Chem reactions

Chemical Reactions

A chemical change is the transformation of one or more substances into different substances, with different properties. A chemical reaction is a process by which chemical change happens. All chemical reactions are also accompanied by changes in energy. Some chemical reactions absorb energy, such as in the chemical reactions that cook food. Other chemical reactions release energy in the form of heat, light, and/or sound, such as the burning of wood in a campfire (Figure 4.41).

Chemical reactions happen at different rates. Some chemical reactions are fast, such as when rocket fuel burns. Other chemical reactions happen slowly, such as the formation of rust on a corroding bicycle chain. The chemical reactions in your own body, which are keeping you alive and allowing you to read the words on this page, are among the fastest chemical reactions known.

Chemical reactions are used in many ways in daily life. For example, chemical reactions produce the glow in glow sticks (Figure 4.42(a)), generate instant cold without ice in a cold pack (Figure 4.42(b)), and are used to manufacture pigments in artists' paints, such as the bright yellow pigment in Figure 4.42(c).

Scientists are constantly working with chemical reactions to produce new substances with useful properties. For example, the chemical reaction shown in Figure 4.42(c) is no longer used commercially because it involves the element lead. Today, lead is avoided as much as possible, because it is toxic to living organisms. Other kinds of chemical reactions have been developed to produce yellow pigments that do not contain lead.

Reactants and Products

All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants. These new reactions may produce substances with different colours or states. Recall that the term "state" refers to solid, liquid, and gas, as well as aqueous, which means

dissolved in water.

Consider the chemical reaction that occurs when a piece of solid magnesium metal is placed into a solution of hydrochloric acid. Bubbles of hydrogen gas are formed during the reaction (Figure 4.43). A second product, aqueous magnesium chloride, also forms.

Chemical Equations

A chemical reaction is often described by writing a chemical equation.

A chemical equation uses either words or symbols and formulas to describe the changes that occur during a chemical reaction. For example, the chemical reaction between solid magnesium metal and

hydrochloric acid is:

Figure 4.43 Some chemical reactions involve a change of state, such as the

formation of bubbles in a liquid.

word equation:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

formula equation:

Mg(s)

+ 2HCl(aq)

→ MgC1, (aq)

+ H,(g)

Notice that the hydrogen is expressed in the formula equation as

H. Recall from Section 4.2 that pure hydrogen exists as a molecule. You will need to know which elements exist as molecules when writing formula equations. The chemical formulas in a formula equation often will include the following:

• the state of matter of each substance. The state of each substance is indicated by placing the appropriate symbol in brackets after the formula. These symbols are (s) for solid, (I) for liquid, (g) for

• gas, and (aq) for aqueous.

• one or more coefficients. A coefficient is an integer that is placed in front of the symbol of an element or a chemical formula. The coefficients show the ratios of the different substances that are present in the chemical reaction. In the formula equation above, a coefficient of 2 is in front of the formula HCl. This means that Mg and HCl combine in a ratio of 1:2

Writing Balanced Chemical Equations

A chemical equation that is complete except for coefficients is called an unbalanced equation or a skeleton equation. Example Problem 4.8 shows a "guess-and-check" method for writing and balancing a chemical equation. Example Problem 4.9 on page 179 shows a more detailed method for balancing an equation. (States are omitted in some examples and shown in others.)

Strategies for Writing Skeleton Equations

Writing the correct formula for certain elements and compounds when you are translating word equations into skeleton equations can sometimes be difficult. Here are some tips:

• When a metal element is not in a compound with other elements, write the symbol of the element from the periodic table. For example, if a word description were to state "an iron nail," it would be translated to Fe.

• Seven common non-metal elements occur as molecules containing two identical atoms. These are hydrogen (H,), nitrogen (N2), oxygen (Oz), fluorine (F2), chlorine (Cl,), bromine (Br), and iodine (I2). One way to help to remember these elements is to think of them as the "-gens": hydrogen, nitrogen, oxygen, and the halogens (consisting of fluorine, chlorine, bromine, and iodine). For example, if a word equation were to state "oxygen reacts with fluorine," the skeleton equation should be written O2 + F2:

• Common compounds that contain hydrogen include water (H,O), ammonia (NH3), and methane (CH,).

• Some common acids are hydrochloric acid (HCI), nitric acid
  (HNO,), sulphuric acid (H,SO4), and phosphoric acid (H,PO,).
  (You will study acids in Chapter 5.)

Strategies for Writing Balanced Equations

Successfully balancing chemical equations can take some practice. Here are some strategies you might try:

• Balance atoms of elements in any complicated-looking formulas first, and balance atoms of pure elements last.

• Never change a subscript in a formula to help make atoms balance. Balance by placing coefficients in front of formulas only.

• Use guess-and-check to balance simple equations. Begin by placing a coefficient where you think it might work, or just take a guess and then count atoms.

• Hydrogen atoms and/or oxygen atoms often will appear in many or all of the formulas of the reactants and products. When this is the case, try to balance other elements first. Balance hydrogen second last, and oxygen last.

• You may be able to treat polyatomic ions as a unit. For example, if NO, appears in the reactants and products of a skeleton equation, count the number of NO, groups rather than the number of N atoms and O atoms separately.

Types of Chemical Reactions

There are many possible ways that elements and compounds may interact. In other words, there are many types of chemical reactions.

When you can identify a reaction type from the reactants, you can predict the products of the reaction. Predicting products is important in the manufacture of many of the consumer goods we use, such as plastics (Figure 6.6). When chemists develop new plastics, they predict the products of the chemical reactions they use.

Each type of chemical reaction can be represented by a general chemical equation. A general chemical equation (GCE) is an equation that uses letters of the alphabet (A, B, C, D) in place of the symbols for elements. A GCE can help you understand and remember a type of chemical reaction more easily. Table 6.1 shows how letters are used when writing a GCE.

Synthesis Reactions

In the simplest type of synthesis reaction, two elements combine to form a compound. Either a metallic element combines with a non-metallic element or a non-metallic element combines with a second non-metallic element to produce one compound. A general statement for a synthesis reaction is: element + element → compound

The general chemical equation for a synthesis reaction is:

A + B → AB

Whenever the reactants of a chemical reaction are two elements and the product is a single compound, you know that the chemical reaction is a synthesis reaction. For example, when the metallic element magnesium burns in the presence of oxygen gas, magnesium oxide is formed (Figure 6.7). Oxygen is a non-metallic element that forms diatomic molecules. The formulas and states of matter at room temperature of some common molecular elements and compounds are shown in Table 6.2.

Skeleton equation: Mg(s) + 02(g) → MgO(s)

Balanced chemical equation: 2Mg(s) + 0,(g) → 2MgO(s)

Magnesium oxide has a number of commercial uses, including making reflective coatings for optical instruments and lining metal and glass furnaces.

Sulphur and oxygen can also undergo a synthesis reaction, producing a poisonous gas called sulphur dioxide. In this case, both reactants are non-metallic elements.

Skeleton equation: Sg(g) + 02(g) → S02(g)

Balanced chemical equation: Sg(g) + 80,(g) → 8S02(g)

As you saw in chapter 5, sulphur dioxide can combine with water in the air to form acid rain. That is why as much sulphur as possible is removed from gasoline during production.

Another synthesis reaction involving only non-metals is the synthesis of ammonia from its elements. The formula of ammonia is NH, so the reactants must be the elements hydrogen and nitrogen.

Both of these elements form diatomic molecules.

Skeleton equation: H, (g) + N2(g) → NH, (g)

Balanced chemical equation: 3H,(g) + N2(g) → 2NH, (g)

Since ammonia is used in the production of fertilizers and other useful compounds, synthesizing ammonia is an important industrial reaction.

Decomposition Reactions

During a decomposition reaction, a compound is broken apart into two or more elements and/or simpler compounds. A decomposition reaction is the reverse of a synthesis reaction. The elements that are produced may be a metallic element and a non-metallic element or two non-metallic elements.

compound → element + element

The GCE for a decomposition reaction is:

AB → A + B

Whenever the reactant of a chemical reaction is a single compound and the products are elements or simpler compounds, you can identify the chemical reaction as a decomposition reaction.

For example, chemists in the 18th century prepared oxygen gas by heating solid mercury II) oxide, leaving globules of mercury metal.

Skeleton equation: HgO(s) → Hg(l) + 0,(g)

Balanced chemical equation: 2HgO(s) → 2Hg() + 0,(g)

The breakdown of water into hydrogen and oxygen is another

example of a decomposition reaction. Figure 6.8 shows an apparatus that is used to decompose water in a laboratory. This apparatus is called the Hoffman apparatus, and it decomposes water by passing electricity through it.

Skeleton equation: H,O(1) - H,(g) + 02(8)

Balanced chemical equation: 2H,0(1) - 2H,(g) + 02(8)

Some vehicles now use hydrogen gas as fuel for at least part of their energy needs. However, it is not practical to use electricity to produce the amounts of hydrogen gas needed by hydrogen-fuelled vehicles.

Large volumes of hydrogen are produced by the decomposition of methane (CH,, natural gas). A device called a reformer or a fuel processor splits the hydrogen atoms from the carbon atoms in the methane. This method of producing hydrogen gas has two main disadvantages. First, it depends on methane gas, which is a non-renewable energy source. Secondly, it also produces carbon dioxide, a greenhouse gas that contributes to global warming.

