Chapter 7

Which change is a chemical change? (a) the evaporation of rubbing alcohol (b) the burning of lamp oil (c) the forming of frost on a cold night

(b) The burning of lamp oil is like the burning of gasoline. The lamp oil is transformed into other substances (primarily carbon dioxide and water). The evaporation of rubbing alcohol and the formation of frost are both changes of state and therefore physical changes.

Chemical Change (7.2)

Changes in matter in which composition changes are chemical changes. changes in matter in which composition does not change are physical changes. we classify the properties of matter into two types: physical and chemical. matter displays its physical properties without changing its composition

Rusting

Chemical Change

The process by which a fossil fuel such as gasoline burns in the presence of oxygen is a

Chemical change

chemical reactions

Chemical changes occur via chemical reactions.

Climate Change and the Combustion of Fossil Fuels (7.1)

Greenhouse gases warm Earth by trapping some of the sunlight that penetrates Earth's atmosphere. Increases in atmospheric carbon dioxide levels (a major greenhouse gas) have led to global warming. The largest source of atmospheric carbon dioxide is the burning of fossil fuels. This can be verified by reaction stoichiometry.

Combustion, Alkali Metal, and Halogen Reactions (7.6)

In a combustion reaction a substance reacts with oxygen- emitting heat and forming one or more oxygen-containing products. The alkali metals react with nonmetals, losing electrons in the process. The halogens react with many metals to form metal halides. They also react with hydrogen to form hydrogen halides and with one another to form interhalogen compounds.

Writing and Balancing Chemical Equations (7.3)

In chemistry, we represent chemical reactions with chemical equations. The substances on the left-hand side of a chemical equation are the reactants, and the substances on the right-hand side are the products. Chemical equations are balanced when the number of each type of atom on the left side of the equation is equal to the number on the right side.

Boiling

Physical Change

Reaction Stoichiometry (7.4)

Reaction stoichiometry refers to the numerical relationships between the reactants and products in a balanced chemical equation. Reaction stoichiometry allows us to predict, for example, the amount of product that can form from a given amount of reactant, or how much of one reactant is required to react with a given amount of another.

EXAMPLE Stoichiometry In photosynthesis, plants convert carbon dioxide and water into glucose (C6H12O6) according to the reaction: 6 CO2(g) + 6 H2O(l) ——-> 6 O2(g) + C6H12O6(aq) Suppose you determine that a particular plant consumes 37.8 g of CO2 in one week. Assuming that there is more than enough water present to react with all of the CO2, what mass of glucose (in grams) can the plant synthesize from the CO2?

SORT: The problem gives the mass of carbon dioxide and asks you to find the mass of glucose that the plant can produce STRATEGIZE: The conceptual plan follows the general pattern of mass A —->amount A (in moles) amount B (in moles) —->mass B. From the chemical equation, deduce the relationship between moles of carbon dioxide and moles of glucose. Use the molar masses to convert between grams and moles. SOLVE: Follow the conceptual plan to solve the problem. Begin with g CO2 and use the conversion factors to arrive at g C6H12O6 CHECK: The units of the answer are correct. The magnitude of the answer (25.8 g) is less than the initial mass of CO2 (37.8). This is reasonable because each carbon in CO2 has two oxygen atoms associated with it, while in C6H12O6 each carbon has only one oxygen atom associated with it and two hydrogen atoms, which are much lighter than oxygen. Therefore, the mass of glucose the plant produces should be less than the mass of carbon dioxide for this reaction.

EXAMPLE Limiting Reactant and Theoretical Yield Ammonia NH3 can be synthesized by the reaction: 2 NO(g) + 5 H2(g) —-> 2 NH3(g) + 2 H2O(g) Starting with 86.3 g NO and 25.6 g H2, find the theoretical yield of ammonia in grams.

SORT: You are given the mass of each reactant in grams and asked to find the theoretical yield of a product STRATEGIZE: Determine which reactant makes the least amount of product by converting from grams of each reactant to moles of the reactant to moles of the product. Use molar masses to convert between grams and moles and use the stoichiometric relationships (from the balanced chemical equation) to convert between moles of reactant and moles of product. Remember that the reactant that makes the least amount of product is the limiting reactant. Convert the number of moles of product obtained using the limiting reactant to grams of product. SOLVE: Beginning with the given Mass of each reactant, calculate the mount of product that can be made in moles. Convert the amount of product made by the limiting reactant to grams-this is the theoretical yield. CHECK: The units of the answer (g NH3) are correct. The magnitude (49.0 g) seems reasonable given that 86.3 g NO is the limiting reactant, NO contains one oxygen atom per nitrogen atom, and NH3 contains three hydrogen atoms per nitrogen atom. Three hydrogen atoms have less mass than one oxygen atom so it is reasonable that the mass of NH3 obtained is less than the mass of NO.

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Stoichiometry

physical changes

The atoms or molecules that compose a substance do not change their identity

Reaction Stoichiometry:

The coefficients in a chemical equation specify the relative amounts in moles of each of the substances involved in the reaction.

View (a) best represents the water after vaporization. Vaporization is a physical change, so the molecules remain the same before and after the change.

The diagram on the right represents liquid water molecules in a pan. Which of the three diagrams below best represents the water molecules after they have been vaporized by boiling?

Summarizing Limiting Reactant and Yield:

The limiting reactant (or limiting reagent) is the reactant that is completely consumed in a chemical reaction and limits the amount of product. The reactant in excess is any reactant that occurs in a quantity greater than is required to completely react with the limiting reactant. The theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant. The actual yield is the amount of product actually produced by a chemical reaction. The percent yield is (actual yield/theoretical yield) x 100%

stoichiometry.

The numerical relationships between chemical amounts in a balanced chemical equation allows us to predict the amounts of products that will form in a chemical reaction based on the amounts of reactants that react.

limiting reactant

The reactant that limits the amount of product in a chemical reaction.

reactants,

The substances on the left side of the equation

chemical equation.

We represent a chemical reaction with a chemical equation.

actual yield,

the amount of product actually produced by a chemical reaction.

percent yield,

the percentage of the theoretical yield that was actually attained, as the ratio of the actual yield to the theoretical yield

products

the substances on the right side of the equation

Balancing Chemical Equations

1. Write a skeletal (unfbalanced) equation by writing chemical formulas for each of the reactants and products. (If a skeletal equation is provided, proceed to Step 2) 2. Balance atoms that occur in more complex substances first. Always balance atoms in compounds before atoms in pure elements. 3. Balance atoms that occur as free elements on either side of the equation last. Always balance free elements by adjusting the coefficient on the free element. 4. If the balanced equation contains coefficient fractions, clear these by multiplying the entire equation by the denominator of the fraction. 5. Check to make certain the equation is balanced by summing the total number of each type of atom on both sides of the equation.

Balancing Chemical Equations Containing lonic Compounds with Polyatomic lons

1. Write a skeletal equation by writing chemical formulas for each of the reactants and products. (If a skeletal equation is provided, proceed to Step 2.) 2. Balance metal ions (cations) first. If a polyatomic cation exists on both sides of the equation, balance it as a unit. 3. Balance nonmetal ions (anions) second. If a polyatomic anion exists on both sides of the equation, balance it as a unit. 4. Check to make certain the equation is balanced by summing the total number of each type of ion on both sides of the equation.

Which quantity or quantities must always be the same on both sides of a chemical equation? (a) the number of atoms of each kind (b) the number of molecules of each kind (c) the number of moles of each kind of molecule (d) the sum of the masses of all substances involved

Both (a) and (d) are correct. When the number of atoms of each type is balanced, the sum of the masses of the substances involved will be the same on both sides of the equation. Since molecules change during a chemical reaction, their number is not the same on both sides, nor is the number of moles necessarily the same.

Limiting Reactant, Theoretical Yield, Percent Yield, and Reactact in Excess (7.5)

When a chemical reaction actually occurs, the reactants are usually not present in the exact stoichiometric ratios specified by the balanced chemical equation. The limiting reactant is the one that is available in the smallest stoichiometric quantity-it is completely consumed in the reaction, and it limits the amount of the product that can be made. Any reactant that does not limit the amount of product is said to be in excess. The reactant in excess occurs in a quantity greater than is required to completely react with the limiting reactant. The amount of product that can be made from the limiting reactant is the theoretical yield. The actual yield-always equal to or less than the theoretical yield- is the amount of product that is usually actually made when the reaction is carried out. The percentage of the theoretical yield that is actually produced is the percent yield.

chemical property

a property that a substance displays only by changing its composition

physical property

a property that a substance displays without changing its composition.

theoretical yield

amount of product that can be made in a chemical reaction based on the amount of limiting reactant.

chemical change

atoms rearrange and the original substances transform into different substances

combustion reaction

involves the reaction of a substance with O2 to form one or more oxygen- containing compounds, often including water. Combustion reactions also emit heat.

combustion reaction

is a particular type of chemical reaction.