Practice Enthalpy in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

A thermodynamic state function H=U+pVH = U + pV; at constant pressure the enthalpy change equals the heat exchanged, ฮ”H=qp\Delta H = q_p, which is negative for exothermic and positive for endothermic processes.

Enthalpy change tells you how much heat a reaction releases or absorbs at constant pressure: ฮ”H<0\Delta H < 0 means heat is released (exothermic, the surroundings warm up) and ฮ”H>0\Delta H > 0 means heat is absorbed (endothermic).

Showing a random 20 of 50 problems.

Example 1

hard
For 4Fe+3O2โ†’2Fe2O34Fe + 3O_2 \rightarrow 2Fe_2O_3, ฮ”H=โˆ’1648\Delta H = -1648 kJ. Find heat released when 11.211.2 g of FeFe (55.8 g/mol) reacts completely.

Example 2

medium
On an enthalpy diagram, an endothermic reaction shows products at a ____ enthalpy than reactants.

Example 3

medium
If 50.0 g of H2OH_2O (c=4.18c = 4.18 J/(gยทK)) warms from 20.020.0 ยฐC to 80.080.0 ยฐC, how much heat (kJ) was absorbed?

Example 4

easy
A reaction has ฮ”H=โˆ’890\Delta H = -890 kJ. Is it exothermic or endothermic?

Example 5

easy
If forward ฮ”H=โˆ’150\Delta H = -150 kJ, what is ฮ”H\Delta H for the reverse reaction?

Example 6

easy
For C+O2โ†’CO2C + O_2 \rightarrow CO_2 with ฮ”H=โˆ’394\Delta H = -394 kJ, how much heat is released per mole of carbon?

Example 7

hard
A reaction's ฮ”H=+85\Delta H = +85 kJ. At constant pressure, 1.001.00 mol reacts in a system that does 5.05.0 kJ of work on the surroundings. Find ฮ”U\Delta U for this process.

Example 8

medium
Burning 1 mol C2H2C_2H_2 (acetylene) releases 13001300 kJ. Find ฮ”H\Delta H for 2C2H2+5O2โ†’4CO2+2H2O2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O.

Example 9

medium
Is bond breaking endothermic or exothermic, and how does that affect ฮ”H\Delta H of a reaction?

Example 10

medium
A reaction has ฮ”H=โˆ’240\Delta H = -240 kJ per mol of AA. How much heat is released using 0.750.75 mol of AA?

Example 11

easy
If the forward reaction has ฮ”H=+200\Delta H = +200 kJ, what is ฮ”H\Delta H for the reverse?

Example 12

hard
For CaCO3(s)โ†’CaO(s)+CO2(g)CaCO_3(s) \rightarrow CaO(s) + CO_2(g), ฮ”Hfโˆ˜\Delta H_f^\circ values (kJ/mol) are CaCO3=โˆ’1207CaCO_3 = -1207, CaO=โˆ’635CaO = -635, CO2=โˆ’394CO_2 = -394. Find ฮ”H\Delta H.

Example 13

medium
Reaction: S+O2โ†’SO2S + O_2 \rightarrow SO_2, ฮ”H=โˆ’297\Delta H = -297 kJ. Find ฮ”H\Delta H for SO2โ†’S+O2SO_2 \rightarrow S + O_2.

Example 14

challenge
Given N2+3H2โ†’2NH3N_2 + 3H_2 \rightarrow 2NH_3, ฮ”H=โˆ’92\Delta H = -92 kJ. Find ฮ”H\Delta H for 12N2+32H2โ†’NH3\frac{1}{2}N_2 + \frac{3}{2}H_2 \rightarrow NH_3 and for 2NH3โ†’N2+3H22NH_3 \rightarrow N_2 + 3H_2.

Example 15

medium
Given ฮ”Hf\Delta H_f: CO2=โˆ’394CO_2 = -394, H2O=โˆ’286H_2O = -286 kJ/mol, and CH4=โˆ’75CH_4 = -75 kJ/mol, set up ฮ”H\Delta H for CH4+2O2โ†’CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O.

Example 16

easy
If a reaction has ฮ”H=+120\Delta H = +120 kJ, classify it as exothermic or endothermic.

Example 17

easy
What is the standard unit of ฮ”H\Delta H in chemistry?

Example 18

medium
A coffee-cup calorimeter (constant pressure) records q=โˆ’2.5q = -2.5 kJ for a reaction with 0.10 mol of limiting reagent. Find ฮ”H\Delta H per mole.

Example 19

challenge
Combustion of 4.0 g of CH4CH_4 (16 g/mol) releases how much heat if ฮ”H=โˆ’890\Delta H = -890 kJ/mol?

Example 20

easy
State Hess's law in one sentence.
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