How Much? The Amount Of Chemical Change
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レッスンノート
Balancing Equations
- A symbol equation uses chemical symbols to show the number and type of atoms in reactants and products; a word equation uses only words.
- Atoms cannot be created or destroyed in a chemical reaction, so the number of each atom must be the same on both sides — the equation must be balanced.
- When balancing: never change any chemical formulae; put numbers in front of formulae; do not split polyatomic ions (like SO₄²⁻ or NO₃⁻) if they appear unchanged on both sides.
- For combustion of organic compounds, balance carbon first, then hydrogen, then oxygen.
- Use state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
- Some elements are diatomic in their natural state and must be written with a subscript 2: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
A balanced chemical equation
Reacting Mass Calculations
- The number of moles is calculated using: number of moles = mass of substance (g) / molar mass (g mol⁻¹).
- Be clear about the particle: 1 mole of CaF₂ contains 1 mole of CaF₂ formula units, but 1 mole of Ca²⁺ ions and 2 moles of F⁻ ions.
- The stoichiometry (mole ratio) from the balanced equation links the amounts of reactants and products.
- To find the mass of product: calculate moles of reactant, use the mole ratio, then convert moles to mass using mass = moles × molar mass.
- You can work in any mass unit (grams, tonnes, etc.) as long as you are consistent, because reacting masses are in proportion to the balanced equation.
Avogadro's Law & Molar Volume of Gas
- Avogadro's Law states that equal volumes of gases contain the same number of molecules, so gas volume ratios equal mole ratios.
- At STP (0 °C and 100 kPa), one mole of any gas occupies 22.7 dm³ mol⁻¹.
- Volume of gas = amount of gas (mol) × 22.7 dm³ mol⁻¹; amount of gas (mol) = volume of gas (dm³) / 22.7 dm³ mol⁻¹.
- To convert cm³ to dm³, divide by 1000.
- In gas reactions, identify the limiting reactant first if volumes are not in the same ratio as the coefficients.
- Quick method to find limiting reactant: divide the moles (or volumes) of each reactant by its coefficient; the smallest result is the limiting reactant.
Concentration Calculations
- Volumetric analysis uses a standard solution of known concentration to find the concentration of an unknown solution, usually by titration.
- In a titration, a known volume (e.g. 20 or 25 cm³) is measured with a pipette into a conical flask; the other solution is added from a burette until the indicator changes colour.
- Multiple trials are carried out until concordant results are obtained (within a small range).
- The four steps: write the balanced equation; calculate moles of the known substance; use the mole ratio; calculate the unknown amount/concentration/volume.
- Concentration (mol dm⁻³) = moles / volume (dm³); moles = concentration × volume (dm³).
- For monoprotic acid–base reactions, the shortcut C₁V₁ = C₂V₂ can be used (volumes in cm³ are fine as units cancel).
- Back titration: react the unknown with an excess of a reagent, then titrate the excess to find the amount of the original substance.
Concentration, moles and volume
Limiting & Excess Reactants
- The limiting reactant is the reactant that is completely used up and determines the amount of product formed.
- The excess reactant is the reactant that is not completely used up and is left over at the end.
- To find the limiting reactant: calculate moles of each reactant, then divide each by its coefficient from the balanced equation; the smallest result is limiting.
- Alternatively, compare the required mole ratio: calculate how much of one reactant is needed to fully react with the other and see which runs out first.
Percentage Yield Calculations
- The theoretical yield is the maximum amount of product possible from the limiting reactant, assuming perfect conversion.
- The experimental (actual) yield is the amount of product actually collected in the lab.
- Percentage yield = (actual yield / theoretical yield) × 100.
- The limiting reactant must be identified first to calculate the correct theoretical yield.
- Differences between theoretical and actual yield arise from side reactions, loss during transfer or purification, and incomplete reaction.
- Ensure actual and theoretical yields are in the same units before calculating percentage yield.
Atom Economy
- Atom economy measures how efficiently reactants are converted into the desired product.
- Atom economy = (molecular mass of desired product / sum of molecular masses of all reactants) × 100.
- Alternatively, atom economy = (mass of desired product / total mass of all products) × 100.
- Addition reactions always have 100% atom economy because all reactant atoms end up in the single product (e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br).
- A low atom economy means more waste and by-products, leading to higher environmental and economic costs.
- Atom economy is calculated from the balanced equation, not from experimental data.
- Other factors for process efficiency include rate, quantities of catalysts and solvents, energy use, and economic efficiency.
スライド
練習問題
無料プレビュー — 65問中8問。すべて見るには登録を。
1.Which of the following is the correct definition of the term 'limiting reactant'?
Easy- AThe reactant that is completely used up in a reaction and determines the amount of product formed
- BThe reactant that is left over at the end of a reaction
- CThe reactant that is present in the largest amount by mass
- DThe reactant that is present in the largest amount by moles
2.In a chemical reaction, atoms are created or destroyed.
EasyTrue or false?
3.Which of the following statements about atom economy are correct? (select all that apply)
Easy- AIt is a measure of how efficiently a reaction converts reactants into the desired product.
- BIt is calculated using experimental data.
- CAddition reactions typically have an atom economy of 100%.
- DA low atom economy means less waste is produced.
- EIt can be calculated using the formula: (molecular mass of desired product / sum of molecular masses of all reactants) × 100.
4.A student calculates the percentage yield of a reaction. The theoretical yield is 6.5 g and the actual yield is 5.8 g. What is the percentage yield?
Medium- A89.2%
- B112%
- C10.8%
- D89.2 g
5.A sample of marble contains calcium carbonate. 27.20 cm³ of 0.200 mol dm⁻³ HCl was added to 0.188 g of marble. The excess acid required 23.80 cm³ of 0.100 mol dm⁻³ NaOH for neutralisation. What is the percentage by mass of calcium carbonate in the marble?
Medium- A81.5%
- B18.5%
- C40.8%
- D95.0%
6.In a titration, concordant results are those that are within a small range of each other, typically within 0.10 cm³.
EasyTrue or false?
7.Match each term with its correct definition.
Easy- Limiting reactant
- Theoretical yield
- Atom economy
- The maximum amount of product that can be formed from a given amount of reactants, assuming perfect conversion.
- A measure of how efficiently a reaction converts reactants into the desired product.
- The reactant that is completely used up in a reaction and determines the amount of product formed.
8.Place the following steps in the correct order for carrying out a titration.
Easy- Rinse the burette with the solution it will contain and fill it.
- Measure a known volume of the solution into a conical flask using a pipette.
- Add a few drops of indicator to the conical flask.
- Slowly add the solution from the burette until the indicator changes colour.
- Repeat the titration until concordant results are obtained.
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