ocr a level chemistry enthalpy question

good evening,
i was doing an ocr a level question on enthalpy changes, and was confused why you had to divide q by the moles in AH = -q/n (if given AH and n and asked to find q), but not when using q=mcAT.
any help would be appreciated, thank you.

Reply 1

Nitrotoluene

Original post by dan_ah

Okay, so here's why you use those different formulas:
What you're measuring: ΔH* = -q/n links the total heat to the moles of the compound. q** = m c ΔT links heat to mass and temperature change.
What's going on: Enthalpy* changes talk about moles of the compound. Heat** calculations can be for any amount of compound.

Ciao,
Sandro

(edited 1 month ago)

Reply 2

dan_ah

thank you, sorry i think i definitely didnt word my question well. this was the question i was doing:

(a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq) as shown below.
Mg(s) + 2AgNO3(aq) 2Ag(s) + Mg(NO3)2(aq) ∆H = –678 kJ mol–1.
A student adds an excess of magnesium to 100.0 cm3 of 0.400 mol dm–3 AgNO3(aq). The initial temperature is 20.0 °C. (i) Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures.

and i found the joules correctly using AH=-q/n, but you then had to divide them by 2 (due to the 2Ag(NO3)) before putting them into q=mcAT and i was wondering why you had to do this and not just use the same value from the AH=-q/n equation.
thank you.

Reply 3

TypicalNerd

Original post by dan_ah

thank you, sorry i think i definitely didnt word my question well. this was the question i was doing:
(a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq) as shown below.
Mg(s) + 2AgNO3(aq) 2Ag(s) + Mg(NO3)2(aq) ∆H = –678 kJ mol–1.
A student adds an excess of magnesium to 100.0 cm3 of 0.400 mol dm–3 AgNO3(aq). The initial temperature is 20.0 °C. (i) Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures.
and i found the joules correctly using AH=-q/n, but you then had to divide them by 2 (due to the 2Ag(NO3)) before putting them into q=mcAT and i was wondering why you had to do this and not just use the same value from the AH=-q/n equation.
thank you.

You can think of the enthalpy change as being per mole of Mg that reacts (since the coefficient of Mg is 1).

The moles of magnesium that react are half the number of moles of silver ions (which you are given the means to calculate), so calculate the moles of Ag^+, halve that number to get the moles of Mg and use it in ΔH = -q/n.

Reply 4

Pigster

Original post by dan_ah

thank you, sorry i think i definitely didnt word my question well. this was the question i was doing:
(a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq) as shown below.
Mg(s) + 2AgNO3(aq) 2Ag(s) + Mg(NO3)2(aq) ∆H = –678 kJ mol–1.
A student adds an excess of magnesium to 100.0 cm3 of 0.400 mol dm–3 AgNO3(aq). The initial temperature is 20.0 °C. (i) Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures.
and i found the joules correctly using AH=-q/n, but you then had to divide them by 2 (due to the 2Ag(NO3)) before putting them into q=mcAT and i was wondering why you had to do this and not just use the same value from the AH=-q/n equation.
thank you.

Where (when) is this Q from? Do you get asked for enthalpy of reaction? i.e. per mol of reaction rather than per mol of any particular reactant. If you have done DH = -q/n what was your n? n(AgNO3)? But there are two of them. If you worked it out per mol of AgNO3, you need to convert that to per mol of reaction, since there are 2 mol of AgNO3, then don't you need to multiply DH by 2? (which is the same as dividing the n by 2)

Reply 5

Nitrotoluene

Original post by dan_ah

thank you, sorry i think i definitely didnt word my question well. this was the question i was doing:
(a) Magnesium reacts with aqueous silver nitrate, AgNO3(aq) as shown below.
Mg(s) + 2AgNO3(aq) 2Ag(s) + Mg(NO3)2(aq) ∆H = –678 kJ mol–1.
A student adds an excess of magnesium to 100.0 cm3 of 0.400 mol dm–3 AgNO3(aq). The initial temperature is 20.0 °C. (i) Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures.
and i found the joules correctly using AH=-q/n, but you then had to divide them by 2 (due to the 2Ag(NO3)) before putting them into q=mcAT and i was wondering why you had to do this and not just use the same value from the AH=-q/n equation.
thank you.

Here, the reaction indicates 2 moles of silver (2Ag) and 1 mole of magnesium nitrate (Mg(NO3)2) are formed. It does not imply that the heat released is evenly distributed among the products. It requires additional energy than that of magnesium nitrate to form the silver.

You can do this by dividing the enthalpy change by 2 to find the heat released for each mole of silver. The enthalpy change represents the heat per mole of the reactant released, but you want to know how much heat is released for each mole of silver produced.

If you skipped this division, you would find the energy released per mole of magnesium, and that is not what you are interested in. Divided by 2 to obtain ΔH per mol of Ag produced.

With the energy per mole of silver, you can easily calculate the maximum temperature reached in the reaction using Q = m c ΔT solving for ΔT. [ΔT = 52.4 °C (to 3 significant figures).]
Ciao,
Sandro

Reply 6

dan_ah

Original post by TypicalNerd

You can think of the enthalpy change as being per mole of Mg that reacts (since the coefficient of Mg is 1).
The moles of magnesium that react are half the number of moles of silver ions (which you are given the means to calculate), so calculate the moles of Ag^+, halve that number to get the moles of Mg and use it in ΔH = -q/n.