Pearson Prentice Hall Physical Science: Concepts in Action Chapter 7 Chemical Reactions

7.1 Describing Reactions Objectives: 1. Interpret chemical equations in terms of reactants, products and conservation of mass 2. Balance chemical equations by manipulating coefficients 3. Explain what a mole is and calculate molar mass

Reactants & Products During a chemical equation, reactants (the stuff you start with) are changed into products (the stuff you finish with) The products actually are composed of the same stuff you started with, but the molecules have been rearranged by chemical bonding to produce completely new compounds with unique physical properties A basic chemical equation looks like this: REACTANTS PRODUCTS

A chemical reaction is when atoms interact with each other to form new substances What you put in = reactant What comes out = product Mass of reactants = mass of products MASS IS CONSERVED

Conservation of Mass Definition: a chemical equation is a chemical reaction in which reactants and products (separated by an arrow) are expressed as formulas The Law of Conservation of Mass states that mass is neither created nor destroyed during a chemical reaction

Balancing Equations/Manipulating Coefficients In order to show that mass is conserved during a reaction, a chemical equation must be balanced Definition: a coefficient is the number that appears BEFORE a formula Recall that subscripts are used to balance a formula Coefficients are used to balance the equation

Steps to balancing a chemical equation: 1. Write out the chemical equation 2. Balance the most unusual element or compound first, use subscripts (charges) as a guide 3. Continue with other elements, ending with the most common Example: Balance the chemical equation ZnCl2 + HF → ZnF + HCl

Moles & Molar Mass Definition: a mole is the counting unit that measures the amount of substance A mole contains 6.02 x 1023 particles (molecules, atoms, ions) of substance Definition: molar mass is the mass of one mole Molar mass is calculated by adding up the atomic masses of each element (multiplied by the subscript) Example: CO2 carbon=12g oxygen x 2 = 32g & 12 + 32= 44g The molar mass of CO2 is 44 g

7.2 Types of Reactions Objectives: 1. Classify chemical reactions as synthesis (also called combination), decomposition (decomp), single replacement (single displacement) double replacement (double displacement) or combustion reactions

Synthesis or Combination Reactions There are millions of substances but only five general types of reactions The first is a synthesis reaction where two substances combine to form a new more complex substance general form A + B → AB Example: 2H2 + O2 → 2H2O

Decomposition The second reaction type is decomposition, where one large substance breaks down to two or more smaller substances general form AB → A + B Example: 2NaClO3 → 2NaCl + 3O2

Single Replacement (Displacement) The next reaction type is a single replacement reaction where atoms of one element take the place of an element in a compound general form XA + B → A + XB example 3CuCl2 + 2Al → 2AlCl3 + 3Cu

Double Replacement (Displacement) A reaction type with a similar name is the double displacement where two compounds exchange their elements general form AX + BY → AY + BX example : AgNO3 + HCl → AgCl + HNO3

Combustion The final type of reaction is a combustion reaction where a compound (usually a hydrocarbon) and oxygen burn With enough oxygen the products are carbon dioxide and water without enough the products are carbon monoxide and water general form A + O2 → H2O + CO2 example 2CH4 + 4O2 → 4H2O + 2CO2

7.3 Energy Changes in Reactions Objectives: 1. Describe the energy changes that take place during chemical reactions 2. Classify chemical reactions as exothermic or endothermic 3. Explain how energy is conserved during chemical reactions

Energy Changes & Chemical Reactions Indications of a chemical reaction include color change, gas formation, and temperature change Atoms are just rearranged as bonds are broken and new ones formed Breaking bonds requires energy Forming bonds releases energy

Definition: chemical energy is the energy stored in the chemical bonds of a substance Chemical reactions involve the breaking of chemical bonds in the reactants & the formation of chemical bonds in the products Q: Does breaking chemical bonds require energy or release energy?

Exothermic & Endothermic Definition: Reactions that release energy are exothermic Definition: Reactions that absorb energy are endothermic ENERGY IS CONSERVED. Example: for an exothermic reaction energy stored in bonds (chemical energy) = energy released during reaction

Conservation of Energy Law of Conservation of Energy: during a chemical reaction, energy is neither created nor destroyed In exothermic reactions, the chemical energy of the reactants is converted to heat plus the chemical energy of the products

7.4 Reaction Rates Objectives: 1. Explain what a reaction rate is 2. Describe factors that affect chemical reaction rates

What a Reaction Rate is Definition: the reaction rate is the rate at which reactants change into products over time Reaction rates tell you how fast a reaction is going

Factors that Affect Reaction Rates Reactions can happen at different speeds Factors influencing reaction rates: Temperature: rxn’s go faster at higher temp Surface area: large surface area speeds up reaction Large particles have less surface area than the same amount of small particles (blocks)

Stirring or agitation (shaking) increases exposure of reactants & increases reaction rate Concentrated solutions react faster (better chance of atoms hitting each other) Rxn’s go quicker at higher pressure Catalysts speed up the reaction by lowering the energy needed (do not influence the products) Enzymes are biological catalysts

7.5 Equilibrium Objectives: 1. Identify and describe physical and chemical equilibria 2. Describe the factors affecting chemical equilibrium

Physical & Chemical Equilibrium Definition: equilibrium is a state in which the forwards and reverse paths take place at the same rate Some reactions are reversible, a double arrow indicates the reaction can go either way Equilibrium is when the amount of reactants being turned into products is the same as the amount of products being turned into reactants reactants ↔ products

When a physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes H2O(l) H2O(g) Definition: a reversible reaction is a reaction in which the conversion of the reactants into products can happen simultaneously (at the same time)

When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions 2SO2(g) + O2(g) 2SO3(g) Q: What happens during chemical equilibrium?

Factors Affecting Chemical Equilibrium LeChatelier’s principle predicts what will happen to equilibrium as a change is introduced to the system LeChatelier’s principle: if a change is made to a system at equilibrium the system will shift to oppose or relieve the change

Increasing temp favors the reaction that absorbs energy Increasing pressure favors the reaction that produces less gas Increasing concentration favors the reaction that produces less of that substance CONDITION Tempera-ture Pressure Concentra-tion

Example: N2 (g)+ H2 (g) ↔ NH3 (g) + heat Which way will the equilibrium shift when the reaction is cooled? When the pressure is raised? When more hydrogen is added?