Chemical Reactions Chapter 7

Physical vs. Chemical Changes A physical change will NOT affect the chemical properties of a substance. [only physical properties change] Example: melting ice. Ice is still H2O

Physical vs. Chemical Changes A chemical change will affect a substance’s physical and chemical properties because it involves a chemical reaction. Example: iron rusting

Chemical Reactions A chemical reaction occurs when substances undergo chemical changes because of their chemical properties to form new substances.

Chemical Reactions Chemical reactions create brand new properties that did not normally exist for any of the reacting substances. Example: Red and blue paint mixed together to make purple (normal blend) PHYSICAL CHANGE. Example: Nickel (silver) and oxygen (transparent) mixed with glass (clear) make purple (abnormal blend) CHEMICAL CHANGE.

Chemical Reactions CHEMICAL CHANGE!

Chemical Reactions A chemical reaction has 2 parts: A reactant is a substance that undergoes a chemical change. A product is a substance that is the result of a chemical change. Reactants Products

Chemical Reactions Signs of a chemical change: the formation of bubbles (production of gas)

Chemical Reactions Signs of a chemical change: formation of a precipitate (solid in a solution)

Chemical Reactions Signs of a chemical change: formation of light

Chemical Reactions Signs of a chemical change: formation of cold

Chemical Reactions Signs of a chemical change: formation of heat

Chemical Reactions Signs of a chemical change: change in color ** change in color does NOT always mean a chemical change took place

Energy in Chemical Reactions Chemical changes always result in a change of energy. Chemical energy is the energy gained or released when a chemical compound reacts to produce new compounds.

Energy in Chemical Reactions When more energy is needed to break bonds in a reaction than to form bonds we call this an endothermic reaction. This reaction will feel COLD. (ABSORBS heat) The bottom of the beaker becomes cold enough to freeze the water and stick to the wood. Ammonium thiocyanate is mixed with barium hydroxide. A drop of water is placed on a block of wood.

Energy in Chemical Reactions ENERGY IS ABSORBED; Change in Heat is Positive

Energy in Chemical Reactions When more energy is released to form bonds than is needed to break bonds we call this an exothermic reaction. This reaction will feel HOT. Heat is released. Ex. Rxn. between Al and Br

Energy in Chemical Reactions ENERGY IS RELEASED; Change in Heat is Negative

Chemical Equations A chemical equation is an equation that uses chemical formulas and symbols to show the reactants and products in a chemical reaction.

reactants “yields” products Chemical Equations Example of a chemical reaction: CH4 + O2 → CO2 + H2O reactants “yields” products Translation: Carbon tetrahydride reacts with oxygen to make (yield) carbon dioxide and water.

Balancing Equations An equation must have the same amount of elements in the products as it does in the reactants. The Law of Conservation of Mass says that matter cannot be created or destroyed, only changed. Count the # of atoms… they are the SAME! +

Balancing Equations Al + S Al2S3 Not Balanced 2Al + 3S Al2S3 Balanced When the number of atoms of each element of the reactants, matches the number of atoms of each element of the product, a chemical equation is called BALANCED. This not only applies to the number of atoms, but also the moles and mass. Al + S Al2S3 Not Balanced 2Al + 3S Al2S3 Balanced

It’s all the same Balancing Equations + + H2 + Cl2 2 HCl Total atoms = Total atoms 2 H, 2 Cl 2H, 2 Cl Total Mass = Total Mass 2(1.0) + 2(35.5) 2(36.5) 73.0 g = 73.0 g + + It’s all the same

How to Balance Equations Coefficients are the ONLY way that you can balance an equation (NO SUBSCRIPTS CAN BE ADDED). Coefficients go in front of formulas balance each type of atom. They multiply ONLY across the molecule that they are in front of. 4NH3 + 5O2 4NO + 6H2O

How to Balance Equations Always check the equation by counting atoms on each side of the equation. For each element, the number of atoms on the left has to be equal to the number of atoms on the right. H2SO4 + 2NaOH 2 H2O + Na2SO4 H S O Na H S O Na 4 1 6 2 4 1 6 2

How to Balance Equations Balance elements that appear in multiple compounds on the same side or very complex compounds last. C6H12O6 + O2 CO2 + H2O 6 6 6 C H O C H O 6 12 18 8 6 1 12 2 18 13 3

