chapter 7: chemical reactions (pg. 190) vocabulary 1.chemical reaction-a process that produces...

Chapter 7: Chemical Reactions (pg. 190)

Vocabulary

1. Chemical reaction-a process that produces chemical change

2. Reactant-substance that exists before the reaction begins

3. Product-substance that forms as a result of the reaction

4. Chemical equation-shorthand form for writing what reactants are used and what products are formed in a chemical reaction; sometimes shows whether energy is produced or absorbed.

5. Endothermic reactions-absorb thermal energy

6. Exothermic reactions-release thermal energy

7. Activation energy-the minimum amount of energy needed to start a chemical reaction

8. Rate of reaction-how fast a reaction occurs after it has started

9. Concentration-the amount of substance present in a certain volume

10. Inhibitor-substance that slows down a chemical reaction

11. Catalyst-substance that speeds up a chemical reaction

12. Enzymes- large protein molecules that speed up reactions needed for your cells to work properly


Physical or Chemical Change?Chemical reactions warm your home, cook your meals, digest your food, and power cars and trucks.Matter can undergo two kinds of changes:1. Physical

• Size• Shape• Solid/liquid/gas

• Ex: when water freezes, changes from liquid to solid but still water

2. Chemical


Physical or Chemical Change?

2. Chemical• Chemical changes produce new substances that have properties different from those of the

original substances.• Ex: rust on bike handlebars has different properties than the metal around it• Ex: the reaction of silver nitrate and sodium chloride forms solid silver chloride and liquid sodium nitrate. (the

combination of two liquids that produce a precipitate which is a solid and a liquid)

• Physical vs. Chemical:• Newspaper example (pg. 190)

• Folding is a physical change because size/shape can change but still a newspaper• burning is a chemical change because new substances result

Review examples on page 191: Visualizing Chemical Reactions


Chemical Equations(pg. 192)• Reactants –substances that react; they exist before the reaction begins• Products- the substances that form as a result of the reaction

Describing What HappensChemical equation- tells chemists (in a shorthand form) the reactants, products, and proportions of each substance present. Some equations also tell the physical state of each substance.

Chapter 7: Chemical Reactions

Using Words. (pg.193)One way to describe what happens is with an equation that uses words to name the reactants and products.

Reactants Products

Baking soda + Vinegar Gas + White solid

Charcoal + Oxygen Ash + Gas + Heat

Iron + Oxygen + Water Rust

Silver + Hydrogen sulfide Black tarnish + Gas

Gas (kitchen range) + Oxygen Gas + Heat

Sliced apple + Oxygen Apple turns brown

Reactants are on the left of the arrow

Products are on the right of the arrow

The arrow means “produces”


Using Chemical Names (pg. 193)Baking soda and vinegar example: Reactants: sodium hydrogen carbonate (baking soda) and acetic acid (vinegar)Products: sodium acetate, water, and carbon dioxide

Word equation for baking soda and vinegar reaction:

Acetic acid + Sodium hydrogen carbonate Sodium acetate + Water + Carbon dioxide


Using Formulas (pg. 194)• Convert a word equation into a chemical formula by substituting chemical

formulas for the chemical names.

Baking soda and vinegar example:

CH3COOH + NaHCO3 CH3COONa + H2O + CO2

Acetic Acid Sodium Sodium water Carbon

hydrogen acetate dioxide

carbonate


Conservation of Mass (pg. 194)The law of conservation of mass- the mass of the products must be the same as the mass of the reactants in a chemical reaction. • Nothing is lost or created• Lavoisier:

• First stated this principle• Showed chemical reactions are like mathematical equations; right and left sides are

numerically equal• Chemical equations must have equal numbers and kinds of atoms on both sides


Balancing Chemical Equations (pg. 195)When we write the chemical equation for a reaction, we must observe the law of conservation of mass. Refer to baking soda and vinegar chemical equation below. The same numbers of each kind of atom are on both sides of the equation.

CH3COOH + NaHCO3 CH3COONa + H2O + CO2

Count: 3 Carbon atoms on left side and 3 Carbon atoms on right side

5 Oxygen atoms on the left side and 5 Oxygen atoms on the right side

2 Hydrogen atoms on the left side and 2 Hydrogen atoms on the right side

1 Sodium atom on the left side and 1 Sodium atom on the right side


Balancing Chemical Equations (pg. 195)Count the atoms See the unbalanced equation that shows what happens when silver tarnishes:

Ag + H2S Ag2S + H2

Count: 1 silver atom on the right and 2 on the left

2 Hydrogen on the right and 2 on the left

1 Sulfur on the right and 1 on the left

This equation does not represent the chemical reaction accurately because a chemical reaction cannot create a silver atom.

In order to balance this equation, we must add a 2 in front of the reactant Ag:

2Ag + H2S Ag2S + H2


Balancing Chemical Equations (pg. 195)In order to balance this equation, we must add a 2 in front of the reactant Ag:

2Ag + H2S Ag2S + H2

The 2 in front of the reactant Ag is a coefficient. • Coefficients change the amount, but not the identity of a substance.

• Ex: 2H2O = two molecules of water; 3H2O = three molecules of water

• Subscripts change the identity of a substance• Subscripts cannot be changed when balancing an equation


Balancing Chemical Equations (pg. 195)

Balance the following equations:

1. Na + AlCl3 NaCl + Al

2. KBr + Cl2 KCl + Br2

3. H2O2 H2O + O2


Energy in Chemical Reactions (pg. 196)

Energy Released• Chemical bonds break and form as atoms gain, lose or share electrons.• Bonds break in the reactants and new bonds form in the products• In reactions that release energy, the products are more stable , and their bonds

have less energy than those of the reactants• The extra energy is released in various forms:

• Light, sound, and thermal energy


Energy in Chemical Reactions (pg. 197)

Energy Absorbed• In reactions that absorb energy, the reactants are more stable and their bonds

have less energy than those of the products.• Reactions can absorb several kinds of energy

• Ex: electricity, light, sound and thermal energy

• Endothermic reactions absorb thermal energy• Exothermic reactions release thermal energy• Ex: burning is an exothermic reaction in which a substance combines with

oxygen to produce thermal energy along with light, carbon dioxide and water.


Energy in Chemical Reactions (pg. 197-198)

Energy Released:Rapid Release

Sometimes energy is released rapidly.Ex: charcoal lighter fluid combines with oxygen in the air and produces

enough thermal energy to ignite a charcoal fire with minutesSlow Release

Sometimes thermal energy is released so slowly, you cannot see it or feel it.Ex: rust, hand warmers


Energy in Chemical Reactions (pg. 198-199)Energy Absorbed:Some chemical reactions and physical processes need t have thermal energy added before they can proceed.

Ex: cold pack is an endothermic process (see example pg. 198)

Energy in the Equation:Energy in the reactant = endothermic reaction (energy absorbed)Energy in the product = exothermic reaction (energy is released)

The word energy written in an equation either as a reactant or a product can draw attention to an important aspect of the equation.


Rates of Chemical Reactions (pg. 200-201)Activation Energy-Starting a Reaction

• To form new bonds in the product, old bonds must break in the reactants, and breaking bonds takes energy.

• The minimum amount of energy needed to start a chemical reaction is called the activation energy of the reaction.• Even reactions that release energy later need energy to start.


Reaction Rate (pg. 202)The rate of reaction tells how fast a reaction occurs after it has started. To determine the rate of reaction you can measure:

• How quickly one of the reactants is consumed OR• How quickly one of the products is created

• Reaction rates are important because speeding up useful reaction rates and slowing down destructive ones can be helpful.

Both measurements tell how the amount of a substance changes per unit of time


Temperature Changes Rate (pg 202-203)Fruit ripens faster at higher temperaturesMeat and fish decompose faster at higher temperatures, producing toxins that can make us sick

Temperature Affects Rate• Most chemical reactions speed up when the temperature increases• Atoms and molecules are always in motion and they move faster at higher

temperatures• Faster molecules collide more often and with greater energy so they are more

likely to provide enough energy to break old bonds- this is the activation energy.


Concentration Affects Rate (pg 203)The amount of substance present in a certain volume is called the concentration of that substance. If you increase the concentration you increase the number of particles of a substance per unit of volume.• Collisions are more frequent in a concentrated solution• Collisions are less frequent in a dilute solution

Collisions by themselves are not enough to achieve a reaction. The atoms or molecules must have correct orientation and sufficient energy to complete the chemical reaction.


Surface Area Affects Rate (pg 204)The exposed surface area of reactant particles also affects how fast the reaction can occur.Only the atoms or molecules in the outer layer of the reactant material can touch the other reactants and react.

Examples on page 204 (twigs vs logs, steel beam vs. steel wool)

Slowing Down ReactionsInhibitor-a substance that slows down a chemical reaction.• Makes the formation of a certain amount of product take longer• Some inhibitors completely stop reactions• Ex: Many cereals contain the compound butylated hydroxytoluene (BHT)

which slows the spoiling of the cereal and increases its shelf life


• Speeding Up Reactions (pg. 205)• A catalyst is a substance that speeds up a chemical reaction.• Catalysts do not appear in chemical equations because they are not changed

permanently or used up• A reaction using a catalyst will not produce more product but it will produce

the same amount of product faster

How do catalysts work?• Many catalysts speed up reaction rates by providing a surface for the reaction

to take place• Sometimes the reacting molecules are held in a position that favors reactions• Others reduce the activation energy needed to start the reaction


Catalytic Converters(pg. 205)• Catalysts are used in exhaust systems of cars and trucks to aid fuel combustion. • Catalysts speed the reactions that change incompletely burned substances that

are harmful such as carbon monoxide, into less harmful substances such as carbon dioxide. • Hydrocarbons are changed into carbon dioxide and water

• The result of these reactions is cleaner air


Enzymes Are Specialists (206)• Enzymes are large protein molecules that speed up reactions needed for our cells

to work properly• They help convert food to fuel, build bone and muscle tissue, convert extra energy to fat, and

even produce other enzymes

• Enzymes make it possible for our bodies to function

• Proteases are a class of enzymes that work within cells to break down large complex molecules called proteins• Ex: meat tenderizer, contact lens cleaning solution

chapter 7: chemical reactions (pg. 190) vocabulary 1.chemical reaction-a process that produces...

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