Unit 4-4 Leftover of Excess Reactant

Flashcards covering the key concepts and calculations related to limiting and excess reactants.

Limiting Reactant (LR)

Gives the least amount of products and is completely consumed in a reaction.

Excess Reactant (ER)

Reactant that is not completely used up after a reaction; there is some leftover.

Finding LR

H2(g) + Cl2(g) -> 2HCl(g); if H2 is used up, calculate moles of HCl; if Cl2 is used up, calculate moles of HCl; the one giving the least product is LR.

Leftover of ER

Moles of initial ER - Moles of consumed ER

Steps to Calculate Leftover ER in Grams

Convert grams of reactants A and B to moles. Find LR by calculating moles of product C produced. The reactant yielding the smallest amount of product C is the LR; the other is ER.

Steps to Calculate Leftover ER in Grams (Continued)

Calculate moles of consumed ER by converting from moles of either LR or product C. Leftover of ER = Initial ER - Consumed ER. Calculate grams of leftover ER.

Conversion

grams of A -> moles of A -> moles of B

Example 4 Concept Plan

9. 05 g NH3, 45.2 g CuO; Convert grams to moles for both reactants, convert moles of each reactant to moles of product (N2), compare the amount of N2 produced to identify LR and ER, determine the smallest moles of N2, and calculate moles of consumed ER and leftover ER.

Step 1 & 2: Finding LR and ER

Convert grams of NH3 and CuO to moles. Use the balanced equation to find moles of N2 produced by each reactant. The smaller amount of N2 indicates the LR.

Step 3: Find Amount of ER

Find amount of ER from the moles of product: 0. 189 mol N2 * (2 mol NH3 / 1 mol N2) * (17.03g NH3 / 1 mol NH3) = 6.44 NH3 consumed; Leftover ER = Initial ER – Consumed ER.

Leftover of ER

Leftover NH3 = 9.05 g – 6.45 g = 2.60 g NH3; Leftover of ER = Initial ER – Consumed ER

HW problem ER calculation

CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(g); if find that O2 is the LR then, 8 moles O2 * (1 moles 4 / 2 moles O2) = 4 moles CH4. Then, 5 moles – 4 moles = 1 moles CH 4