How to Balance Equations Whole number coefficients are preferred. But use fractions during the balancing process. The equation can be scaled to remove them later. C2H6 + O2 CO2 + H2O 2 7 7/2 4 2 6 3 C H O C H O 4 2 12 6 14 2 7 1 2 4 12 2 6 14 5 3 7

How to Balance Equations When you notice that the same polyatomic ions appear on both sides of the equation, it's much easier to balance the equation as though the polyatomic ions were 'atoms'. For example: (NH4)3PO4 + CaCl2 Ca3(PO4)2 + NH4Cl 2 3 6 NH4 PO4 Ca Cl NH4 PO4 Ca Cl 3 6 1 2 1 3 2 6 6 1 2 3 1 6

How to Balance Equations Write the equation so that the coefficients are the smallest set of integers possible. For example, if all of the coefficients are divisible by 2, divide them all by 2. H2O O2 + H2 2 4 2 2 4 H O H O 8 4 8 4 Technically both are correct 4 2 4 2

Learning Check Balance each equation. A. Mg + N2 Mg3N2 B. Al + Cl2 AlCl3 C. Fe2O3 + C Fe + CO2 D. Al(OH)3 + FeO Fe(OH)2 + Al2O3

How did you do? Balance each equation. A. 3Mg + N2 Mg3N2 B. 2Al + 3Cl2 2AlCl3 C. 2Fe2O3 + 3C 4Fe + 3CO2 D. 2Al(OH)3 + 3FeO 3Fe(OH)2 + Al2O3

Types of Reactions Synthesis reactions are reactions of at least two substances that form a new more complex compound. A+B→AB 2Na+Cl2→2NaCl

Types of Reactions Decomposition reactions are reactions in which one compound breaks into at least two products. AB→A+B 2H2O→2H2+O2

Types of Reactions Single-Replacement (single-displacement) reactions: reactions in which atoms of one element take the place of atoms of another element in a compound. XA+B→BA+X 3CuCl2+2Al→2AlCl3+3Cu

Types of Reactions Double-Replacement (double-displacement) reactions: a reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of ions between two compounds. AX+BY→AY+BX Pb(NO3) 2+K2CrO4→PbCrO4 + 2K(NO3)

Types of Reactions Combustion reactions are reactions in which a compound and oxygen burn. An equation with oxygen in the reactant and water in the product is a combustion reaction. Example: isooctane and oxygen.

Types of Reactions Reduction/Oxidation (redox) reactions occur when electrons are transferred from one reactant to another.

Types of Reactions Example: When iron rust to form Fe2O3, each iron atom loses three electrons to form Fe3+ ions and each oxygen atom gains two electrons to form O2- ions. Which element is oxidized and which is reduced?

Rates of Reactions Chemical changes can occur at different rates depending on the factors involved during the reaction.

Rates of Reactions Factors that affect a chemical reaction: Surface Area: With increased surface area there are more chances that particles will collide and react with each other.

Rates of Reactions vs. Factors that affect a chemical reaction: Higher Temperature: the higher the temperature of a substance, the faster its particles move, and the higher the chances of a collision and subsequent reaction between particles. vs.

Rates of Reactions Factors that affect a chemical reaction: Size: the smaller the molecule of a substance, the faster its particles move, and the higher the chances of a collision and subsequent reaction between particles.

Rates of Reactions Factors that affect a chemical reaction: Concentration: the higher the concentration the more particles of a substances and a higher likelihood for collision and reaction.

Rates of Reactions Factors that affect a chemical reaction: Pressure: Increasing the pressure of a gas is exactly the same as increasing its concentration. Under higher pressure or at a higher concentration, gas molecules collide more frequently and react at a faster rate.

Rates of Reactions Factors that affect a chemical reaction: Presence of a Catalyst: These are substances that change the rate of a chemical reaction without being consumed. They usually lower the activation energy (the energy required for a chemical reaction to occur) in chemical reactions. The opposite of a catalyst is an inhibitor which will actually slow down the rate of a reaction.

Rates of Reactions Enzymes are complex proteins in our body that work as catalysts.

Equilibrium During chemical equilibrium a state of balance in which the rate of a FORWARD reaction equals the rate of the REVERSE reaction and the concentrations of products and reactants remain unchanged. CaCO3 CaO + CO2

Equilibrium Le Châtelier’s principle states that if a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached.