surface area

[/0]

(Insight 2008 Unit 4)
Which of the following changes will always ensure an increase in the rate of a chemical
reaction?
I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV




The answer is B, but why doesn't IV also increase the rate of reaction :/?

[Over9000]

(Insight 2008 Unit 4)
Which of the following changes will always ensure an increase in the rate of a chemical
reaction?
I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV




The answer is B, but why doesn't IV also increase the rate of reaction :/?

Thats weird, increasing temperature doesnt always increase the rate of reaction, there are some reactions which favour colder temperatures (thats what my chem teacher told me).
I also would have thought increased surface area would increase rate of reaction. I thought increased surface area -----> increased interaction with particles -----> increased rate.

[fady_22]

Is it possible to increase/ decrease the surface area of a liquid/ solution? Because the surface area of a liquid would not affect the rate of reaction, I think (i.e in different containers, where the surface area is different, the rate is not changed).

[Edmund]

Try have a look here. Someone asked this before

[/0]

Thanks edmund
sorry for the repost

[QuantumJG]

(Insight 2008 Unit 4)
Which of the following changes will always ensure an increase in the rate of a chemical
reaction?
I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV




The answer is B, but why doesn't IV also increase the rate of reaction :/?

Thats weird, increasing temperature doesnt always increase the rate of reaction, there are some reactions which favour colder temperatures (thats what my chem teacher told me).
I also would have thought increased surface area would increase rate of reaction. I thought increased surface area -----> increased interaction with particles -----> increased rate.

I think this may be wrong! Since I know 1 and 4 definately increase the rate of reaction. Increasing the temperature will increase the rate of reaction, you are right about exothermic and endothermic reactions to a certain degree. Increasing the temperature will increase the rate of reaction it will just be favouring the endothermic direction. As for concentration I am indifferent! I would personally say it's either C or D!

[Over9000]

(Insight 2008 Unit 4)
Which of the following changes will always ensure an increase in the rate of a chemical
reaction?
I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV




The answer is B, but why doesn't IV also increase the rate of reaction :/?

Thats weird, increasing temperature doesnt always increase the rate of reaction, there are some reactions which favour colder temperatures (thats what my chem teacher told me).
I also would have thought increased surface area would increase rate of reaction. I thought increased surface area -----> increased interaction with particles -----> increased rate.

I think this may be wrong! Since I know 1 and 4 definately increase the rate of reaction. Increasing the temperature will increase the rate of reaction, you are right about exothermic and endothermic reactions to a certain degree. Increasing the temperature will increase the rate of reaction it will just be favouring the endothermic direction. As for concentration I am indifferent! I would personally say it's either C or D!

I wasn't talking about exothermic or endothermic reactions or wateva, I remember either from my teacher or from the chem book that there are some reactions in which decreasing the temperature increases the rate, I was really shocked when I saw it, oh well dont expect everyone to believe it coz it sounds weird.

[arthurk]

In the production of ammonium one of the processes doesnt have a conflict with yield and rate as the lower the temperature the faster the reaction
At least i think it was ammonium
one of the 4 detailed studies

[Over9000]

In the production of ammonium one of the processes doesnt have a conflict with yield and rate as the lower the temperature the faster the reaction
At least i think it was ammonium
one of the 4 detailed studies

Thats probs where I saw it.

[arthurk]

VCAA can't ask us something like that though
it would only favour the people doing that detailed study

[Mao]

(Insight 2008 Unit 4)
Which of the following changes will always ensure an increase in the rate of a chemical
reaction?
I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV




The answer is B, but why doesn't IV also increase the rate of reaction :/?

Thats weird, increasing temperature doesnt always increase the rate of reaction, there are some reactions which favour colder temperatures (thats what my chem teacher told me).
I also would have thought increased surface area would increase rate of reaction. I thought increased surface area -----> increased interaction with particles -----> increased rate.

