Limiting Reactants
Use:
2H2+O2→2H2O
A mixture contains 5.00 g of H2 and 20.0 g of O2. Which reactant is limiting?
2H2+O2→2H2O
5.00 g H2, 20.0 g O2
n(H2)=5.00 / 2.016≈2.48 mol
n(O2)=20.0 / 32.00=0.625 mol
Need 2 mol H2 per 1 mol O2. For 0.625 mol O2, required H2 = 1.25 mol (we have 2.48), so:
Limiting reactant: O2
Convert 745 mmHg to atm.
P=745/760≈0.980 atm
Is the following reaction endothermic or exothermic?
ΔH=−125 kJ
Exothermic
Balance the reaction:
__ C3H8+__ O2→__ CO2+__ H2O
C3H8+5O2→3CO2+4H2O
What is the electron geometry and molecular shape of CO2?
linear, linear
2K+Cl2→2KCl
A reaction mixture contains 12.0 g K and 15.0 g Cl2. Identify the limiting reactant.
Limiting reactant
Moles of K:
n(K)=12.0/39.1≈0.307 mol
Moles of Cl2:
n(Cl2)=15.0/70.9≈0.212 mol
Stoichiometric ratio: 2 mol K:1 mol Cl2.
For 0.212 mol Cl2, K required:
n(K) needed=2×0.212=0.424 mol
You only have 0.307 mol K → K is limiting.
A 3.00 liter container holds 0.120 mol of nitrogen gas at 325 K. Find the pressure.
n=0.120 mol, V=3.00 L, T=325 K
P=(nRT)/V=(0.120⋅0.08206⋅325)/3.00≈1.07 atm
An energy diagram shows products at a lower energy than reactants. Is the reaction exothermic or endothermic?
Exothermic
What type of reaction is this?
Zn+CuSO4→ZnSO4+Cu
Single replacement
Is BF3 polar or nonpolar?
nonpolar
N2+3H2→2NH3
A container has 5.00 g N2 and 2.00 g H2.
Which reactant is limiting?
Limiting reactant
Moles of N2:
n(N2)=5.00/28.0≈0.179 mol
Moles of H2:
n(H2)=2.00/2.02≈0.990 mol
For 0.179 mol N2, H2 required:
n(H2) needed=3×0.179=0.537 mol
You have 0.990 mol H₂ → N₂ is limiting.
A gas has a volume of 2.0 L at 1.0 atm and 300 K. What is the new volume at 2.0 atm and 300 K?
(P1V1)/T1=(P2V2)/T2
P1=1.0 atm, V1=2.0 L, T1= 300 K, T2=300 K, P2=2.0 atm
V2=(P1V1)/P2=(1.0⋅2.0)/2.0=1.0 L
An energy diagram shows a large activation energy and products higher in energy than reactants. Is the reaction exothermic or endothermic?
Endothermic
What mass of calcium chloride, CaCl2, contains 0.750 mol?
0.750 mol CaCl2, MW≈110.98 g/mol
m=0.750⋅110.98≈83.2 g
What intermolecular forces are present in CH3OH?
dispersion, dipole-dipole, hydrogen bonding
2Al+3Br2→2AlBr3
A mixture contains 4.50 g Al and 20.0 g Br2.
Determine the limiting reactant.
Limiting reactant
Moles of Al:
n(Al)=4.50/26.98≈0.167 mol
Moles of Br₂:
n(Br2) =20.0/159.8≈0.125 mol
Stoichiometric ratio: 2Al:3Br2.
For 0.167 mol Al, Br₂ required:
n(Br2) needed=32×0.167≈0.251 mol
You only have 0.125 mol Br₂ → Br₂ is limiting.
A gas at 1.0 atm is heated from 300 K to 450 K. What is the new pressure?
P1=1.00 atm, T1=300 K, T2=450 K
P2=1.00⋅(450/300)=1.50 atm
A reaction has ΔH=−92 kJ. Does the reaction release or absorb heat?
ΔH<0 → Exothermic., therefore heat is released.
Use:
N2+3H2→2NH3
What mass of NH3 is produced from 10.0 g of H2 (assuming excess N2)?
N2+3H2→2NH3
From 10.0 g H2:
n(H2)=(10.0)/(2.016)≈4.96 mol
n(NH3)=4.96⋅(2/3)≈3.31 mol
m(NH3)=3.31⋅17.03≈56.4 g
Rank the following compounds from lowest to highest boiling point:
CH4, H2O, NH3
CH4 < NH3 < H2O
CaCO3→CaO+CO2
A decomposition reaction starts with 50.0 g CaCO3 and produces 18.0 g CO2.
Identify the limiting reactant.
Hint: don't overthink!
You start with 50.0 g CaCO₃ and obtain 18.0 g CO₂.
Limiting reactant
Only one reactant (CaCO₃), so CaCO₃ is the limiting reactant.
A gas sample occupies 4.50 L at 0.980 atm and 310 K. How many moles of gas are present?
P=0.980 atm, V=4.50 L, T=310 K
n=(PV)/(RT)=(0.980⋅4.50)/(0.08206⋅310)≈0.173 mol
An energy diagram shows:
Reactants at 50 kJ, Products at 120 kJ, Activation energy of 80 kJ. What is the ΔH?
ΔH=Eproducts−Ereactants=120−50=+70 kJ
Endothermic.
Write and balance the reaction for solid magnesium reacting with aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas.
Mg+2HCl→MgCl2+H2
What is the electron geometry and molecular shape of CO32-?
trigonal planar, trigonal planar