sourmango1
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#1
Report Thread starter 4 years ago
#1
Q: Equal masses of barium carbonate and magnesium carbonate powders are mixed together. The mixture is then heated using a Bunsen burner flame until there is no further change. A gas is given off. Which statements are correct?
1) The residue left after heating reacts with aqueous hydrochloric acid to produce carbon dioxide.
2) The percentage decrease in mass after heating is 26% (to 2 significant figures).
3) The gas given off during heating relights a glowing splint.

Is this question referring to the decomposition of the two carbonates? If so, shouldn't the residue produce MgCl2, BaCl2 and H20 with the reaction with HCl, since MgO and BaO were produced after decomposition? I also couldn't calculate the precentage decrease when using the formula for decomposition.

The answer is 1) and 2).
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charco
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#2
Report 4 years ago
#2
(Original post by sourmango1)
Q: Equal masses of barium carbonate and magnesium carbonate powders are mixed together. The mixture is then heated using a Bunsen burner flame until there is no further change. A gas is given off. Which statements are correct?
1) The residue left after heating reacts with aqueous hydrochloric acid to produce carbon dioxide.
2) The percentage decrease in mass after heating is 26% (to 2 significant figures).
3) The gas given off during heating relights a glowing splint.

Is this question referring to the decomposition of the two carbonates? If so, shouldn't the residue produce MgCl2, BaCl2 and H20 with the reaction with HCl, since MgO and BaO were produced after decomposition? I also couldn't calculate the precentage decrease when using the formula for decomposition.

The answer is 1) and 2).
I really do not like this question as it seems to suggest that barium carbonate is stable to heat, when this is not the case.

1. The fact that some carbon dioxde is produced when the residue is treated with acid means that there is a carbonate present.

2. If 50% of the mixture is magnesium carbonate (relative mass = 84) and it decomposes to MgO (relative mass 40), then the 50% loses 44/84 x 100 % of its mass = 52.4%
so the mass loss of the 100% mixture = 26.2%

BUT, this assumes that barium carbonate does not decompose on heating.
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sourmango1
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#3
Report Thread starter 4 years ago
#3
(Original post by charco)
I really do not like this question as it seems to suggest that barium carbonate is stable to heat, when this is not the case.

1. The fact that some carbon dioxde is produced when the residue is treated with acid means that there is a carbonate present.

2. If 50% of the mixture is magnesium carbonate (relative mass = 84) and it decomposes to MgO (relative mass 40), then the 50% loses 44/84 x 100 % of its mass = 52.4%
so the mass loss of the 100% mixture = 26.2%

BUT, this assumes that barium carbonate does not decompose on heating.
Ah, I see, so barium carbonate did not decompose at all, which explains the acid observation. Thanks a lot.
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Uzairmuhammad436
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#4
Report 1 year ago
#4
(Original post by sourmango1)
Q: Equal masses of barium carbonate and magnesium carbonate powders are mixed together. The mixture is then heated using a Bunsen burner flame until there is no further change. A gas is given off. Which statements are correct?
1) The residue left after heating reacts with aqueous hydrochloric acid to produce carbon dioxide.
2) The percentage decrease in mass after heating is 26% (to 2 significant figures).
3) The gas given off during heating relights a glowing splint.

Is this question referring to the decomposition of the two carbonates? If so, shouldn't the residue produce MgCl2, BaCl2 and H20 with the reaction with HCl, since MgO and BaO were produced after decomposition? I also couldn't calculate the precentage decrease when using the formula for decomposition.

The answer is 1) and 2).
But how barium carbonate not decompose
I searched on Google and book
All says barium carbonate decompose on heating
But why in this case barium carbonate not decomposing
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Uzairmuhammad436
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#5
Report 1 year ago
#5
(Original post by charco)
I really do not like this question as it seems to suggest that barium carbonate is stable to heat, when this is not the case.

1. The fact that some carbon dioxde is produced when the residue is treated with acid means that there is a carbonate present.

2. If 50% of the mixture is magnesium carbonate (relative mass = 84) and it decomposes to MgO (relative mass 40), then the 50% loses 44/84 x 100 % of its mass = 52.4%
so the mass loss of the 100% mixture = 26.2%

BUT, this assumes that barium carbonate does not decompose on heating.
But how barium carbonate not decompose
I searched on Google and book
All says barium carbonate decompose on heating
But why in this case barium carbonate not decomposing
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Uzairmuhammad436
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#6
Report 1 year ago
#6
But how barium carbonate not decomposeI searched on Google and bookAll says barium carbonate decompose on heatingBut why in this case barium carbonate not decomposing
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Aloe Striata
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#7
Report 1 year ago
#7
(Original post by Uzairmuhammad436)
But how barium carbonate not decomposeI searched on Google and bookAll says barium carbonate decompose on heatingBut why in this case barium carbonate not decomposing
in these questions when they mean something can't decompose when heated or something is thermally stable that means that substance will not decompose when heated with a Bunsen flame in the LAB. The temperature at which BaCO3 decomposes is 1100°C which, despite being in the Bunsen flame's range, is not something that would be used in a school laboratory. if this is a gcse or a level question it's safe to assume they're talking about decomposition at school. for example, we study at GCSE level that na2co3 is thermally stable. however it decomposes at 550°C. we are told that because if we were to heat a sample of it in our school lab, it WOULD effectively be thermally stable. thermal stability of course depends on the ionic character of the compound, and since barium is sufficiently non-polarizing I think it is safe to assume it won't decompose if heated by a typical Bunsen burner at school
Last edited by Aloe Striata; 1 year ago
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charco
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#8
Report 1 year ago
#8
(Original post by ilovephysmath)
in these questions when they mean something can't decompose when heated or something is thermally stable that means that substance will not decompose when heated with a Bunsen flame in the LAB. The temperature at which BaCO3 decomposes is 1100°C which, despite being in the Bunsen flame's range, is not something that would be used in a school laboratory. if this is a gcse or a level question it's safe to assume they're talking about decomposition at school. for example, we study at GCSE level that na2co3 is thermally stable. however it decomposes at 550°C. we are told that because if we were to heat a sample of it in our school lab, it WOULD effectively be thermally stable. thermal stability of course depends on the ionic character of the compound, and since barium is sufficiently non-polarizing I think it is safe to assume it won't decompose if heated by a typical Bunsen burner at school
There appears to be a major discrepancy between the requirements of the various boards and the actual literature data.

