Chemistry Flashcards

Physical vs. Chemical Change

• Physical Change:

  • Affects only physical properties.
  • The substance remains the same.
  • Examples: cutting, coloring, crushing, melting, boiling.

• Chemical Change:

  • Affects chemical properties.
  • A chemical reaction occurs.
  • A new substance is formed.
  • Examples: burning, rusting, decomposing.

Chemical Reactions and Equations

• Chemical Equation Example: 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)
  • Reactants: Starting substances/ingredients.
  • Products: Ending substances.
  • Yield sign: Indicates the direction of the reaction (points to the products).

Coefficients and Subscripts

• Coefficient:
  • The big number in front of an element or compound.
  • Can be changed to balance equations.
  • Can be quite large, especially in combustion reactions.
• Subscripts:
  • The little number within a compound.
  • Cannot be changed because changing it alters the compound.
  • Example: H2O (water) vs. H2O_2 (hydrogen peroxide).
• If only one of anything is needed (subscript or coefficient), it is not written but understood to be 1.
• The plus sign separates reactants or products.

Law of Conservation of Matter

• Matter cannot be created or destroyed, only changed in form.
• The mass of the reactants must equal the mass of the products.
  • Example: 50 grams of reactant yields 50 grams of product.
• Only coefficients are used to balance reactions and must go in front of the entire formula.

Balancing Equations: Steps

1. Write the equation with correct formulas for all compounds.
2. List the symbols of each element in a column under the reactants and products, keeping polyatomic ions together if possible.
3. Change coefficients to balance the number of atoms of each element.
4. Ensure the balanced equation is in its lowest form (reduce coefficients if necessary).

Balancing Equations: Example and Tips

• Example Equation: Fe + Cl2 \rightarrow FeCl3
  • Write down elements in the same order on both sides (e.g., Fe and then Cl).
  • Count the number of each atom on both sides.
    • Reactant side: 1 Fe, 2 Cl
    • Product side: 1 Fe, 3 Cl
  • Balance chlorine first by aiming for a common multiple (6).
    • Add a coefficient of 3 in front of Cl2 and a 2 in front of FeCl3.
    • This changes the equation: Fe + 3Cl2 \rightarrow 2FeCl3
  • Update counts: 1 Fe, 6 Cl on the reactant side; 2 Fe, 6 Cl on the product side.
  • Balance iron by adding a coefficient of 2 in front of Fe on the reactant side: 2Fe + 3Cl2 \rightarrow 2FeCl3
  • Final balanced equation. Check: 2 Fe and 6 Cl on both sides.
• If an element is alone, balance it last as it only affects one element.
• If a coefficient changes multiple things, address that first.

Writing Equations from Names

• Aqueous: Denoted as (aq).

• Example: Aqueous sodium hydroxide + Aqueous calcium bromide produces solid calcium hydroxide + aqueous sodium bromide.

• Translate the names into formulas, including states:

  • NaOH (aq) + CaBr2 (aq) \rightarrow Ca(OH)2 (s) + NaBr (aq).

Balancing with Polyatomic Ions

• When balancing, keep polyatomic ions together if they remain unchanged in the reaction.
• Example using NaOH (aq) + CaBr2 (aq) \rightarrow Ca(OH)2 (s) + NaBr (aq):
  • List elements and polyatomic ions: Na, OH, Ca, Br.
  • Count each on both sides to determine what needs balancing.
  • Balance by adjusting coefficients.
  • In this case, place a 2 in front of NaOH and NaBr to balance the equation: 2NaOH (aq) + CaBr2 (aq) \rightarrow Ca(OH)2 (s) + 2NaBr (aq).

Evidence of Chemical Reactions

• Release of light.
• Temperature change (not just heating on a hot plate).
• Odor change (not just having an odor).
• Sudden color change - mixing to clear things and getting a color.
• A gas is given off ( наблюдается fizzing or bubbles).
• Sudden appearance of a solid called a precipitate.

Practice Problems and Balancing Tips

• H2 + O2 \rightarrow H_2O
  • Balanced: 2H2 + O2 \rightarrow 2H_2O
  • Write "balance" if the equation is already balanced.
• When water ($\H_2O) is involved, rewrite it as HOH, especially if H and OH are separate on the reactant side; this makes balancing easier.
• H3PO4 + KOH \rightarrow K3PO4 + H_2O
  • Rewrite \H_2O as HOH.
  • Balanced: H3PO4 + 3KOH \rightarrow K3PO4 + 3HOH$$
• Potassium + Boron trioxide + p two zero three Heels k two o before I write the no.
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