Chemical reactions that produce energy or that require energy

Introduction: (Initial Observation)

We have all seen a burning candle or burning wood. Burning is a chemical reaction between the burning object and the oxygen gas. Burning is the most well known chemical reaction that creates heat energy. During a burning process, chemical energy will be converted to heat energy.

Of course not all chemical reactions create energy as much as burning. Most heat producing chemical reactions may have no flame at all.

In this project we will research the chemical reactions that produce or require energy. We will also attempt to find a general rule that helps us to determine if a chemical reaction will produce or will absorb energy.

You will not need to perform all the proposed experiments. Please select the safest experiments that you can perform and have access to it’s material.

Information Gathering:

To find out about the chemical reactions that produce energy or require energy we simply review some chemical reactions to see if they will fit in any of these categories.

Before searching for information, think yourself. Write down a list of chemical reactions that you can remember:

1. Water electrolysis
2. Acid – Base reaction
3. Acid Metal reaction
4. Sodium – Water reaction
5. …..
   …..
   Write as many as you can. Then try to determine (Hypothesize) which reaction produces energy and which one requires and gets energy. Also think what form of energy each reaction receives or produces.

Now you can start searching for more information. While searching, you will find that there are four types of chemical reactions.

SYNTHESIS REACTION

A synthesis reaction is a reaction in which two or more substances combine to produce a single, more complex substance. The components involved in the reaction may be elements, compounds or both.

Several examples of synthesis reactions are:

2H2 + O2 ==> 2H2O * The burning of hydrogen to form water

C + O2 ==> C O2 * The burning of carbon to form carbon dioxide

2CO + O2 ==> 2CO2 * The burning of carbon monoxide to form carbon dioxide

CaO + H2O ==> Ca(OH)2 * The reaction of calcium oxide and water to form calcium hydroxide

2Mg + O2 ==> 2MgO * The oxidation of magnesium

The reaction of sodium metal with water is among the chemical reactions that creates extreme heat and fire. This is a dangerous experiment that must be performed at the smallest possible scale at a fire-proof lab and protective clothing for experimenter and observers are required.

 

 

 

2 Na + 2 H2O —-> 2 NaOH + H2

 

 

 

DECOMPOSITION REACTION

In a decomposition reaction a single substance is broken down into two or more simpler substances. These simpler substances may either be elements or compounds. Most decomposition reactions are endothermic (absorb energy during the reaction). The energy for the endothermic reactions is usually supplied as heat or electricity. Sometimes, however, the reaction can be exothermic. An example of this is the breakdown of unstable compounds.

Several examples of decomposition are:

2H2O ==> 2H2 + O2

* Decomposition of water by an electric current, a reaction also called Electrolysis

2HgO + HEAT ==> 2Hg + O2 (g)

*Mercury(II) oxide decomposed by heat

SINGLE REPLACEMENT REACTION

A single replacement reaction is a reaction in which a free element takes the place of a less active element in a compound, setting the less active element free.

Some examples of single replacement reactions are:

Zn + H2SO4 ==> Zn SO4 + H2

* This is the replacement of hydrogen in an acid with and active metal, in this case, zinc.

Cl2 + 2NaBr ==> Br2 + 2NaCl

* The replacement of bromide ions in a compound, by the element chlorine

DOUBLE REPLACEMENT REACTION

In this type of reaction one ionic compound is added to another ionic compound. The anions of one compound react with the cations of the other. The result of this reaction can be a precipitate, water or a gas.

A precipitate is a solid substance formed by a physical or chemical change in a liquid medium. In a double replacement reaction, the anion of one compound and the cation of the other compound react to form a substance that is insoluble in the liquid medium. This solid is denser than the solution and therefore separates by settling to the bottom.

Some examples of double replacement reactions are:

NaCl + AgNO3 ==> NaNO3 + AgCl(s)

* The formation of solid silver chloride in an aqueous solution

3NaOH + FeBr3 ==> 3NaBr + Fe(OH)3 (s)

* The formation of solid iron hydroxide in an aqueous

ENERGY OF CHEMICAL REACTIONS

Chemical reactions always involve a change in energy. Energy is neither created or destroyed. Energy is absorbed or released in chemical reactions. Chemical reactions can be described as endothermic or exothermic reactions.

Endothermic Reactions

Chemical reactions in which energy is absorbed are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy. Adding electrical energy to metal oxides can separate them into the pure metal and oxygen. Adding electrical energy to sodium chloride can cause the table salt to break into its original sodium and chlorine parts.

Exothermic Reactions

Chemical reactions in which energy is released are exothermic. The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product(s) to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.

When we speak about chemical reactions that produce energy, we are talking about converting the chemical energy to heat energy. Chemical energy is stored in the form of chemical bonds between the atoms of a substance. Breaking the bonds will release that energy in the form of heat.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to identify the chemical reactions that produce or require (absorb) energy.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable is the chemical reaction. This may include the names of two substances that react with each other.

Dependent variable is the direction of reaction energy. Possible values are produce or absorb.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

My hypothesis is that creation of any chemical bond will release (or produce) some energy and breaking such chemical bond requires some energy. The amount of energy depends on the strength of a chemical bond.

For example when we burn hydrogen gas, we are creating a bond between oxygen and hydrogen. That is why we get some heat energy. But when we do electrolysis of water, we are breaking the bonds, that is why we use some electrical energy to achieve that.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

We design an experiments to test each hypothesis.

