chapter 7 – chemical reactions. 7.1 – describing reactions

Chapter 7 – Chemical Reactions

7.1 – Describing Reactions

What is a chemical change?

• Chemical Change: occurs when a substance reacts and forms one or more new substances– EXAMPLES: baking a cake, leaves changing color

in the fall, food digesting in your stomach, paper burning

Evidence of a Chemical Change

• Some clues that a chemical change has taken place:

– change in color– production of gas– formation of precipitate

Click HERE for video (production of a gas)

Chemical Reactions• When a substance undergoes a chemical

change, a chemical reaction is said to have taken place.

• In order to understand chemical reactions, you must be able to describe them!

Describing Chemical Reactions

• Identify what is present before and after the change– Reactants: the substances that undergo change– Products: the new substances formed as a

result of that change

• During a chemical change, the reactants change into products– Reactants Products

Chemical Reaction

Chemical Reaction

• Example: Burning charcoal

• Word Equation: carbon + oxygen carbon dioxide

• Chemical Equation: C + O2 CO2

Chemical Equation: C + O2 CO2

• Chemical Equation: a representation of a chemical reaction in which the reactants and products are expressed as formulas

• Read aloud as: carbon and oxygen react to form carbon dioxide, or the reaction of carbon and oxygen yields carbon dioxide

Mass is Conserved!• As the charcoal burns, what happens to the

mass of the piece of charcoal? Where does it go? It does not just disappear!

• Would the mass of the charcoal before you burn it equal the mass after it is burned?

Mass is Conserved• If you were to measure the mass of the charcoal

before the reactions and the mass of the CO2 (carbon dioxide gas) what would you notice?– THEY WOULD BE EQUAL!!! – During a chemical reaction, the mass of the products

is ALWAYS equal to the mass of the reactants.

• This is known as the Law of Conservation of Mass- mass is neither created nor destroyed in a chemical reaction.

Showing Mass is Conserved• In order to ensure that the Law of

Conservation of Mass is being obeyed, in other words, that mass is being conserved, you must write a balanced chemical equation.

• This means that the number of atoms of an element in the reactants MUST EQUAL the number of atoms of that element in the products

Rules to Balancing a Chemical Equation1. Count the number of atoms of each element on

each side of the equationa. Example: N2H4 + O2 N2 + H2O

b. N = 2 H = 4 O = 2 N=2 H=2 O=1

2. Change one or more coefficients until the equation is balanced

a. Coefficient- the numbers that appear before the formula

b. NEVER change the subscripts! Only the coefficients! c. Example: ____N2H4 + ____O2 ____N2 + ____ H2O

Example of Balancing

• ____N2H4 + ____O2 ____N2 + ____ H2O

• How would you correctly balance the problem above?

Correctly Balanced

• ____N2H4 + ____O2 ____N2 + 2 H2O

7.3 – Energy Changes in Reactions

Chemical Bonds & Energy• Some reactions yield more than just the

products (elements). • For example, when propane burns in the

presence of oxygen, it creates carbon dioxide (CO2) and water (H2O) – but what else??? – HEAT AND LIGHT! – The heat that is released in the reaction comes

from the reactants.

Chemical Energy• Chemical Energy: the energy stored in the

chemical bonds of a substance.

Chemical Energy

• Each of the bonds in propane has stored energy.

• Energy changes in chemical reactions are determined by changes that occur in chemical bonding.

• In other words… – Chemical reactions involve the breaking of

chemical bonds in the reactants and the formation of chemical bonds in the products.

Chemical Energy• Breaking chemical bonds REQUIRES ENERGY.– That’s why a grill requires an igniter to produce a

spark -- to give enough energy for the bonds in reactants to break, starting the reaction.

• Forming bonds RELEASES ENERGY.– The heat and light that are given off by propane

stoves are the result of the formation of new chemical bonds.

– The bonds are formed as carbon, hydrogen, and oxygen atoms rearrange.

Energy Exchange

• Remember how physical changes deal with an exchange of energy?

– Example of endothermic phase change: melting.– Example of exothermic phase change: freezing.

Endothermic

Exothermic

Energy Exchange in Chemical Reactions

• Energy also flows into and out of chemical changes.

• During a chemical reaction energy is either released or absorbed. – Endothermic Reaction (feels cool)– Exothermic Reaction (feels warm)

Exothermic Reaction• Exothermic reaction: chemical reaction that

releases energy to its surrounding– Feels WARM– Energy released, as the products form is greater

than the energy required to break the bonds in the reactants.

– Example: combustion reactions (burning)

Endothermic Reaction

• Endothermic Reaction: a chemical reaction that absorbs energy from its surroundings– Feels COLD– More energy is required to break the bonds in the

reactants than is released by the formation of the products

– In other words, the energy of the products is greater than the energy of the reactants.

– Example: decomposition reactions (breaking down)

Decomposition Reaction

No matter the reaction…energy is conserved!

• In both types of reactions, the amount of energy from the reactants side must equal the amount of energy from the products side.– Energy cannot be created nor destroyed!– You cannot end up with more energy than was

put into the reaction, and vice versa.

• This is called the Law of Conservation of Energy.

7.4 Reaction Rates

Reactions over Time

• Progress of a chemical reaction is measured over time and expressed as a rate.

• Reaction Rate: rate at which reactants change into products over time

• Reaction rates tell you how fast a reaction is going

Factors Affecting Reaction Rates

• Factors that affect reaction rates include:– Temperature– Surface Area – Concentration– Stirring – Catalysts.

Temperature• An increase in temperature will increase the

reaction rate• A decrease in temperature will decrease the

reaction rate– Cooking on stove top: turn up heat faster the food

cooks– Store milk in refrigerator to slow down reaction that

causes milk to spoil– Reason: Increased temperature = Increased particle

motion = Higher number of particle collisions = Increased reaction rate

Surface Area

• The smaller the particle size of a given mass, the larger is its surface area

• Example: Using newspaper to cover the floor of a room– Keeping all the sections folded together versus separating

all the pages. If you separate the newspaper into open pages you can cover a much larger are with the same mass of paper

– Increase in surface area increases the exposure of reactants to one another

– Reason: Increased surface area = Greater exposure = More collisions = More particles reacting = Increased reaction rate

LESSSURFACE AREA

Surface Area

GREATER SURFACE AREA

GREATER SURFACE AREA

LESSSURFACE AREA

Stirring• By stirring you increase the exposure of

reactants to each other• Example: Washing your clothes in the washer– Leave clothes to just soak in tub of water and

detergent reaction proceeds more slowly– Placing clothes and detergent in washer – reaction

sped up by stirring the contents back and forth– Reason: Stirring =Collisions more likely to occur

=Increase in reaction rate

Concentration• Concentration: number of particles in a given

volume– More reacting particles = Greater chance for

particles to collide = Faster reaction rate

Pt

Catalyst• Catalyst: a substance that affects the reaction rate

without being used up in the reaction– Use a catalyst to:

• Speed up a RXN or• Enable a RXN to occur at a lower temperature• Lower the amount of energy required for a reaction to occur

– Catalyst is neither a reactant or a product, so it is written over the arrow of the chemical equation (it is NOT used up)• Platinum is a catalyst for breaking hydrogen peroxide

into water and oxygen

Example: 2H2O2 2H2O + O2

Catalyst