Q&A for How to Calculate the Enthalpy of a Chemical Reaction

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Will increasing pressure in the Haber process produce more or less ammonia?

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More ammonia will be produced. With pressure, entropy will reduce and gas molecules will interact effectively to produce more ammonia.

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How can I solve this problem: "The half-life of element X is 5 days. If we have 5g of X initially, what is the mass of X after 5 days, 20 days and 40 days"?

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After 5 days, there will be 2.5 g remaining. Every 5 days we divide by 2. Therefore after 10 days we have 1.25, after 15 we have 0.625, after 20 we have 0.3125 grams. You can do the same thing for 40 days. Heres a formula which is easier to use: A(t) = Ainitial*(1/2)^(t/k), where k is the half life, in this case 5, and t is the duration you are calculating for.

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How can I calculate the percentage error?

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100*[(absolute value of theoretical value - actual value) ÷ theoretical value]

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How do I know what the product should be when two reactants react?

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You should receive a reaction type with it if not the products directly- (EG: double/single replacement) if not, it's usually safe to assume one of those two when solving.

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How do I calculate a change in entropy?

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Entropy is nothing but change in the randomness of molecules. If a solid changes to vapor by sublimation of any other process, the tight molecules of the solid are released and they become free.

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Do I subtract the reactants from the products, or the product from the reactant?

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Subtract your final temperature (products) from your initial temperature (reactants).

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How do I calculate enthalpy change?

Kipkemei Metto

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A solid 30.2 cm3 block of KClO3 is heated in the laboratory and decomposes according to the following equation. What is the change in enthalpy in kJ when all the KClO3 decomposes? The density of KClO3 is 2.34 g/cm3. 2 KClO3 (s) --> 2 KCl (s) + 3 O2 (g) ΔH = -89.4 kJ.

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How I use specific heat and temperature in the equation for enthalpy?

Monster Hack

Community Answer

∆H = ms ∆T. Multiply the temperature changes with specific heat value of product and mass of reactant.

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