Experiment 1:

In this experiment we test creation of new chemical bonds. Burning charcoal that is almost pure carbon is an example of creating new bonds. We have all seen or experimented burning charcoal in the past and we know that it creates heat. We use it to make barbeque. The chemical reaction of charcoal and oxygen is:

C + O2 => CO2

Sulfur is another pure substance that can be burned to create heat:

S + O2 => SO2

For our experiment we will try something that does not create any fire. We test the reaction of a strong acid and a strong base. I suggest to do this experiment in school lab or any other equipped lab with proper supervision. Sulfuric acid also known as battery acid is a strong acid and can be purchased from auto part stores. For this experiment you just need about 2 to 3 milliliter (4 to 6 grams).

Either caustic soda or caustic potash can be used as strong base. Caustic soda or sodium hydroxide flakes are used to open clogged drains and can be purchased from hardware stores. When you buy this, read on the package. Some of these products are caustic soda and some others are caustic potash. For our purpose, either product will work, but it is also good to know what you are using.

Warning:

protective clothing and goggles are required while working with chemicals. Also make sure you have access to water for emergency washing.

Procedure:

1. place about 10 grams of caustic soda in a beaker and add about 20 ml water. This may not completely dissolve the caustic soda, but should give you enough caustic soda solution to work with. You may carefully and slowly stir the solution for a few seconds. Dissolving caustic soda in water will create heat, so leave the solution in room temperature for about one hour until it cools off.
2. If your sulfuric acid is more than 50%, you will need to dilute that too. To do that add 20 ml water to a beaker and add 10 ml sulfuric acid to that. You may use a pipette to transfer acid. (If you do not have experience in using pipettes, do some practice using regular water before starting your experiment). Also leave this solution in room temperature for about one hour.
3. Use a thermometer to read the temperature of each solution separately and then record it. Make sure you avoid cross contamination. So if you are only using one thermometer, you need to wash and dry it after each test.
4. Use a pipette to transfer 5 ml caustic soda solution to a test tube
5. Use another pipette to gradually add about 5 ml acid solution to the test tube
6. Feel the test tube to see if it gets hot.
7. Measure the temperature and record it.

Notes: You can use muriatic acid or hydrochloric acid instead of sulfuric acid and you will not need to dilute it at all. You may also try citric acid that can be purchased from grocery stores. Even acetic acid can be used for this experiment, but you can not use vinegar because it only has 5% acetic acid and 95% water, so the generated heat will be distributed among a lot of water and will not easily be noticeable.

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Experiment 2:

In this experiment we use a catalyst enzyme to break the bonds in Hydrogen Peroxide to see if it produces or absorbs energy.

Catalysts accelerate chemical reactions that otherwise proceed slowly. The enzyme called

catalase is a catalyst. It exists in plant and animal cells and breaks down hydrogen peroxide, H2O2.

The chemical reaction accelerated by catalase is written

2(H2O2) ==catalase==> 2H2O + O2

Under favorable conditions, the reaction occurs very fast. The maximum catalytic rate for one catalase molecule is 6 million molecules of hydrogen peroxide converted to water and oxygen per minute. The reaction product is 6 million molecules of water and 3 million molecules of oxygen.

Procedure:

1. Fill 1/2 of a small beaker or jar with hydrogen peroxide.
2. Test and record the temperature
3. drop a small piece of cow or lamb liver in the jar
4. Observe oxygen gas coming out of the solution
5. measure and record the temperature again.

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Experiment 4: Citric acid powder and Sodium bicarbonate powder

Updated: I have just updated this experiment in order to avoid unexpected results caused by using commercial chemicals or powders that are not fine.

Introduction: In this experiment we will test the chemical reaction between citric acid powder and sodium bicarbonate (baking soda) powder. Both powders are initially at room temperature.

Question: Is the reaction of citric acid and sodium bicarbonate endothermic or exothermic?

Hypothesis: During the reaction between sodium bicarbonate and citric acid CO2 or Carbon Dioxide will be released. Breaking the bonds in carbon dioxide may result a considerable temperature decrease.

Material

• Thermometer;
• Styrofoam cup or any hard plastic cup;
• Sodium bicarbonate and citric acid (both in powder form) and water.

Procedure:

Temperature changes are easily visualized using a thermometer.

• Add 12 grams sodium bicarbonate to 8 grams citric acid in a cup.
• Start recording the temperature.
• Mix the compounds by stirring with the thermometer.
• After a short time read the temperature and record it.
• To start or expedite the reaction add some water to the mixture while stirring.

The temperature will decrease, also note the development of carbon dioxide.

Results of the measurements

Record the temperature change during this reaction.

Variations of this experiment:

Use a solution of citric acid and a solution of baking soda. Make sure that both solutions are at room temperature before you start.

Use a solution of citric acid and a solution of sodium carbonate. Make sure that both solutions are at room temperature before you start

 

Reaction of sodium bicarbonate and citric acid cause a huge temperature drop after adding water. Before adding water, the temperature drop is slow, but it is detectable by high precision thermometers if you are using very fine powders.With Sodium bicarbonate, citric acid and water the temperature dropped from 75 to 45.
	Before Reaction	After Reaction

With a solution of sodium carbonate and citric acid the temperature dropped from 75 to 65.

Mixing sodium carbonate and water increased the temperature from 75 to 110. That is why you cannot mix citric acid and sodium carbonate and then add water to show an endothermic reaction.

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Experiment 5:

You can repeat the previous experiment by mixing barium hydroxide (powder) with ammonium chloride or ammonium thiocyanate at room temperature leads to a spontaneous reaction that is so endothermic as to cause water to freeze.

Materials and Equipment:

List of materials can be extracted from the experiment section.

Material may be purchased from ChemicalStore.com or other science suppliers.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

Description

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious

References:

List of References

Type of Chemical Reactions

General Chemistry online

Reaction Energy