Maxwell Curve

hoay SG

Given the Maxwell-Boltzmann curve for the distribution of molecular energies which statements are correct?

1 the maximum of the curve is dispalced to the right.

2 the proportion of molecules with energies above any given value increaes.

3 the proportion of molecules with any energy increases.

Explanation:

Statement 1 is correct. When temperature increases the proportion of molecules having energy greater than the Ea increases.

Statement 2 It means that if a molecule at lower temperature (before the T is increased) has some energy, when Temperature is increased its energy increases …..In statement 2 this energy is called above any given value.

Statment 3 is corret also....Not able to distinguish between 2 and 3?

UltimaOnline SG

Given the Maxwell-Boltzmann curve for the distribution of molecular energies which statements are correct?

1 the maximum of the curve is dispalced to the right.

2 the proportion of molecules with energies above any given value increaes.

3 the proportion of molecules with any energy increases.

Explanation:

Statement 1 is correct. When temperature increases the proportion of molecules having energy greater than the Ea increases.

Statment 3 is corret also....Not able to distinguish between 2 and 3?

If this is a CIE MCQ, pls try to provide an online link.

Statement 3 is wrong, because if you choose a very low energy value, increasing temperature will result in a decrease of % of molecules with such a low energy value.

hoay SG

this is the link:

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w03_qp_1.pdf

hoay SG

Q 33

UltimaOnline SG

this is the link:

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w03_qp_1.pdf

As I expected, the question is about the change with results during an increase in temperature (a detail you omitted, which is why it's always a good idea to provide the direct online link whenever possible).

As such, statement 3 is wrong, as I explained in my earlier post.

hoay SG

but when we increase the temperature the energies of the molecules increases. In statement 3 when it says energies with any given value increases....does it mean that these molecules will NOT be near to Ea?

UltimaOnline SG

Yes. Statement 3 says *any* energy value. If you choose a high energy value (eg. Ea), then as you said, increasing temperature will increase the % of molecules with this energy (eg. Ea). But if you choose a low energy value, then increasing temperature will *decrease* the % of molecules with this energy (because increasing temperature provides energy).

As an analogy, statement 3 = the no. of pple with any amount of money will increase. If u let any amount = $0, and you increase the wealth of everyone (eg. giving everyone $1million), then statement 3 = "the no. of pple with $0 will increase after giving everyone $1million" is obviously false.

hoay SG

ok.

The most probable energy is the energy that is conatined by most of the molecules at low temperature. It is different from Ea..is this correct?

When we increase the temperature more molecules will have most probable energy so the reaction occurs quickly. How do we relate this to Ea?

UltimaOnline SG

ok.

The most probable energy is the energy that is conatined by most of the molecules at low temperature. It is different from Ea..is this correct?

When we increase the temperature more molecules will have most probable energy so the reaction occurs quickly. How do we relate this to Ea?

Ea is defined as a particular energy value required for a particular reaction to occur. If reactant molecules do not meet this energy value requirement, even if they collide, the reaction doesn't occur.

Increasing temperature boosts the energy of all reactant molecules, so that at a higher temperature, a higher % of molecules will have energy equals or exceeding activation energy required for the reaction to occur. Hence rate of reaction increases.

hoay SG

Yes i know. But what is most probable energy then?

UltimaOnline SG

Yes i know. But what is most probable energy then?

Probability = Abundance (think gambling at the casino, the more numbers you buy at the roulette table, the higher your probability of winning... of course, if you still lose, you'll also lose more money... so overall in the long run, the 5.26% house edge remains). Most probable = most abundant, ie. the mode (not the median or the mean).

Hence if Cambridge asks, "On the Maxwell Boltzmann distribution graph, indicate the most probable energy or speed of the molecules", you have to draw a horizontal line on the y-axis touching the highest peak (ie. most abundant = most probable) of the curve, together with a vertical line from this same highest peak down to the x-axis, which gives you the x-axis value for the most probable energy or speed.

To address your question more directly :

The most probable energy is the energy that is contained by most of the molecules at low temperature. It is different from Ea..is this correct?

Yes, but not at 'low temperature', but at whatever the given temperature is (each temperature will have it's own graph-curve). So if more than 1 temperature-graph-curve (eg. 300K, 350K, 400K) is given in the diagram, then each of these temperature-graph-curves will have its own most probable energy or speed, ie. the x-axis value corresponding to the highest peak on each of the temperature-graph-curves.

It's different from Ea, because Ea is a fixed value for each chemical reaction, that doesn't change with temperature.