Chemical Reactions

Chapter 7

A time to Review

• What are Physical Properties of Matter?

• What are Chemical Properties of Matter?

• What does a Chemical Formula tell us?

A Time to Review

• What is an atom?• What does the term mass mean?• What is atomic mass?• What is a chemical change?• What is the evidence for a chemical

change?

A Time to Review

• What is a precipitate?• What is an element?• What is a compound?• What is an electron?• What is an ion?• What are the two main types of

chemical bonds?– How do they form?

A Time to Review

• What is temperature?• What is surface area?• What is pressure?• What is concentration?

Think About It

• What happens to a piece of charcoal when it burns?

• How do air bags inflate?• Why does a propane stove need a spark in

order for the propane to ignite?• How does a refrigerator keep food fresh?• What changes take place inside a closed

container of water?

Describing Chemical Reactions

• What type of change is happening in the picture to the left?

• Chemical Reaction• New Products

being formed• Heat and Light

being produced

Chemical Equations

• What is present before and after a chemical change takes place?

• Reactants – the substances undergoing the chemical change

• Products – the new substances formed as a results of the chemical change

• In the case of the burning charcoal• Reactants are carbon and oxygen and the

product is carbon dioxideC + O2 → CO2

Using Equations to Represent Reactions• Reactants → Products• Chemical Equation - a representation

of a chemical reaction in which the reactants and products are expressed as formulas

Conservation of Mass

• What happens to the charcoal as it burns?– Get smaller until it is actually just a pile of ash

• If you measured the amount of carbon dioxide produced it would equal the amount of carbon and oxygen consumed in the reaction

• The mass of the products is always equal to the mass of the reactants

• Called the Conservation of Mass– Principal was establish by Antoine Lavoisier– Mass is neither created nor destroyed in a

chemical reaction

Conservation of Mass

Balancing Equations

• In rockets– Hydrazine + Oxygen yields Nitrogen +

Water– N2H4 + O2 → N2 + H2O

– Does the number of atoms of each element on the left side of the arrow equal the number of atoms on the right side of the arrow?

• The equation is not in balance

Balancing Equations

• We balance equations by adding coefficients (a whole number) before the formula itself– We never change subscripts when

balancing an equations

• N2H4 + O2 → N2 + 2H2O

Some Practice Balancing Equations• H2 + O2 → H2O

• HgO → Hg + O2

• Zn + HCl → ZnCl2 + H2

Inquiry Activity

• Procedure1. Pour 100 mL of water into a resealable plastic

bag. Flatten the bag and remove all of the air and then seal the bag. Measure and record the mass of the bag and the water

2. Measure and record the mass of 1 Alka Seltzer tablet.

3. Add the masses of the bag with water and the Alka Seltzer tablet and record the number

4. Slightly open one side of the bag and drop the tablet into the water. Quickly reseal the bag. After the bubbling has stopped, measure and record the mass of the bag and its contents

Answer these questions

• Does a chemical change take place?• How do you know?• What happened to the mass of the

plastic bag and its contents after the bubbling stopped?

• What might this information tell you about a chemical change?

Counting with Moles

• Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance

• 1 mole = 6.02 X 1023 particles of that substance– Particles might be atoms, ions,

molecules

Molar Mass

• Molar mass – the mass of one mole of a substance– the atomic mass expressed in grams– 1 mole of Na = 23 grams– 1 mole of Cl = 35 grams– 1 mole of NaCl = 58 grams– 1 Mole of Hydrogen = 1 gram– 1 Mole of Oxygen = 16 grams– 1 Mole of H2O = 18 grams

Mole-Mass Conversions

• Express molar mass as a conversion factor– ie: CO2 has a molar mass of 44 grams

• 1 mole of Carbon = 12 grams• 2 moles of Oxygen = 32 grams (16 g x 2)

– 44.0g CO2/1 mole of CO2

– 1 mole of CO2/44.0 g CO2

– How many moles of CO2 do I have if I have 55 grams of CO2

The Calculation

• 55.0 g CO2 X 1 mole of CO2/44 g CO2

= 1.25 moles of CO2

• If you know how many grams of a substance you have divide by the molar mass to determine how many mole you have

Types of Reactions

• Synthesis Reactions• Decomposition Reactions• Single-Replacement Reactions• Double-Replacement Reactions• Combustion Reactions

Synthesis Reactions

• A synthesis reaction is a reaction in which two or more substances react to form a single substance – the reactants may be either elements or compoundsA + B → AB

2 Na + Cl2 → 2 NaCl

2 H2 + O2 → 2 H2O

This reaction generates electricity for satellites and spacecraft

Decomposition Reactions

• A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances – The reactant in a decomposition reaction must be a compound – the products may be elements or compounds

AB → A + B

2 H2O → 2H2 + O2

CaCO3 → CaO + CO2

Making cement

2NaN3 → 2 Na + 3 N2 (Air bags)

Single-Replacement Reactions

• A Single-replacement reaction is a reaction in which one element takes the place of another element in a compoundA + BC → B + AC (A and B have switched

places)

Cu + 2Ag NO3 → Cu(NO3)2 + 2Ag

2K + 2H2O → H2 + 2 KOH

Double-Replacement Reactions

• A double-replacement reaction is one in which two different compound exchange positive ions and form two new compoundsAB + CD → AD + CB

Pb(NO3)2 + 2 KI → PbI2 + 2KNO3

CaCO3 + 2 HCl → CaCl2 + H2CO3

Combustion Reactions

• A combustion reaction is one in which a substance reacts rapidly with oxygen, often (not always) producing heat and light.