All group 1 and 2 carbonates decompose with heating according to the literature. However, as stated above, the traditional information deals with Bunsen burners. In fact, the Bunsen burners traditionally used in schools operated using town gas, a mixture of hydrogen and carbon monoxide, which burned at a higher temperature than the current natural gas burners.

The classical posture was that all group 2 carbonates decompose with heat, whereas only lithium carbonate decomposes from group 1. This is rather inaccurate.

Reference: Flame temperatures for gases burning in air:
carbon monoxide 2,121 °C (air)
hydrogen 2,045 °C (air)
methane 1,957 °C (air)

Helmenstine, Anne Marie, Ph.D. "Flame Temperatures Table for Different Fuels." ThoughtCo, Aug. 27, 2020, thoughtco.com/flame-temperatures-table-607307.
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Aloe Striata
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#9
Report 1 year ago
#9
(Original post by charco)
There appears to be a major discrepancy between the requirements of the various boards and the actual literature data.

All group 1 and 2 carbonates decompose with heating according to the literature. However, as stated above, the traditional information deals with Bunsen burners. In fact, the Bunsen burners traditionally used in schools operated using town gas, a mixture of hydrogen and carbon monoxide, which burned at a higher temperature than the current natural gas burners.

The classical posture was that all group 2 carbonates decompose with heat, whereas only lithium carbonate decomposes from group 1. This is rather inaccurate.

Reference: Flame temperatures for gases burning in air:
carbon monoxide 2,121 °C (air)
hydrogen 2,045 °C (air)
methane 1,957 °C (air)

Helmenstine, Anne Marie, Ph.D. "Flame Temperatures Table for Different Fuels." ThoughtCo, Aug. 27, 2020, thoughtco.com/flame-temperatures-table-607307.
Oh alright, thank you. I always wonder why exam boards can’t stick to the facts, it’s not like it would make it more difficult for us to understand things like this. For example, in CIE IGCSE we were taught that catalysts work by lowering the activation energy of a reaction, while that’s not true. What we have learnt in IAL is certainly not difficult for an IGCSE student to understand. How much thinking goes into understanding that catalysts increase the rate of reaction by providing an alternate route for the reaction to occur, and that route has a lower activation energy?
Last edited by Aloe Striata; 1 year ago
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Pigster
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#10
Report 1 year ago
#10
(Original post by ilovephysmath)
Oh alright, thank you. I always wonder why exam boards can’t stick to the facts, it’s not like it would make it more difficult for us to understand things like this. For example, in CIE IGCSE we were taught that catalysts work by lowering the activation energy of a reaction, while that’s not true. What we have learnt in IAL is certainly not difficult for an IGCSE student to understand. How much thinking goes into understanding that catalysts increase the rate of reaction by providing an alternate route for the reaction to occur, and that route has a lower activation energy?
Sooo much of chemistry is simplified at lower levels.

First you learn 2.8.8.2
Then you learn that it's not that simple and spdf pops up
Then you get to university and again it's not so simple.
Eventually you might learn what is really going on.

There is much educational psychology involved. I blame Piaget.
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Aloe Striata
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#11
Report 1 year ago
#11
(Original post by Pigster)
Sooo much of chemistry is simplified at lower levels.

First you learn 2.8.8.2
Then you learn that it's not that simple and spdf pops up
Then you get to university and again it's not so simple.
Eventually you might learn what is really going on.

There is much educational psychology involved. I blame Piaget.
And often times it doesn’t even need to be simplified so much. I know for a fact that if I had been taught the actual thing I wouldn't have been confused about a lot of stuff.
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Pigster
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#12
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#12
(Original post by ilovephysmath)
And often times it doesn’t even need to be simplified so much. I know for a fact that if I had been taught the actual thing I wouldn't have been confused about a lot of stuff.
What are your thoughts on this line from the AQA GCSE spec?

"For metallic bonding the particles are atoms which share delocalised electrons"

It somewhat contradicts the AQA A Level spec:

"Metallic bonding involves attraction between delocalised electrons and positive ions arranged in a lattice."

But since neither is an especially good way of explaining it. Which is wrong?
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Aloe Striata
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#13
Report 1 year ago
#13
(Original post by Pigster)
What are your thoughts on this line from the AQA GCSE spec?

"For metallic bonding the particles are atoms which share delocalised electrons"

It somewhat contradicts the AQA A Level spec:

"Metallic bonding involves attraction between delocalised electrons and positive ions arranged in a lattice."

But since neither is an especially good way of explaining it. Which is wrong?
the first one is strange because 1. if these are atoms where did the electrons come from 2. if these are atoms why would they be attracted to electrons? atoms are supposed to be electrically neutral
I'm not sure about the A level spec because I'm doing A levels myself (Edexcel). I'm sure this is still over-simplified, as is almost everything in our spec.
Edit: also, ‘sharing’ electrons is a non-metal thing, but we’re talking about metals? 🤦*♀️
Last edited by Aloe Striata; 1 year ago
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