Fundamentals of Chemical Reactions

Welcome to Introductory Chemistry. In this video we'll begin exploring the basic principles of chemical reactions. We'll first learn how to recognize when a reaction is taking place based on various clues that may be present. We'll then examine the conventions that are used to write chemical equations. One of the most important conventions is that we write our chemical equations so that they're consistent with the law of conservation of mass. We do this in a process called balancing equations, which is a skill that requires practice. We begin this chapter by considering the nature of chemical reactions. Any given chemical reaction involves substances that exist in some particular state of matter that depends on the conditions. We can indicate the physical states of the components of a reaction in the same way we describe the states of matter for substances in a mixture. For instance, suppose we have a glass of salt water and ice. We can describe the components of this mixture as solid ice, or H2O parentheses S. floating in liquid water which we specify as H2O parentheses L. Above the surface of the liquid is evaporated water or H2O parentheses G. We indicate the sodium chloride is dissolved in the aqueous solution by writing NaCl parentheses Aq. Now having specified the nature of the components of this mixture, Let's determine the nature of the process that occurs when we created it. Did the process of mixing the components of the mixture involve a chemical reaction? To answer this question, we must ask whether the substances involved are being transformed into new substances. In this case, the sodium chloride can be recovered as the water evaporates, and the water is simply changing from one phase to another as the ice melts. Therefore, no chemical reaction is involved in the creation of this mixture. So how does one know when there is a chemical reaction? It turns out that chemical reactions often give visual signals. Sometimes it's an obvious indication, as in the case of potassium metal reacting vigorously with water as shown here. But reactions are not always visible or obvious, even though they must involve transformations of one substance each. into another. In any case, one can frequently observe clues that indicate a reaction has occurred. These clues may include changes in color, the formation of a solid, or the generation of bubbles. Other clues include the absorption or generation of heat and possibly flame. For example, the reaction of hydrogen and oxygen to form water vapor gives off significant energy in the form of heat and flame. This picture of the Hindenburg disaster indicates that the magnitude of the energy given off by this reaction is quite large. We can describe this particular reaction in detail with a chemical equation. The equation shows the specific phases for each component, including the fact that water is produced as a vapor, or gas, because of the intense heat of the reaction. The equation also provides precise information about the ratios of each substance that's reacting and being produced, as well as their states of matter at the time of the reaction. The ratios of the components are given by numbers, called coefficients, that are written in front of the chemical formulas. If no number is written, it's understood that the number is 1. So the chemical equation for this reaction specifies that two parts hydrogen gas react with one part oxygen gas to form two parts water vapor. In a different chemical reaction, energy from a battery discharging in water produces bubbles, demonstrating that a reaction is occurring. The input of energy from the battery drives a reaction in which water is transformed into hydrogen and oxygen gas. The chemical equation for this process is given. and is almost the exact reverse of the previous reaction except that it involves liquid water rather than water vapor another difference is that the process requires energy to proceed instead of releasing energy as a ball of fire However, in both cases the ratios of the components of the reaction are the same, but in the reverse sense. That is, two parts water produce two parts hydrogen and one part oxygen. Reactions can take place in a variety of forms. Some reactions occur upon mixing of two solutions containing different substances. In the reaction depicted here, A colorless solution of hydrochloric acid is added to a red solution of cobalt nitrate. The resulting solution turns blue and we can conclude that a chemical reaction has taken place. In some cases, mixing two solutions may cause more than a color change. Here, a yellow solid forms when two clear solutions are mixed. The yellow solid results from the mixing of a solution of sodium dichromate and a solution of lead nitrate. The solid that's formed sinks to the bottom of the flask and is evidence that a reaction has occurred. Previously we saw that the reaction of potassium metal with water is quite vigorous. The equation describing this reaction provides details about the chemical transformations that occur. At this point, however, information about the ratios of the components of the reaction are not yet included, and we call this an unbalanced equation. Nevertheless, the equation does indicate that solid potassium metal combines with liquid water to produce hydrogen gas and potassium hydroxide, which is dissolved in the water. Heat is also clearly generated in this reaction. Now, to indicate how much of each component is unbalanced, component reacts and is produced, we must provide the ratios for each component. It can be demonstrated experimentally that this particular reaction involves two parts potassium and two parts water to produce one part hydrogen gas and two parts potassium hydroxide. The equation is then completed by adding these ratios as coefficients to the formulas. The result is a balanced chemical equation. We can now describe all of the basic components of a chemical equation. First, chemical reactions involve changing atom groupings, so the components on the right side of the reaction will always differ in some way from the components on the left. The left side of the reaction represents the reactants, or starting materials, while the products are presented on the right. The arrow indicates that a change has occurred such that new chemical substances are formed. The phases of the components are also indicated. In addition, in order to have a complete and balanced chemical equation, we must indicate the ratios of reactants and products that must be present for this chemical process. For this particular reaction, one part methane combines with two parts oxygen to produce carbon dioxide gas and two parts water vapor. How do we know that this particular ratio is required and how do we determine these coefficients? The answer comes from the law of conservation of mass which states that for any closed system the mass must be constant over time. This means that in a chemical reaction atoms are neither created nor destroyed. The result is that the mass is conserved throughout the chemical reaction such that the total mass of the reactants must equal the total mass of the products. In other words, the number and types of atoms must be the same for both reactants and products, even though the atoms have recombined to give new substances. Take for example the reaction of sodium with chlorine, which is represented by an unbalanced equation. We've also simplified the equation by omitting the phases of the components. When we take an accounting of the number and types of atoms on each side of the arrow, we can see that we have two chlorine atoms from our diatomic chlorine on the reactant side, but only one chlorine atom on the product side. This discrepancy is resolved by the addition of a coefficient of 2 in front of the formula for sodium chloride. Our equation is now balanced with respect to chlorine atoms. However, in our effort to balance the chlorine atoms, we now have two atoms of sodium on the product side and only one atom of sodium on the reactant side. To correct for this newly created imbalance, we add a second coefficient in front of the formula for elemental sodium on the reactant side. Our equation is now balanced because the same number of atoms of each type are represented for both the reactant and the product sides of the equation, and mass has been conserved. We can also consider balancing equations in terms of the individual atoms and molecules. The combustion of a hydrocarbon such as methane, which is CH4, produces carbon dioxide and water. We can interpret the balanced chemical equation as a depiction of individual reactant molecules transforming into new product molecules. The coefficients indicate the numbers of molecules that are required as reactants and products in order to conserve mass. So in this reaction we have one molecule of methane, reacts with two molecules of oxygen to give one molecule of carbon dioxide and two molecules of water. At the same time, however, this reaction also provides a macroscopic depiction indicating that two parts of oxygen gas are required for every one part of methane gas to produce one part of carbon dioxide gas and two parts of water gas or water vapor. Note! that the identity of the components of a reaction must not be changed when balancing chemical equations. This is because the identity of the components, which is determined by experimental observation, must remain properly depicted throughout the process of balancing the equation. So as we transform the unbalanced equation for the combustion of methane, we only add coefficients in front of the chemical formula. We don't change the subscripts within the chemical formula to maintain mass balance because this would give different compounds. Because the products of a chemical reaction are determined experimentally, we must be able to write chemical equations from depictions of such experiments. Consider the following experiment in which blue light shines on a mixture of hydrogen and chlorine gas. The resulting reaction is explosive and produces gaseous hydrogen chloride. Can you select the proper unbalanced equation that describes this chemical process? Take a moment by pausing the video and see if you can correctly answer the question. For this reaction you must remember that hydrogen and chlorine are diatomic gases under standard conditions. So our unbalanced equation must give the reactants as H2 and Cl2 both in the gas phase. the product hydrogen chloride is also formed in the gas phase we can go further and balance the equation by simply adding a coefficient of two to the hydrogen chloride product this gives us a proper accounting of all the atoms on both the reactant and product side the basic steps for writing a chemical reaction from a written or verbal description begins with the identification of the reactants and products followed by the proper depiction of their chemical formulas and physical states. Once the formulas and states are determined, an unbalanced reaction can be written. The final step is to provide a balanced chemical equation. This step is often an iterative process requiring the successive addition of new coefficients. Let's examine how we might balance the reaction between hydrogen and oxygen gas. Remember, to balance an equation, we use coefficients, which essentially adds molecules, or more ionic units, in the case of ionic compounds. Begin by accounting for the number and type of each atom in the equation. When we do this for oxygen, we see that we have two oxygens on the reactant side, and only one on the product side. We can balance the number of oxygens by adding a coefficient in front of water to indicate two water molecules are produced. We now have two oxygens on the product side as needed, but we've changed the number of hydrogens from two to four, given that we now have two water molecules, each with two hydrogen atoms. We can see that there are only two atoms of hydrogen on the reactant side. We compensate for this by changing the number of hydrogens on the reactant side by placing a coefficient of 2 in front of the hydrogen molecule. We now have 4 hydrogens on the reactant side and product side. The reaction is now balanced with respect to all the atoms on both sides of the arrow. Once you've balanced an equation you should always double check your work to make sure that all the atom types and numbers are the same for both sides of the equation. Also it's generally preferred to give a balanced equation with coefficients that are the simplest whole number ratios possible. So for these two balanced equations The first equation is preferred, even though both equations give the same atom type of numbers for reactants and products. Balancing equations takes practice. Let's consider the following equation and determine the sum of all the coefficients. Before you jump in to answer this, you'll need to go through the iterative process of balancing the equation. Only then can you properly add up all the coefficients from each of the components. We begin by iteratively counting up all the atoms of each type on both sides of the arrow, starting with oxygen. We have a total of three oxygens on the left side of the reaction, one from the iron 2 oxide and two from oxygen. On the right side, we can see that we also have three oxygens based on the chemical formula of iron 3 oxygen. However, we have two iron atoms on the right and only one on the left. We also note that there is an odd number of oxygens on either side. In order to balance this particular equation, we'll want the number of oxygens to be an even number for both sides. This is achieved by adding a coefficient of 4 to the iron oxide on the left. We pick the number 4 because this also transforms the number of oxygens on the left to an even number. We now have 4 oxygens from 4 iron oxides and 2 from oxygen to give a total of 6 oxygens on the left. To achieve a balanced number of oxygens on the right, we use the coefficient of 2 in front of the iron oxide. This gives us a new oxygen atom count of 2 times 3. for a total of 6 oxygens on the right. Our equation is now balanced, which we confirm by counting up atoms. 4 irons, 6 oxygens, versus 4 irons and 6 oxygens. When we add up all the coefficients for all the components, we have 4 plus 1 plus 2, which gives us 7, our answer. As mentioned earlier, Balancing equations requires practice, so let's work another example. Here we have an unbalanced equation describing the reaction between calcium oxide and carbon to give calcium carbide and carbon dioxide. We can see at first glance that there is a discrepancy in the number of oxygen atoms and carbon atoms in the reactants and products. After we tally up the atoms, we focus on the oxygen atoms because they're present in a relatively complex formula. We'll balance the elemental carbon last because doing so will not disrupt our iterative process. We begin by balancing the oxygen atoms with a coefficient in front of the calcium oxide. We now have two oxygens on both sides of the arrow, but we also now have two calcium atoms on the left side and only one. on the right. We therefore add a coefficient of 2 to the formula for calcium carbide so that we have two calciums on both sides. This gives us a total of 2 times 2 or 4 carbons from CaC2 plus the one carbon from CO2 for a total of 5 carbons on the right. We complete the process by placing a coefficient on elemental carbon. Checking our work confirms that we have a balanced chemical equation with two calciums, two oxygens, and five carbons on each side. As you can see, balancing each new chemical reaction provides new challenges. You should work as many problems as you can and remember the requirements for a properly balanced equation. First, the number and type of atoms must be the same on each side of the arrow. This is achieved using coefficients and not by changing the subscripts of chemical formulas. Remember the balanced equation tells us the ratio of molecules or atoms that react and are produced and describes the process from both the macroscopic perspective of a laboratory experiment and the microscopic perspective of the reacting atoms and molecules themselves. To maintain this dual perspective, try to create a balanced equation that involves the smallest whole number integers possible. Balancing equations is a fundamental skill that requires significant practice. Your ability to balance equations will help ensure success in a great number of chemistry challenges yet to come.

