Chemistry Test Study Guide: Chemical Reactions

Chemical Equations and Recipes

• A chemical equation is like a recipe because it shows the ingredients (reactants) and what they produce (products).

Parts of a Chemical Equation

• Reactants: The substances that are initially involved in a chemical reaction.
• Products: The substances that are produced as a result of the chemical reaction.
• +: Separates reactants from each other or products from each other.
• Coefficients: Numbers placed in front of reactants or products to balance the equation, indicating the ratio of moles.
• (s), (l), (g), or (aq): Symbols indicating the state of matter:
  • (s) - solid
  • (l) - liquid
  • (g) - gas
  • (aq) - aqueous (dissolved in water)

Thermodynamics of Chemical Reactions

• Exothermic: A reaction that releases energy, usually in the form of heat. The energy of the products is lower than the energy of the reactants.
• Endothermic: A reaction that absorbs energy, usually in the form of heat. The energy of the products is higher than the energy of the reactants.

Observations and Chemical Equations

• Observations can tell us about a reaction that a chemical equation might not, such as:
  • Heat released (exothermic)
  • Light produced
  • Color change
  • Evolution of a gas
  • Formation of a precipitate.
• Physical changes vs. chemical changes

Indicators of a Chemical Change

• Evolution of a gas.
• Formation of a precipitate.
• Color change.
• Change in temperature (either giving off heat or absorbing heat).
• Production of light.

Energy Flow in Chemical Reactions

• Bonds Breaking: Requires energy input (endothermic).
• Bonds Forming: Releases energy (exothermic).

Ions, Electrons, and Batteries

• How a Battery Works:
  • A battery consists of an anode, a cathode, and an electrolyte.
  • Chemical reactions at the anode and cathode create a flow of electrons through an external circuit.
  • Ion movement through the electrolyte maintains charge balance.
• Common Battery Anode Elements (Lithium, Sodium, Magnesium):
  • These elements are easily oxidized, meaning they readily lose electrons.
  • They have a high energy density, allowing for smaller and lighter batteries.
• Ion Movement and Electricity Production:
  • Ions move from the anode to the cathode through the electrolyte.
  • This ion movement balances the electron flow in the external circuit, enabling the battery to produce electricity.

Electron Movement Vocabulary

• LEO the lion says GER:
  • Lose Electrons = Oxidation
  • Gain Electrons = Reduction
• Reduction: The gain of electrons by a substance.
• Oxidation: The loss of electrons by a substance.

Conservation of Mass

• The total amount of matter in a system remains constant, regardless of chemical and physical changes.

Balancing Chemical Equations

• Balancing Equations: Ensuring that the number of atoms of each element is the same on both sides of the equation.
• Atom Inventory/Model Representation: A method to keep track of the number of atoms of each element on both sides of the equation.
• Example
  • $C<em>2H</em>2 + O<em>2 \rightarrow CO</em>2 + H_2O$
  • $FeS<em>2 + Cl</em>2 \rightarrow FeCl<em>3 + S</em>2Cl_2$
  • $Al + HAsO<em>3 \rightarrow H</em>2 + Al(AsO<em>3)</em>3$

Types of Chemical Reactions

• Synthesis: Two or more reactants combine to form a single product. $A + B \rightarrow AB$
• Decomposition: A single reactant breaks down into two or more products. $AB \rightarrow A + B$
• Single Exchange/Displacement: One element replaces another element in a compound. $A + BC \rightarrow AC + B$
• Double Exchange/Displacement: Two compounds exchange ions or elements. $AB + CD \rightarrow AD + CB$
  • Note: An insoluble precipitate may form as a product.
• Combustion: A substance reacts with oxygen, usually producing heat and light. Often involves hydrocarbons. $C<em>xH</em>y + O<em>2 \rightarrow CO</em>2 + H_2O$

Classifying Chemical Reactions

• Examples:
  • $16Ag + S<em>8 \rightarrow 8Ag</em>2S$ (synthesis)
  • $2KI + Br<em>2 \rightarrow 2KBr + I</em>2$ (single exchange)
  • $BaCl<em>2 + Na</em>2SO<em>4 \rightarrow BaSO</em>4 + 2NaCl$ (double exchange)
  • $2KClO<em>3 \rightarrow 2KCl + 3O</em>2$ (decomposition)
  • $2KI + Pb(NO<em>3)</em>2 \rightarrow PbI<em>2 + 2KNO</em>3$ (double exchange)
  • $Zn + F<em>2 \rightarrow ZnF</em>2$ (synthesis)
  • $2C<em>3H</em>7OH + 9O<em>2 \rightarrow 6CO</em>2 + 8H_2O$ (combustion)

Predicting Products and Balancing Equations

• Using the understanding of common chemical reaction types to predict the products and balance the equation.
• Examples:
  • $HgO \rightarrow$
  • $Mg + Ag<em>2SO</em>4 \rightarrow$
  • $C<em>5H</em>{10} + O_2 \rightarrow$
  • $K<em>2SO</em>4 + Ca(NO<em>3)</em>2 \rightarrow$
  • $Al + O_2 \rightarrow$
  • $F_2 + NaI \rightarrow$