Chemical Reactions

Chemical Reactions Processes in which one or more substances are converted into new substances with different physical and chemical properties

Chemical Reactions Processes in which one or more substances are converted into new substances with different physical and chemical properties Ex: rusted metal, burned toast

Chemical Reactions Parts of a chemical reaction: Reactants – substances before reaction occurs Products – substances produced by the reaction

Chemical Reactions Reason why reactions occur: Increase in stability for products

Types of Reactions Combination - two substances combine to form new substance A + B  AB Ca + Cl2  CaCl2

Types of Reactions Combustion – substance reacts with oxygen A + O2  AO CH4 + O2  CO2 + H2O

Types of Reactions Decomposition – substance breaks into two or more substances AB  A + B MnSO4  MnO + SO3

Types of Reactions Single-replacement – uncombined element “trades places” with an element in a compound A +BX  AX + B Cd + 2 HCl  CdCl2 + H2

Types of Reactions Double-replacement – two binary compounds “trade partners” AX +BY  AY + BX CaCO3 + 2 HCl  CaCl2 + H2CO3

Chemical Equations Sentences representing a chemical reaction

Chemical Equations 2 types Word equations - gives names of reactants & products Formula equations – shows chemical formulas and symbols

Chemical Equations Formula equations… Symbols used: + reacts/combines with, and  yields/produces  precipitate formed  gas emitted

Chemical Equations Formula equations… Symbols used: (s) solid state (l) liquid state (g) gaseous state (aq) aqueous state

Chemical Equations Diatomic molecule reminder… Hydrogen H2 Nitrogen N2 Oxygen O2 Fluorine F2 Chlorine Cl2 Bromine Br2 Iodine I2

Chemical Equations Unbalanced Examples (aka Skeleton Equations): Calcium reacts with oxygen to produce calcium oxide

Chemical Equations Unbalanced Examples (aka Skeleton Equations): Carbon reacts with oxygen to produce carbon dioxide

Chemical Equations Unbalanced Examples (aka Skeleton Equations): Solid sodium reacts with chlorine gas to produce solid sodium chloride

Chemical Equations Unbalanced Examples (aka Skeleton Equations): Nitrogen gas reacts with hydrogen gas to produce ammonia gas

Chemical Equations Unbalanced Examples (aka Skeleton Equations): Sodium hydroxide reacts with hydrogen chloride to produce water and sodium chloride

Chemical Equations Can include heat Endothermic – heat, from surroundings, enters the system, has positive heat of reaction (DH) value, reactants have lower potential energy than products

Chemical Equations Can include heat Endothermic reaction

Chemical Equations Can include heat Endothermic reaction: C(s) +2 S(s) + 89.3 kJ  CS2(l) C(s) +2 S(s)  CS2(l) DH = 89.3 kJ

Chemical Equations Can include heat Endothermic – heat, from surroundings, enters the system Exothermic – heat exits the system and goes to surroundings, has negative heat of reaction (DH) value , reactants have higher potential energy than products

Chemical Equations Can include heat Exothermic reaction

Chemical Equations Can include heat Exothermic reaction: CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ CaO(s) + H2O(l)  Ca(OH)2(s) DH = -65.2 kJ

Chemical Equations Can include heat For systems with a constant pressure, heat is also called enthalpy (so DH is also change in enthalpy)

Chemical Equations Can include heat Enthalpy diagrams – visual representation of a thermochemical equation

Chemical Equations Enthalpy diagrams – Endothermic reaction

Chemical Equations Enthalpy diagrams – Exothermic reaction

Chemical Equations How to balance: Write formula equation with correct formulas & symbols Count number of atoms on each side of arrow (make table if needed) Mass must be conserved (number of atoms on each side of arrow are equal), use coefficients to adjust Work with one atom or ion at a time Save H & O for last (they tend to be in more than one place & can be difficult) Recount to ensure that number of atoms are equal on each side

Chemical Equations Balancing examples: NaOH + HCl  H2O + NaCl Ca + O2  CaO

Chemical Equations Balancing examples: N2 + H2  NH3 CH4 + O2  CO2 + H2O

Chemical Equations Balancing examples: FeCl3 + Na2CO3  NaCl + Fe2(CO3)3 C2H6 + O2  CO2 + H2O

Chemical Reactions Reaction Rates Reactions happen if particles have enough energy.

Chemical Reactions Reaction Rates Factors Temperature Concentration Surface area/Particle Size Catalysts – substances that speed up reaction without being used

Chemical Reactions Reactions are reversible When rate of forward and reverse reaction are equal, system is at equilibrium.

Chemical Reactions Reactions are reversible Equilibrium is described by Le Chatelier’s principle If stress is applied to a system, the system changes to relieve the stress

Chemical Reactions Reactions are reversible Factors affecting equilibrium: Concentration Temperature Pressure Increasing pressure shifts equilibrium to whichever side has fewer particles. Decreasing pressure has opposite effect.

Chemical Reactions Reactions are reversible Equilibrium is described by a constant Keq, which is a ratio of products to reactants. Keq > 1, products favored Keq < 1, reactants favored