We'll then examine the conventions that are used to write chemical equations. One of the most important conventions is that we write our chemical equations so that they're consistent with the law of conservation of mass. We do this in a process called balancing equations, which is a skill that requires practice. We begin this chapter by considering the nature of chemical reactions. Any given chemical reaction involves substances that exist in some particular state of matter that depends on the conditions.

We can indicate the physical states of the components of a reaction in the same way we describe the states of matter for substances in a mixture. For instance, suppose we have a glass of salt water and ice. We can describe the components of this mixture as solid ice, or H2O parentheses S.

floating in liquid water which we specify as H2O parentheses L. Above the surface of the liquid is evaporated water or H2O parentheses G. We indicate the sodium chloride is dissolved in the aqueous solution by writing NaCl parentheses Aq.

Now having specified the nature of the components of this mixture, Let's determine the nature of the process that occurs when we created it. Did the process of mixing the components of the mixture involve a chemical reaction? To answer this question, we must ask whether the substances involved are being transformed into new substances.

In this case, the sodium chloride can be recovered as the water evaporates, and the water is simply changing from one phase to another as the ice melts. Therefore, no chemical reaction is involved in the creation of this mixture. So how does one know when there is a chemical reaction? It turns out that chemical reactions often give visual signals. Sometimes it's an obvious indication, as in the case of potassium metal reacting vigorously with water as shown here.

But reactions are not always visible or obvious, even though they must involve transformations of one substance each. into another. In any case, one can frequently observe clues that indicate a reaction has occurred.

These clues may include changes in color, the formation of a solid, or the generation of bubbles. Other clues include the absorption or generation of heat and possibly flame. For example, the reaction of hydrogen and oxygen to form water vapor gives off significant energy in the form of heat and flame. This picture of the Hindenburg disaster indicates that the magnitude of the energy given off by this reaction is quite large.

We can describe this particular reaction in detail with a chemical equation. The equation shows the specific phases for each component, including the fact that water is produced as a vapor, or gas, because of the intense heat of the reaction. The equation also provides precise information about the ratios of each substance that's reacting and being produced, as well as their states of matter at the time of the reaction.

The ratios of the components are given by numbers, called coefficients, that are written in front of the chemical formulas. If no number is written, it's understood that the number is 1. So the chemical equation for this reaction specifies that two parts hydrogen gas react with one part oxygen gas to form two parts water vapor. In a different chemical reaction, energy from a battery discharging in water produces bubbles, demonstrating that a reaction is occurring. The input of energy from the battery drives a reaction in which water is transformed into hydrogen and oxygen gas. The chemical equation for this process is given.

and is almost the exact reverse of the previous reaction except that it involves liquid water rather than water vapor another difference is that the process requires energy to proceed instead of releasing energy as a ball of fire However, in both cases the ratios of the components of the reaction are the same, but in the reverse sense. That is, two parts water produce two parts hydrogen and one part oxygen. Reactions can take place in a variety of forms.

Some reactions occur upon mixing of two solutions containing different substances. In the reaction depicted here, A colorless solution of hydrochloric acid is added to a red solution of cobalt nitrate. The resulting solution turns blue and we can conclude that a chemical reaction has taken place. In some cases, mixing two solutions may cause more than a color change.

Here, a yellow solid forms when two clear solutions are mixed. The yellow solid results from the mixing of a solution of sodium dichromate and a solution of lead nitrate. The solid that's formed sinks to the bottom of the flask and is evidence that a reaction has occurred. Previously we saw that the reaction of potassium metal with water is quite vigorous.

The equation describing this reaction provides details about the chemical transformations that occur. At this point, however, information about the ratios of the components of the reaction are not yet included, and we call this an unbalanced equation. Nevertheless, the equation does indicate that solid potassium metal combines with liquid water to produce hydrogen gas and potassium hydroxide, which is dissolved in the water.

Heat is also clearly generated in this reaction. Now, to indicate how much of each component is unbalanced, component reacts and is produced, we must provide the ratios for each component. It can be demonstrated experimentally that this particular reaction involves two parts potassium and two parts water to produce one part hydrogen gas and two parts potassium hydroxide.

The equation is then completed by adding these ratios as coefficients to the formulas. The result is a balanced chemical equation. We can now describe all of the basic components of a chemical equation.

First, chemical reactions involve changing atom groupings, so the components on the right side of the reaction will always differ in some way from the components on the left. The left side of the reaction represents the reactants, or starting materials, while the products are presented on the right. The arrow indicates that a change has occurred such that new chemical substances are formed. The phases of the components are also indicated.

In addition, in order to have a complete and balanced chemical equation, we must indicate the ratios of reactants and products that must be present for this chemical process. For this particular reaction, one part methane combines with two parts oxygen to produce carbon dioxide gas and two parts water vapor. How do we know that this particular ratio is required and how do we determine these coefficients? The answer comes from the law of conservation of mass which states that for any closed system the mass must be constant over time. This means that in a chemical reaction atoms are neither created nor destroyed.

The result is that the mass is conserved throughout the chemical reaction such that the total mass of the reactants must equal the total mass of the products. In other words, the number and types of atoms must be the same for both reactants and products, even though the atoms have recombined to give new substances. Take for example the reaction of sodium with chlorine, which is represented by an unbalanced equation.

We've also simplified the equation by omitting the phases of the components. When we take an accounting of the number and types of atoms on each side of the arrow, we can see that we have two chlorine atoms from our diatomic chlorine on the reactant side, but only one chlorine atom on the product side. This discrepancy is resolved by the addition of a coefficient of 2 in front of the formula for sodium chloride. Our equation is now balanced with respect to chlorine atoms. However, in our effort to balance the chlorine atoms, we now have two atoms of sodium on the product side and only one atom of sodium on the reactant side.

To correct for this newly created imbalance, we add a second coefficient in front of the formula for elemental sodium on the reactant side. Our equation is now balanced because the same number of atoms of each type are represented for both the reactant and the product sides of the equation, and mass has been conserved. We can also consider balancing equations in terms of the individual atoms and molecules.

The combustion of a hydrocarbon such as methane, which is CH4, produces carbon dioxide and water. We can interpret the balanced chemical equation as a depiction of individual reactant molecules transforming into new product molecules. The coefficients indicate the numbers of molecules that are required as reactants and products in order to conserve mass.

So in this reaction we have one molecule of methane, reacts with two molecules of oxygen to give one molecule of carbon dioxide and two molecules of water. At the same time, however, this reaction also provides a macroscopic depiction indicating that two parts of oxygen gas are required for every one part of methane gas to produce one part of carbon dioxide gas and two parts of water gas or water vapor. Note! that the identity of the components of a reaction must not be changed when balancing chemical equations. This is because the identity of the components, which is determined by experimental observation, must remain properly depicted throughout the process of balancing the equation.

So as we transform the unbalanced equation for the combustion of methane, we only add coefficients in front of the chemical formula. We don't change the subscripts within the chemical formula to maintain mass balance because this would give different compounds. Because the products of a chemical reaction are determined experimentally, we must be able to write chemical equations from depictions of such experiments. Consider the following experiment in which blue light shines on a mixture of hydrogen and chlorine gas.

The resulting reaction is explosive and produces gaseous hydrogen chloride. Can you select the proper unbalanced equation that describes this chemical process? Take a moment by pausing the video and see if you can correctly answer the question.

For this reaction you must remember that hydrogen and chlorine are diatomic gases under standard conditions. So our unbalanced equation must give the reactants as H2 and Cl2 both in the gas phase. the product hydrogen chloride is also formed in the gas phase we can go further and balance the equation by simply adding a coefficient of two to the hydrogen chloride product this gives us a proper accounting of all the atoms on both the reactant and product side the basic steps for writing a chemical reaction from a written or verbal description begins with the identification of the reactants and products followed by the proper depiction of their chemical formulas and physical states.

Once the formulas and states are determined, an unbalanced reaction can be written. The final step is to provide a balanced chemical equation. This step is often an iterative process requiring the successive addition of new coefficients. Let's examine how we might balance the reaction between hydrogen and oxygen gas. Remember, to balance an equation, we use coefficients, which essentially adds molecules, or more ionic units, in the case of ionic compounds.

