6.3 Conserving Mass in Chemical Reactions Th ink again about Investigation 6.2: Is Mass Gained or Lost During a Chemical Reaction? In Part A, you saw that when two solutions are combined to form a product in a sealed container, the total mass of reactants equals the total mass of products. No mass is gained; none is lost. In other words, mass is conserved during the chemical reaction.

Is mass conserved in other chemical reactions as well? Th e answer to this questionmaynotbeimmediatelyobvious.Afterall,everydayexperiencemay suggest that mass does change during a chemical reaction. For example,acampfireburnsdowntoapileoffluffyashesthathavemuchless mass than the original wood (Figure 1). One of the fi rst scientists to study this question was the eighteenth-century French chemist Antoine Lavoisier. Until this time, chemists had not considered the possibility that some reactions could involve gases. Gases, like other forms of matter, have mass. In his experiments, Lavoisier used equipment designed to trap all the reactants and products, including any gases produced during the reactions. Lavoisier found that mass was always conserved if all the reactants and products were considered.

Lavoisier’s conclusion has been supported by the work of many other scientists and is now considered to be a scientifi c law. It is known as the law of conservation of mass:

In any given chemical reaction, the total mass of the reactants equals the total mass of the products.

Th e law of conservation of mass makes sense when you consider what is occurring at the atomic level. (Recall the Bohr–Rutherford model inSection5.4.)Experimentshaveshownthat,duringachemicalreaction, the atoms in reactant molecules are rearranged to form

products. Th erefore, all the atoms that existed in the reactants are still present in the products of the reaction. Atoms cannot be created or destroyed. Th at explains why the total mass of reactants is equal to the total mass of products.

Reactions, Equations, and the Conservation of Mass In Section 6.1, you learned that chemical equations can

provide a lot of information about chemical reaction. Equations give the formulas and physical state of the reactants and products.

To be a completely accurate description of the chemical reaction, an equation must also follow the law of conservation of mass. It must show

an equal number of each kind of atom on both sides of the equation. Th is indicates that there are equal numbers of each kind of atom before and a� er the chemical reaction takes place.

Let’s consider the reaction in which carbon and oxygen react to form carbon dioxide (Figure 2):

C(s) + O2(g) → CO2(g)

Figure 1 Wood disappears to almost nothing when it burns. How does this reaction obey the law of conservation of mass?

Figure 2 There are two (red) oxygen atoms and one (black) carbon atom on either side of the scale. Similarly, there are two oxygen atoms and one carbon atom on either side of the arrow in the chemical equation.

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To be a completely accurate description of the chemical reaction, an

law of conservation of mass the statement that, in any given chemical reaction, the total mass of the reactants equals the total mass of the products

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Note that the same numbers and kinds of atoms are present on both sides of the equation. Th e product (carbon dioxide) contains exactly the same numbers and kinds of atoms as the reactants (carbon and oxygen). Th erefore, the chemical equation obeys the law of conservation of mass.

Let’s look at another chemical equation: H2(g) + Cl2(g) → HCl(g) Th is equation does not accurately describe the reaction between hydrogen

and chlorine. Can you tell why? Count the atoms on each side of the arrow. In this reaction, two atoms of hydrogen react with two atoms of chlorine. (Remember that hydrogen and chlorine both exist as diatomic molecules.) Th e product, however, contains only one atom of hydrogen and one of chlorine (Figure 3(a)). One atom of hydrogen and one atom of chlorine remain unaccounted for. An equation in which the reactants and products are not balanced is sometimes called a “skeleton equation.” Since atoms cannot just vanish, we can assume that two molecules of HCl(g) are produced in this reaction. Laboratory investigations indicate that this is the case (Figure 3(b)).

To show that two hydrogen chloride molecules are produced in this reaction, the coeffi cient “2” is placed before HCl(g) in the chemical equation:

H2(g) + Cl2(g) → 2 HCl(g) Th e coeffi cient in a chemical equation applies to all the atoms in the

molecule. “2 HCl” means that there are two molecules of hydrogen chloride, each containing one hydrogen atom and one chlorine atom.

Now the chemical equation obeys the law of conservation of mass. Chemical equations must always be balanced, with the same kinds and same numbers of atoms on both sides of the arrow.

