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
- Write the equation with correct formulas for all compounds.
- List the symbols of each element in a column under the reactants and products, keeping polyatomic ions together if possible.
- Change coefficients to balance the number of atoms of each element.
- 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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