Combustion Reactions of Hydrocarbons

A combustion reaction is a chemical reaction in which a compound or element rapidly combines with oxygen gas. These reactions usually give off a large amount of heat and light. A hydrocarbon is a compound made of only carbon and hydrogen. Hydrocarbons have the formula C,H, where the letters "x" and "y" stand for the number of atoms (i.e., 2, 3, or higher). The simplest example of this kind of compound is methane (CH,) (Figure 6.12). Some of these compounds are fossil fuels, such as methane and petroleum. Fossil fuels are hydrocarbons formed underground over millions of years from the remains of once-living organisms. The general chemical equation for the combustion reaction of a hydrocarbon is:

С,Н, + 02 → СО, + H,0

(where C,H, stands for a hydrocarbon molecule).

We write C,H, in the general chemical equation because there are many different compounds made up of only carbon and hydrogen atoms. The products of a hydrocarbon combustion reaction are always carbon dioxide and water.

For example, the combustion reaction of methane gas can be written

this way:

Skeleton equation: CH,(8) + 02(g) → CO2(g) + H,0U)

Balanced chemical equation: CH,(g) + 20,(g) → COz(8) + 2H,00)

Single Displacement Reactions

In a single displacement reaction, an element reacts with an ionic compound. During the reaction, the element becomes part of the ionic compound, while one of the elements in the ionic compound becomes an element by itself. The elements that switch places may be either two metals or two non-metals.

In the first type of displacement reaction, a metallic atom trades places with a metallic ion in a compound. The general chemical equation for this single displacement reaction is:

AB + 0→ CB + A

(where the metallic element A is replaced by another metal, C).

In this type of single displacement reaction, the reactants will always be a compound and a metallic element, and the products also will always be a compound and a metallic element.

For example, a single displacement reaction can take place between

copper II) chloride and aluminum. This reaction is written as:

Skeleton equation: CuClaq) + Al(s) → All,(aq) + Cu(s)

Balanced chemical equation: 3CuCl, (aq) + 2Al(s) → 2AIClaq) + 3Cu(s)

In the second kind of displacement reaction, a non-metal in a compound is displaced by another non-metal. The general chemical equation for this single displacement reaction is:

AB + C → AC + B

(where the non-metallic element B is replaced by another non-metal, C).

In this form of single displacement reaction, the reactants will always be a compound and a non-metallic element, and the products will also always be a compound and a non-metallic element.

For example, a single displacement reaction can take place between

potassium iodide and bromine. This reaction is written as:

Skeleton equation: KI(aq) + BrD) → KBraq) + I,(aq)

Balanced chemical equation: 2KI(aq) + Br,) → 2KBraq) + 1, (aq)

Double Displacement Reactions

In a double displacement reaction, the positive or negative ions in two dissolved ionic compounds switch places. The general chemical equation for a double displacement reaction is:

AB + CD → AD + CB

Take It Eurther

The combustion of hydrocarbons provides energy for many human activities. Prepare a poster or another form of display for your classroom using information and

In a double displacement reaction, the reactants will always be

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compounds. Therefore, a positive ion will only switch places with a ScienceSource.

positive ion, and a negative ion will only switch places with a negative ion. Two positive ions will never pair up to form a new compound, because their charges will repel one another. Two negative ions will never pair up to form a new compound for the same reason. Double displacement reactions may result in the formation of a precipitate.

For example, when aqueous magnesium hydroxide combines with aqueous silver nitrate, the products are aqueous magnesium nitrate and silver hydroxide.

Skeleton equation: Mg(OH), (aq) + AgNOg(aq) → Mg(NO:), (aq) + AgOH(s)

Balanced chemical equation: Mg(OH), (aq) + 2AgNOg(aq) → Mg(NO3), (aq) + 2AgOH(s)

You are not expected to be able to predict the states of the products

in a double displacement reaction.

Neutralization and Double Displacement

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A neutralization reaction between an acid and a base also follows the same general pattern as a double-displacement reaction. Recall that in a neutralization reaction, an acid reacts with a base to form an ionic salt and water. The general chemical equation for this reaction is:

HB + XOH → XB + H,0

(where HB is an acid, XOH is a base, and XB is an ionic salt).

Another way of looking at this reaction is that the H+ ion in the acid switches places with the positive ion (X) in the base, or the OH ion in the base switches places with the negative ion B) in the acid. Since neutralization reactions are so common and so important, they are classified on their own and are not usually called double displacement reactions.

Summary: Types of Chemical Reactions

Table 6.3 summarizes the types of chemical reactions you explored in this chapter and in chapter 5. The general chemical equation and an example of a matching balanced chemical equation are shown for each.