Diatomic MoleculesRemember that some elements exist as diatomic molecules. Refer back to Table 1 in Section 5.10.

LeaRning Tip

Figure 3 In (a), there are two (white) hydrogen atoms and two (green) chlorine atoms on the left side of the scale, but only one hydrogen atom and one chlorine atom on the right side. In (b), there are two hydrogen atoms and two chlorine atoms on each side of the scale. Reactants and products are balanced.

Coeffi cients versus SubscriptsDo not confuse coeffi cients with subscripts in chemical formulas. Coeffi cients give the ratio of reactants and products in a reaction. Subscripts give the ratio of elements in a chemical formula; they cannot change in a given chemical. Thus, chemical equations can be balanced only by using coeffi cients.

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Adjusting Inferences Sometimes as you continue reading, you come across information that confl icts with an inference that you have already made. For example, you read that an equation must obey the law of conservation of mass. Later you read a chemical equation that appears to break this fundamental law. You infer that exceptions are possible. Later you read that the equation as stated is incorrect. You revise your inference and conclude that the law of conservation of mass is unbreakable.

Reading Tip

6.3 Conserving Mass in Chemical Reactions 231NEL

SKILLS: Observing, Predicting, Analyzing

In this activity, you will use molecular models to visualize the law of conservation of mass.

Equipment and Materials: molecular model kit

1. Build one molecule of hydrogen, H2. Build one molecule of bromine, Br2. Predict how many molecules of hydrogen bromide, HBr, can be made from these two models. Verify your prediction by making the “product” from the “reactants.”

2. Build two molecules of hydrogen and one molecule of oxygen. Predict how many molecules of water, H2O, can be made from them. Verify your prediction by making the product.

3. Build two molecules of hydrogen peroxide, H2O2. Predict how many molecules of oxygen and water can be made from them. Verify your prediction.

4. Build two molecules of ammonia, NH3. Imagine that this is the product of a reaction between hydrogen and nitrogen. Predict how many molecules of hydrogen and nitrogen are required to make two molecules of ammonia. Check your prediction.

A. Write the word equations and chemical equations for each of these four reactions. K/U

B. Explain how the results of this activity illustrate the law of conservation of mass. T/I

TTRY THIS MOdeLLing BaLanCed CHeMiCaL eQUaTiOnS

in SUMMARY•Th elawofconservationofmassstatesthatin

any given chemical reaction, the total mass of the reactants equals the total mass of the products.

•Chemicalequationsobeythelawofconservationof mass. Th ey show that all the atoms in the reactants are still present in the products.

•Coefficientsareaddedbeforechemicalformulasin a chemical equation to ensure that the numbers of atoms on each side of the arrow are equal (balanced).

CHECK YOUR LeaRningCHECK

1. (a) The idea that gases have mass can be diffi cult to accept. How has this reading helped your understanding of this concept?

(b) Identify what remains unclear. Discuss this with your teacher. C

2. (a) State the law of conservation of mass.

(b) Explain the law of conservation of mass by referring to the atoms involved in a chemical reaction.

(c) Which best represents the law of conservation of mass: a skeleton equation or a balanced chemical equation? Explain. K/U

3. Are the following situations exceptions to the law of conservation of mass? Justify your answer in eachcase. K/U

(a) The mass of a hamburger decreases as it is barbecued.

(b) A tree’s mass is continually increasing as the tree grows.

(c) The mass of a copper penny increases if it is heated in a Bunsen burner fl ame.

(d) You are often lighter in the morning than you were when you went to bed.

4. You might have noticed that new copper roofs turn green over time. This occurs because copper reacts with substances in the air to form a hard, protective coating. Will the mass of the new copper roof increase or decrease over time? Explain. Does this prediction violate the law of conservation of mass? Explain. T/I A

5. Design an experiment involving the reaction of vinegar and baking soda to test the law of conservation of mass. T/I

6. A 20 g sample of compound A is mixed with 45 g of compound B. A chemical reaction occurs in which a gas is produced. Once the reaction is complete, the fi nal mixture has a mass of 55 g. T/I

(a) What is the mass of the gas?

(b) What assumption did you make in (a)?

7. Soon after learning about the work of Lavoisier, John Dalton proposed that atoms are never created or destroyed in chemical reactions, only rearranged. Explain how this statement applies to the law of conservation of mass. A

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