Teacher's Overview
Summary
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.
Student Investigation
Energy Changes in Chemical Reactions
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:
- It takes energy to break bonds.
- 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.
- 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.
- 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
- 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.
- While the thermometer is in the cup, add about ½ teaspoon of baking soda to the cup.
- 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
- 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.
- 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.
- While the thermometer is in the cup, add ½ teaspoon of calcium chloride to the cup.
- 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.
| Process | Ti | Tf | Exothermic or Endothermic? | Other observations? | ΔH(+/−) |
|---|---|---|---|---|---|
| Baking soda + vinegar | |||||
| Baking soda solution + calcium chloride |
- 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.
- 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.
- 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.
- 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?
- 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?
- Use your answers from questions 1 and 2 to help you write the chemical equation for:
- the chemical reaction between baking soda and vinegar
- the chemical reaction between baking soda and calcium chloride
- Using the language of breaking and making bonds, explain the net energy change for the chemical reaction between baking soda and calcium chloride.
- Draw energy profiles for both chemical reactions. Refer to the exothermic energy profile shown previously as an example. Are they the same or different?
- What is the sign of the heat of reaction (ΔH) for an exothermic reaction? Why?
- 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.
- Is dissolving calcium chloride in water a chemical change? Explain your reasoning.
- Using the language of breaking and making bonds, how can you describe the temperature change you observed when you dissolved calcium chloride in water?
- 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.
Calculating Lattice Energies Using the Born-Haber Cycle
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FAQs
What are exothermic and endothermic reactions answers? ›
In simple terms, the endothermic reactions absorb energy from the surrounding that is in the form of heat. On the other hand, an exothermic reaction releases energy into the surrounding of the system.
What are exothermic and endothermic chemical changes? ›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.
Can a chemical change be both exothermic and endothermic? ›Answer and Explanation: When considering an overall chemical reaction, it cannot be both exothermic and endothermic in general.
What is an example of an exothermic reaction answer? ›The chemical reaction in which the heat is released is known as an Exothermic reaction. Examples are the burning of a candle and the reaction of a strong acid with water.
What are good examples of exothermic and endothermic reactions? ›Examples of exothermic reactions are combustion, most oxidation reactions and neutralisation. Exothermic reactions are used in things like self-heating cans and hand warmers. Examples of endothermic reactions are thermal decomposition reactions and the reaction of citric acid with sodium hydrogencarbonate.
What are 3 examples of exothermic reactions? ›- Making of an Ice Cube. Making an ice cube is a process of liquid changing its state to solid. ...
- Snow Formation in Clouds. ...
- Burning of a Candle. ...
- Rusting of Iron. ...
- Burning of Sugar. ...
- Formation of Ion Pairs. ...
- Reaction of Strong Acid and Water. ...
- Water and Calcium Chloride.
Endothermic reactions: Heat is absorbed.
1) Photosynthesis: Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen. 2) Cooking an egg: Heat energy is absorbed from the pan to cook the egg.
In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases (gets cold). A chemical reaction is exothermic if heat is released by the system into the surroundings.
Which chemical change is always exothermic? ›Adsorption is always exothermic.
What is the difference between exoergic and endoergic reactions? ›An exothermic (exoergic) nuclear reaction is a reaction that releases energy while an endothermic (or endoergic) reaction is one that requires an input of energy to take place. We define the change in kinetic energy during a nuclear reaction as the reaction energy (or Q-value).
What is a chemical change that is exothermic? ›
An exothermic reaction is a chemical reaction in which less energy is needed to break bonds in the reactants than is released when new bonds form in the products. During an exothermic reaction, energy is constantly given off, often in the form of heat. All combustion reactions are exothermic reactions.
Is melting ice endothermic or exothermic? ›Where an exothermic reaction releases heat, an endothermic reaction absorbs heat. One common endothermic reaction is ice melting.
Is freezing endothermic or exothermic? ›In fact, water freezing into ice is also exothermic. As water releases heat into its surroundings, its temperature lowers to its freezing point and ice is formed.
Is Boiling water endothermic or exothermic? ›Because we must add heat, boiling water is a process that chemists call endothermic.
What is a real life example of an endothermic reaction? ›Endothermic reactions: Heat is absorbed.
1) Photosynthesis: Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen. 2) Cooking an egg: Heat energy is absorbed from the pan to cook the egg.
An exothermic reaction is a chemical reaction that releases energy by light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation: reactants → products + energy.
What are common exothermic reactions? ›Examples are numerous: combustion, the thermite reaction, combining strong acids and bases, polymerizations. As an example in everyday life, hand warmers make use of the oxidation of iron to achieve an exothermic reaction: 4Fe + 3O2 → 2Fe2O3 ΔH⚬ = - 1648 kJ/mol.
What is the most common exothermic reaction? ›Combustion Examples
One of the most common exothermic reaction examples in everyday life is combustion. Combustion reactions involve the complete burning of a hydrocarbon substance.
Some examples of exothermic reactions are: burning. Neutralization reactions between acids and alkalis. the reaction between water and calcium oxide.
What is an example of an exothermic reaction in everyday life? ›Everyday uses of exothermic reactions include self-heating cans and hand warmers. When energy is taken in from the surroundings, this is called an endothermic reaction and the temperature of the surroundings decreases.
What are the three endothermic changes? ›
Fusion, vaporization, and sublimation are endothermic processes; they occur only with the absorption of heat.
What happens in a chemical reaction? ›chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.
Which event is endothermic? ›An endothermic reaction is any chemical reaction that absorbs heat from its environment. The absorbed energy provides the activation energy for the reaction to occur. A hallmark of this type of reaction is that it feels cold.
What is an example of a chemical reaction with a change in temperature? ›CaO ( s ) + H 2 O ( l ) → Ca ( OH ) 2 ( aq ) + Heat.
Which of the 5 reaction types are always endothermic? ›Detailed Solution. Option 2 is the correct answer: Decomposition reactions are always endothermic in nature. In a decomposition reaction, a chemical compound is broken into its constituent components. The process takes place through the breaking of bonds between the constituent atoms of the compound.
Is burning exothermic or endothermic? ›Combustion is a high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.
How do you know if a reaction is exergonic or endergonic? ›Endergonic reactions require energy input to take simple, low energy reactants and build complex, high energy products. Exergonic reactions release the energy bound up in the reactants and yield simpler, low energy products.
Can a reaction be both exothermic and exergonic? ›Yes, all exergonic reactions are exothermic.
How do you tell if an equation is endergonic or exergonic? ›Endergonic and exergonic reactions
Exergonic reactions are also called spontaneous reactions, because they can occur without the addition of energy. Reactions with a positive ∆G (∆G > 0), on the other hand, require an input of energy and are called endergonic reactions.
Eg:Cooking an egg is an endothermic process. The egg absorbs the heat from the water and does not release it.
Is rusting metal endothermic or exothermic? ›
Incredibly, the reaction between iron and moist air that produces rust is a very exothermic process and generates lots of heat.
Is fireworks endothermic or exothermic? ›This reaction is very fast and exothermic, which means it gives off energy as heat—and anytime you have a very fast and hot reaction, you get an explosion. This launches the fireworks into the sky and the heat from this explosion is what provides the energy to create the colours.
Is a gas to water exothermic? ›Answer and Explanation: The phase transition from a gas to a liquid is an exothermic process. The molecules or atoms in the gas phase have sufficient kinetic energy to have overcome the attractive forces between the molecules or atoms.
Is Sweating endothermic or exothermic? ›Sweating reduces skin temperature because the sweat that is secreted to the skin evaporates, which is an endothermic process. Thus, heat is absorbed from the body and skin to drive the evaporation of the sweat on the skin.
Is burning a candle exothermic or endothermic? ›Because it involves a combustion event, lighting a candle is an exothermic reaction. When combustion occurs, energy is produced in the form of heat, and smoke is produced as a byproduct.
Is Gold endothermic or exothermic? ›... the H 2 S produced by the deposition of gold (an endothermic process) is used to react with sider- ite to produce pyrite (an exothermic process). The pyrite-forming process releases c. 246 kJ of heat, which can be used to amplify the gold deposition process that requires c. 20 000 kJ of heat.
Is burning paper endothermic or exothermic? ›Burning paper is exothermic. The ash formed by the burning is not flammable and will not burn.
Is digesting food endothermic or exothermic? › These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. Hence, digestion is an exothermic process.
What is an endothermic reaction short answer? ›An endothermic reaction is any chemical reaction that absorbs heat from its environment. The absorbed energy provides the activation energy for the reaction to occur.
What is an endothermic reaction answer? ›What is an Endothermic Reaction? Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect.
What is exothermic and endothermic simple? ›
Exothermic reactions transfer energy to the surroundings and the temperature of the surroundings increases. Endothermic reactions take in energy and the temperature of the surroundings decreases.
What is endothermic and exothermic quizlet? ›endothermic - thermal energy transformed into chemical energy in the products. a chemical reaction that feels warm. exothermic - thermal energy is released into the surroundings with the products in a chemical reaction.
What is 1 example of endothermic reaction? ›Endothermic reactions: Heat is absorbed.
1) Photosynthesis: Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen. 2) Cooking an egg: Heat energy is absorbed from the pan to cook the egg.
Example - Melting of ice is an endothermic change in which ice cubes are melted by heat absorbed from the surrounding.
Is endothermic hot or cold? ›Endothermic reactions are the opposite of exothermic reactions. They absorb heat energy from their surroundings. This means that the surroundings of endothermic reactions are colder as a result of the reaction.
How do you tell if a reaction is exothermic or endothermic from an equation? ›So if the sum of the enthalpies of the reactants is greater than the products, the reaction will be exothermic. If the products side has a larger enthalpy, the reaction is endothermic.
What are some endothermic chemical reactions? ›Melting ice cubes, melting solid salts, and sublimation of dry ice into carbon dioxide gas are three instances of endothermic reactions.
Which reaction is exothermic? ›The exothermic reaction generally takes place in the case of a spontaneous process. Examples could be from burning or fuel to nuclear fusion in the burning of the Sun. Combustion reactions are generally considered an exothermic reactions.
What is the difference between endothermic and exothermic with example? ›An exothermic reaction is a chemical reaction that involves the release of energy in the form of heat or light. Example: Carbon dioxide and heat energy are produced when carbon combines with oxygen. An endothermic reaction is a chemical reaction that involves the absorption of energy in the form of heat or light.
What are the parts of a chemical equation? ›There are two parts of a chemical equation. The participating elements or molecules in a chemical reaction are called reactants and are represented on the left side of the arrow $ ( \to ) $ . The compound that forms after the changes in the participating reactants are called products.
Is melting endothermic or exothermic? ›
Final Answer: Melting is an endothermic process because when heat absorbs then the physical state will change from solid to liquid.