Begin by accounting for the number and type of each atom in the equation. When we do this for oxygen, we see that we have two oxygens on the reactant side, and only one on the product side. We can balance the number of oxygens by adding a coefficient in front of water to indicate two water molecules are produced.

We now have two oxygens on the product side as needed, but we've changed the number of hydrogens from two to four, given that we now have two water molecules, each with two hydrogen atoms. We can see that there are only two atoms of hydrogen on the reactant side. We compensate for this by changing the number of hydrogens on the reactant side by placing a coefficient of 2 in front of the hydrogen molecule.

We now have 4 hydrogens on the reactant side and product side. The reaction is now balanced with respect to all the atoms on both sides of the arrow. Once you've balanced an equation you should always double check your work to make sure that all the atom types and numbers are the same for both sides of the equation. Also it's generally preferred to give a balanced equation with coefficients that are the simplest whole number ratios possible. So for these two balanced equations The first equation is preferred, even though both equations give the same atom type of numbers for reactants and products.

Balancing equations takes practice. Let's consider the following equation and determine the sum of all the coefficients. Before you jump in to answer this, you'll need to go through the iterative process of balancing the equation. Only then can you properly add up all the coefficients from each of the components. We begin by iteratively counting up all the atoms of each type on both sides of the arrow, starting with oxygen.

We have a total of three oxygens on the left side of the reaction, one from the iron 2 oxide and two from oxygen. On the right side, we can see that we also have three oxygens based on the chemical formula of iron 3 oxygen. However, we have two iron atoms on the right and only one on the left.

We also note that there is an odd number of oxygens on either side. In order to balance this particular equation, we'll want the number of oxygens to be an even number for both sides. This is achieved by adding a coefficient of 4 to the iron oxide on the left. We pick the number 4 because this also transforms the number of oxygens on the left to an even number.

We now have 4 oxygens from 4 iron oxides and 2 from oxygen to give a total of 6 oxygens on the left. To achieve a balanced number of oxygens on the right, we use the coefficient of 2 in front of the iron oxide. This gives us a new oxygen atom count of 2 times 3. for a total of 6 oxygens on the right. Our equation is now balanced, which we confirm by counting up atoms.

4 irons, 6 oxygens, versus 4 irons and 6 oxygens. When we add up all the coefficients for all the components, we have 4 plus 1 plus 2, which gives us 7, our answer. As mentioned earlier, Balancing equations requires practice, so let's work another example.

Here we have an unbalanced equation describing the reaction between calcium oxide and carbon to give calcium carbide and carbon dioxide. We can see at first glance that there is a discrepancy in the number of oxygen atoms and carbon atoms in the reactants and products. After we tally up the atoms, we focus on the oxygen atoms because they're present in a relatively complex formula.

We'll balance the elemental carbon last because doing so will not disrupt our iterative process. We begin by balancing the oxygen atoms with a coefficient in front of the calcium oxide. We now have two oxygens on both sides of the arrow, but we also now have two calcium atoms on the left side and only one.

on the right. We therefore add a coefficient of 2 to the formula for calcium carbide so that we have two calciums on both sides. This gives us a total of 2 times 2 or 4 carbons from CaC2 plus the one carbon from CO2 for a total of 5 carbons on the right.

We complete the process by placing a coefficient on elemental carbon. Checking our work confirms that we have a balanced chemical equation with two calciums, two oxygens, and five carbons on each side. As you can see, balancing each new chemical reaction provides new challenges.

You should work as many problems as you can and remember the requirements for a properly balanced equation. First, the number and type of atoms must be the same on each side of the arrow. This is achieved using coefficients and not by changing the subscripts of chemical formulas. Remember the balanced equation tells us the ratio of molecules or atoms that react and are produced and describes the process from both the macroscopic perspective of a laboratory experiment and the microscopic perspective of the reacting atoms and molecules themselves.

To maintain this dual perspective, try to create a balanced equation that involves the smallest whole number integers possible. Balancing equations is a fundamental skill that requires significant practice. Your ability to balance equations will help ensure success in a great number of chemistry challenges yet to come.

Transcript for:Fundamentals of Chemical Reactions

Transcript for:
Fundamentals of Chemical Reactions