calcium chloride in water a chemical change? Explain your reasoning.
3. Using the language of breaking and making bonds, how can you describe the temperature change you observed when you dissolved calcium chloride in water?
4. How might you use exothermic or endothermic processes to solve a real-world problem? Are there any instances when it would be useful to quickly make something hot or cold? Explain how it is useful to know which processes absorb or release energy.

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