Chemical Reactions Basics

Chemical Reactions

• A chemical reaction is a process where two or more substances interact, causing atoms to rearrange and form new substances with different properties.
• A physical change alters the form of matter, but not its chemical identity. Examples include changes of state (solid, liquid, gas), dissolving, and changing the size or shape of an object.
• A chemical change involves a chemical reaction, producing new substances with different properties. Examples include rusting and burning.
• Reactants are the substances you start with, and products are what you end up with. Bonds break in reactants and atoms rearrange to form new bonds in products.
• Evidence of chemical reactions includes: color change, light emission, formation of gas, and temperature change.

Classifying Reactions

• Synthesis: Two simple compounds combine to form a more complex compound.
  • Example: 2NO + O₂ → 2NO₂
• Decomposition: One reactant breaks down into two or more products.
  • Example: CH₄ + 2O₂ → CO₂ + 2H₂O
• Combustion: A substance reacts with oxygen, often producing heat and light.
  • Example: 2H₂O₂ → 2H₂O + O₂

Factors Affecting Reaction Rate

• Temperature: Higher temperatures increase reaction rates, as particles move faster and collide more frequently.
• Catalyst: A substance that speeds up a reaction but is not consumed in the process.
• Surface Area: Increasing surface area exposes more reactants, promoting faster collisions and thus faster reactions.
• Concentration: Higher concentrations increase reaction rates by increasing the number of reactant particles available for collisions.

Law of Conservation of Mass

• The Law of Conservation of Mass states that atoms are neither created nor destroyed in chemical reactions; rather, they are rearranged.
• Lavoisier discovered this law by showing that reactions that appear to gain or lose mass are actually involving interactions with gases in the air.

Writing Chemical Equations

• To write a balanced chemical equation, you need: the reactants, the products, the reactants' and products' chemical formulas/atomic symbols, and the direction of the reaction.
• Chemical equations are balanced when the number of each type of atom is the same on both sides of the equation.

Exothermic vs Endothermic Reactions

• Exothermic reactions release energy (often as heat or light). Less energy is required to break bonds in the reactants compared to the energy released when new bonds form in the products.
• Endothermic reactions absorb energy. More energy is needed to break bonds in the reactants compared to the energy released when new bonds form in the products. Examples include photosynthesis and some phase changes, Examples of Endothermic reactions include some phase changes, like melting ice.