• One of the reactants must be oxygenCH4 + 2 O2 → CO2 + H2O

2H2 + O2 → 2H2O

Reactions as Electron Transfers (Redox Reactions)• Oxidation – A loss of electrons during a

chemical reaction• Reduction – a gain of electrons during a

chemical reaction• 2 Ca + O2 → 2 CaO

– Calcium is oxidized– Oxygen is reduced

• Na + Cl →NaCl– Sodium is oxidized– Chlorine is reduced

Lab: Using Single-Replacement Reactions to Compare Reactivities

1. Read the entire lab and answer the Pre-Lab Discussion Questions before you do anything. Be sure you read the background information (This should take AT LEAST 15 minutes)

2. No group begins without showing Mr. W that you have answered the pre-lab discussion questions and understand the lab

3. YOU MUST WEAR GOGGLES TODAY!4. ARRANGE IN 8 LAB GROUPS – NONE

MORE THAN 3

Lab Today

• Metal + HCl → Metal Chloride (An Ionic Compound soluble in water) + H2 (Hydrogen Gas)

• Some of the reactions may not take place!

• You need to do the go further in the lab today!

Energy Changes in Reactions

• C3H8 + 5O2 → 3CO2 + 4 H2O + Heat

• Chemical energy is the energy stored in the chemical bonds of a substance

• Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products

Breaking Bonds

• Breaking bonds requires energy– This is why we need a spark to light the

propane because it doesn’t happen spontaneously

• Forming bonds requires energy• During a chemical reaction energy is

either released or absorbed

Exothermic Reactions

• A chemical reaction in which energy is released to the surroundings

• Combustion reactions are extreme exothermic reactions– 1 mole of Propane produces 2220

kilojoules of heat– In an exothermic reaction the kilojoules

would be written to the right of the arrow

Endothermic Reactions

• A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction– 2HgO + Heat → 2 Hg + O2

– In an endothermic reaction heat appears on the left side of the arrow

Conservation of Energy

• The total amount of energy before and after a reaction must always be the same

Reaction Rates

• The progress of a chemical reaction can be measured over time

• Different reactions have different durations– TNT– Leaves turning color

• Any reaction that takes places over time can be expressed as a rate– A reaction rate is the rate at which reactants

change into products over time – in other words how fast the reaction is going

Factors affecting Reactions Rates• Temperature

– Generally an increase in temperature will increase the reaction rate

• Surface area– Increasing the surface area of a reactant

tends to increase the reaction rate• Stirring

– Stirring the reactants will generally increase the reaction rate

Factors affecting Reactions Rates• Concentration

– Increasing the concentration generally increases the reaction rate

• Catalysts– A substance that affects the reaction rate

without being used up in the reaction• Generally speed up a reaction

Equilibrium

• Equilibrium is a state in which the forward and reverse paths of a change take place at the same rate

• Physical Equilibrium– H2O (l) ↔ H2O (g)

– When physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes

Equilibrium

• Most chemical reactions are reversible to some extent

• A reversible reaction is a reaction in which the conversion of reactants into products and the conversion of products into reactants can happen simultaneously– 2 SO2 (g) + O2 (g) ↔ 2 SO3 (g)– When a chemical reaction does not go to

completion, a chemical equilibrium is established between the forward and reverse reactions

Factors Affecting Chemical Equilibrium• When a change is introduced to a system in

equilibrium, the equilibrium shifts in the direction that relieves the change– Discovered by Henri Le Chatlier

N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat→ This reaction is exothermic← This reaction would be endothermic

• Temperature– If you add heat to the system the equilibrium would shift

in the direction that removes heat from the system (In the above reaction you would decrease the amount of ammonia produced

Factors Affecting Chemical Equilibrium• Pressure

– Increasing the pressure would shift the equilibrium in the direction that reduces the pressure• More gas molecules = more pressure• Increasing the pressure in the ammonia

production would produce more ammonia because the right side of the equation has fewer ( 2 vs. 4 ) gas molecules

N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat

Factors Affecting Chemical Equilibrium• Concentration

– If we removed ammonia from the system the reaction would favor producing more ammonia

– If we removed either of the reactants the reaction would favor breaking down the ammonia and producing more of the reactants

N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat