Reacting masses and yield
Introduced| English | Meaning |
|---|---|
| mole ratio/məʊl ˈreɪʃɪəʊ/ | The ratio of amounts of substances measured in moles, often given by the coefficients in a balanced equation. |
| stoichiometry/ˌstəʊɪkɪˈɒmətri/ | The quantitative relationships between reactants and products specified by a balanced chemical equation. |
| percentage yield/pəˈsentɪdʒ jiːld/ | Actual product yield divided by the theoretical maximum yield, multiplied by one hundred. |
| excess reagent/ekˈses rɪˈeɪdʒənt/ | A reactant present in more than the amount needed to react completely with the limiting reactant. |
| limiting reagent/ˈlɪmɪtɪŋ rɪˈeɪdʒənt/ | The reactant that is used up first and therefore limits the amount of product formed. |
How much product will you get?
- The numbers in a balanced equation give the mole ratio 摩尔比 — the stoichiometry 化学计量.
- From it we work out reacting masses.
- Real reactions give less than the maximum, and one reactant runs out first.
Reacting mass route
Follow a balanced equation from known mass to predicted product mass.
Percentage yield is the actual yield divided by the theoretical yield, times 100.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
It measures how much product you actually obtained compared with the maximum possible.
The stoichiometry of a reaction comes from:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The balancing numbers give the mole ratio in which species react and form.
The reacting-mass method
To find a reacting mass:
- change the known mass → moles ($n = m/M$).
- use the mole ratio to find the moles you want.
- change those moles → mass.
Put the steps of a reacting-mass calculation in order.
Difficulty 3/5 · 200 coins + a small bonus on first completion today
mass → moles → ratio → moles → mass. The mole ratio is the bridge between the two substances.
In 2H₂ + O₂ → 2H₂O, how many moles of O₂ are needed to react with 4 mol of H₂?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
The ratio H₂ : O₂ is 2 : 1, so 4 mol H₂ needs 4 ÷ 2 = 2 mol O₂.
Percentage yield 产率
- You usually get less product than the maximum.
A reaction makes 8 g of product when the maximum possible is 10 g. What is the percentage yield?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
percentage yield = (actual / maximum) × 100 = (8 / 10) × 100 = 80%.
Limiting and excess reagent 过量试剂
- The limiting reagent 限量试剂 runs out first and decides how much product forms.
- The excess reagent is left over.
- To find it: divide each reactant's moles by its number in the equation — the smallest result is limiting.
The reactant that runs out first (the limiting reagent) sets how much product forms.
The limiting reagent in a reaction is the one that:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The limiting reagent is used up first, so it sets the amount of product; the other is in excess.
The reactant that runs out first and decides how much product forms is the ______ reagent.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The other reactant is "in excess" — some of it is left over.
You've got it
- stoichiometry = mole ratio from the balanced equation
- reacting mass: mass → moles → ratio → moles → mass
- $\text{percentage yield} = \dfrac{\text{actual}}{\text{maximum}} \times 100\%$
- the limiting reagent runs out first and sets the product; base the calculation on it