Chemical Reactions

1 – Nature of Chemical Reactions2 – Chemical Equations

3 - Reaction Types4 – Reaction Rates and Equilibrium

State Standards

• CLE.3203.1.9 – Apply the Laws of Conservation of Mass/Energy to balance chemical equations

• CLE.3202.Inq.4 – Apply qualitative and quantitative measures to analyze data and draw conclusions that are free of bias

• CLE.3202.Inq.6 – Communicate and defend scientific findings

1 - Nature of Chemical Reactions

Key Questions : • When do chemical reactions take place?

• What is the role of energy in chemical reactions?

Chemical Reactions• Everyday occurrences are reactions ( rxns )– Growing, ripen, decay, burn

• Chemical reactions stem from chemical changes

• How do you tell that a chemical change happens?

Chemical Reactions

• Atoms are rearranged ( to form new substance )

• Reactant – substance participating in rxn

• Product – substance being formed by rxn

Demonstration

• Add vinegar ( acetic acid ) to baking soda

• CO2 is produced

• What evidence of a reaction is obsered?

Energy & Chemical Reactions

• Energy factors into the state of matter

• Energy also plays a role in changes of state

• Chemical changes use energy also

• Same for chemical rxns

Chemical Reactions• Involve changes in energy ALWAYS

• Energy is required to break bonds

• Forming bonds releases energy

Chemical Reactions• Energy is conserved in rxns

• Exothermic – Rxns that release energy

• Endothermic – Rxns that absorb energy

Endothermic - Exothermic

Photosynthesis

• An endothermic rxn

1 - Nature of Chemical Reactions

Key Questions : • When do chemical reactions take place?

• What is the role of energy in chemical reactions?

2 - Chemical Equations

Key Questions : • What is a chemical equation?

• What can a balanced chemical equation tell you?

Describing Reactions

• Can write a word equation• A chemical equation uses symbols to represent a

chemical reaction and shows the relationship between the reactants and products

Conservation of Mass

• An equation must be balanced• CH4 + O2 CO2 + H2O ( NOT BALANCED )

• CH4 + 2O2 CO2 + 2H2O ( BALANCED )

• Balance by adding coefficients so that there are EQUAL NUMBERS OF EACH ELEMENT on both sides of the “Yield” sign

Balanced Equations & Mole Ratios

• A balanced equation indicates the molar ratio– This is the proportion of reactants and products

• Or relative amounts of a reactant to product

• CH4 + 2O2 CO2 + 2H2O

• Molar Ratio of methane to carbon dioxide?• Molar ratio of methane to oxygen?

Molar Ratios can be shown as MASS

• How do we do this?

2 - Chemical Equations

Key Questions : • What is a chemical equation?

• What can a balanced chemical equation tell you?

3 – Reaction Types Key Questions : • How does learning about reaction types help in

understanding chemical reactions?

• In which kinds of chemical reactions do the numbers of electrons in atoms change?

Classifying Reactions

• Can use patterns to identify kinds of chemical reactions and to predict the products of the chemical reactions

• For example : small molecules join to form a larger one by a certain type of reaction

Synthesis Reactions

• Synthesis reactions occur when multiple substances combine to form a new compound

• General form : A + B AB

• Single compound formed

• Example : plastic ( polymers – remember GOOP? )

Decomposition Reactions• Decomposition occurs when substances are

broken apart ( Opposite of Synthesis )

• Cracking – large molecules of C and H (hydrocarbons) are broken apart

• Digestion – similar to cracking

• General form : AB A + B

Combustion Reactions

• CH4 + 2O2 CO2 + 2H2O ( methane combustion )

• When oxygen reacts with a substance ( burning )

• Usually an organic material– Hydrocarbon– Plant matter ( wood ) or clothing ( cotton/polymeric )

• Not always though – ammonia ( NH3 )– NH3 + O2 NO2 + H2O

Displacement Reactions

• Single displacement rxn occurs when one atom appears to take the place of another

• General form : AX + B BX + A

• Double displacement rxn occurs when two compounds appear to exchange ions

• General form : AX + BY BX + AY

Electrons & Chemical Reactions

• Free radical reactions and redox reactions can be understood as changes in the numbers of electrons that atoms have

• Free radical is an atom or a group of atoms that has one unpaired electron

• Redox reactions ( oxidation-reduction ) occur when one substance loses electrons and another substance gains electrons

3 – Reaction Types

Key Questions : • How does learning about reaction types help in

understanding chemical reactions?

• In which kinds of chemical reactions do the numbers of electrons in atoms change?

4 – Reaction Rates & Equilibrium

Key Questions : • What speeds up a reaction?

• What does a catalyst do?

• What happens when a reaction goes both directions

What is Reaction Rate?

• How fast a reaction proceeds

• Can explain by finding the amount of products produced in a certain time interval

What Speeds up Reactions?

• Higher temperatures

• More surface area

• Higher reactant concentration – ( amount / vol )

• Pressure / Size of compound or molecule

What is a catalyst?

• Speeds up or slows down a reaction but is not altered by the reaction

• It does not participate as a reactant or a product

• In your body – ENZYMES are catalysts

Equilibrium

• Like balance when you are walking

• Balance between products and reactants when a reaction goes forward and backwards

4 – Reaction Rates & Equilibrium

Key Questions : • What speeds up a reaction?

• What does a catalyst do?

• What happens when a reaction goes both directions