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AS Level chemistry question

"Calcium and strontium are Group 2 metals. They both react with water. A chemist reacts 0.200 g of strontium with 250 cm3 water, leaving a colourless solution containing strontium ions. The volume remains at 250 cm3 ."

"Write an equation for the reaction between strontium and water.
Include state symbols."

Why is the answer "Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)" and not "Sr(s) + H2O(l) -> SRO (aq) + H2 (g)"?

Also
"A student mixes 100 cm3
of 0.200 mol dm–3 NaCl(aq) with 100 cm3 of 0.200 mol dm–3 Na2CO3(aq). What is the total concentration of Na+ ions in the mixture formed?"
Why is the answer 0.3 moldm^-3?
Original post by Coolerthanapples
"Calcium and strontium are Group 2 metals. They both react with water. A chemist reacts 0.200 g of strontium with 250 cm3 water, leaving a colourless solution containing strontium ions. The volume remains at 250 cm3 ."

"Write an equation for the reaction between strontium and water.
Include state symbols."

Why is the answer "Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)" and not "Sr(s) + H2O(l) -> SRO (aq) + H2 (g)"?

Also
"A student mixes 100 cm3
of 0.200 mol dm–3 NaCl(aq) with 100 cm3 of 0.200 mol dm–3 Na2CO3(aq). What is the total concentration of Na+ ions in the mixture formed?"
Why is the answer 0.3 moldm^-3?


The Sr is reacting with water, and all group 2 metals form hydroxides with water. If it was burning in air it would form an oxide.

In Nacl there is an equal ratio of NA : CL so there is 0.2 mol. In Na2Co3, there are 2 x 0.2 mol so 0.4 mol.

0.4 + 0.2 = 0.6 mol

There are 200cm^3 in total so 0.6/0.02 dm^3 = 0.3 mol dm-3
https://chemistry.stackexchange.com/questions/6212/why-do-the-reactions-of-magnesium-in-steam-and-cold-water-differ

According to this link, the first 'stage' of the reaction between water and a group 2 element (such as magnesium) generally gives a hydroxide. This would be the reaction that requires less energy because it only removes one hydrogen from a water molecule, whereas the second one removes two. However, if given enough energy, water's two hydrogens could be removed and you could get your proposed answer. However, water would only have enough energy to break off its two hydrogens if it was in the form of steam, so only the peroxide would form. Moreover, the reaction would stop after a short time because the peroxide itself is a protective, insoluble layer which halts further reaction. You should just know that water reacts with group 2 metals differently with steam than with water. Also, some group 2s like barium can make peroxides :redface:

For your second question, convert 100cm^3 to 0.1dm^3. You can treat the Na in NaCl and Na2CO3 as their own, separate species because it's aqueous. The concentration would be kept the same in both cases. In the end you get:

0.1dm^3 of 0.2 mol dm^-3 Na makes 0.02 moles of Na (n=c*v)
and
0.1dm^3 of 0.2 mol dm^-3 Na2 = 0.1dm^3 of 0.4 mol dm^3 Na (from treating the Na as separate rather than joined up) that makes 0.04 moles of Na (n=c*v)

Adding them together gets

0.06 moles of Na in 0.2dm^3 hence 0.06/0.2 = 0.3 moldm^-3 Na (c=n/v)

Are you doing Edexcel? Exam is tomorrow, scary @_@
Original post by Latimaster
https://chemistry.stackexchange.com/questions/6212/why-do-the-reactions-of-magnesium-in-steam-and-cold-water-differ

According to this link, the first 'stage' of the reaction between water and a group 2 element (such as magnesium) generally gives a hydroxide. This would be the reaction that requires less energy because it only removes one hydrogen from a water molecule, whereas the second one removes two. However, if given enough energy, water's two hydrogens could be removed and you could get your proposed answer. However, water would only have enough energy to break off its two hydrogens if it was in the form of steam, so only the peroxide would form. Moreover, the reaction would stop after a short time because the peroxide itself is a protective, insoluble layer which halts further reaction. You should just know that water reacts with group 2 metals differently with steam than with water. Also, some group 2s like barium can make peroxides :redface:

For your second question, convert 100cm^3 to 0.1dm^3. You can treat the Na in NaCl and Na2CO3 as their own, separate species because it's aqueous. The concentration would be kept the same in both cases. In the end you get:

0.1dm^3 of 0.2 mol dm^-3 Na makes 0.02 moles of Na (n=c*v)
and
0.1dm^3 of 0.2 mol dm^-3 Na2 = 0.1dm^3 of 0.4 mol dm^3 Na (from treating the Na as separate rather than joined up) that makes 0.04 moles of Na (n=c*v)

Adding them together gets

0.06 moles of Na in 0.2dm^3 hence 0.06/0.2 = 0.3 moldm^-3 Na (c=n/v)

Are you doing Edexcel? Exam is tomorrow, scary @_@


OCR. Thanks for the help, I misunderstand some really simple chemistry stuff so making sure I cover up the cracks.
Original post by Coolerthanapples
OCR. Thanks for the help, I misunderstand some really simple chemistry stuff so making sure I cover up the cracks.


No problem! ^^ That was me before I made sure that I understood everything that they put on the specification. Good luck for the exam!
Whenever a Group 2 metal reacts with water it forms a hydroxide.

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