I believe you are confusing equilibrium yield with rate of reaction.

The following explanation is beyond the VCE course
Reaction rates follow the rate laws , where A, B, C, ... are reactants, and n_x are the order of reaction (from empirical data). The reaction constant 'k' is a factor describing the rate of reaction in at certain conditions. k follows the Arrhenius equation , where A is a constant dependant on the reaction mechanism. It can be seen here that as T increases, the reaction constant k always increases, hence the rate of reaction must increase.
Back to VCE Land

The result of this is important: Increase in temperature ALWAYS increases rate of reaction. This can be understood in terms of the Kinetic Theory (part of the VCE syllabus), increase in T --> increase in Energy --> more frequent collisions and more collisions at sufficient energy.

However, when increasing temperature, both forward and backward reaction will be affected (rate of both will be increased), but the amount of increase of these two rates will be different. In an endothermic reaction, forward rate increases more than the backwards rate, which gives a net forward reaction initially (proceeding to the right until equilibrium is re-established), and vice versa for exothermic reaction (backward reaction increases more than forward reaction, hence net backward initially).



On the other hand, increasing surface area doesn't always necessarily increase rate of reaction. Surface area does not directly affect the rate of reaction, the factor that does matter is the total contact area between the reactants. If the total contact area is limited by the surface area of the solid immersed in the liquid, having a greater surface area for the liquid by changing to a wider beaker would not increase the rate of reaction (even though SA is increased). Similarly, if a liquid reactant is added on top to a large solid object (i.e. a few drops of acid on a metal sheet), having a flatter, larger sheet of metal (SA of solid increases) does not affect the total contact area, hence does not increase reaction rate.

However, grinding the solid into powder (if the solid is going to be immersed in liquid) will increase the total contact area --> increase in rate of reaction.

[/0]

Thanks Mao, you give some great reasons there...

When a question asks: "Why does the rate of reaction slow as the reaction proceeds" (say this is a 1-2 mark question)

Do we need to go into detail about how the forward and back reactions become equal, or is that too much depth? Do we need to only talk about the net reaction?

Because what happens is that as the reaction proceeds the back reaction increases and the forward reaction slows, but technically forward and back reactions still occur at equilibrium, right?

[Mao]

Thanks Mao, you give some great reasons there...

When a question asks: "Why does the rate of reaction slow as the reaction proceeds" (say this is a 1-2 mark question)

Do we need to go into detail about how the forward and back reactions become equal, or is that too much depth? Do we need to only talk about the net reaction?

Because what happens is that as the reaction proceeds the back reaction increases and the forward reaction slows, but technically forward and back reactions still occur at equilibrium, right?

That is how equilibrium is established, but not why the rate decrease.

Kinetic Theory: more collisions --> higher rate, and higher concentration --> more collisions

In a system with only reactants at the start, the initial concentrations of reactants are high, hence initial rate of forward reaction is high. As the reaction proceeds, the concentration of reactants decrease, hence rate of forward reaction decrease.


As for equilibrium: also as the reaction proceeds, the concentration of products increase, and the rate of backward reaction increase. will continue to decrease and will continue to increase until a point where the rate of reactions are equal, and the system reaches a dynamical equilibrium, i.e. the backwards reaction is fast enough that enough reactants are produced such that will not decrease, and the rate of forward reaction is slow enough that the amount of products produced won't increase the concentration of products hence stops increasing.

[sorry if that was confusing]

[/0]

ah ok, so by "rate of reaction", they imply "rate of forward reaction"?

[Mao]

ah ok, so by "rate of reaction", they imply "rate of forward reaction"?

That kind of question ("Why does the rate of reaction slow as the reaction proceeds") is only usually asked for irreversible reactions, such as , and they ask you why the rate of carbon dioxide evolution decreases over time.

Otherwise, it's highly situational. Rate of reaction is given in mol/s, and usually only have meaning when assigned a specific direction.