Chem coursework Help

Need help - The experiment is how does the concentration of hcl affect the rate of reaction (with Mg) ~ measuring volume of h2 produced over given time intervals.
I'm struggling to get my head around the variables.
Ik the independent is the concentration of hcl.
But is the dependent the volume of h2 produced and if so what variable is time? or is it just the rate of reaction.
Thanks in advanced.

(edited 8 years ago)

Reply 1

Dupin

So you'll start off doing the experiment using one concentration of hcl. You'll measure volume of h2 produced over time. So you can make a graph with time on the x axis (this is an independent variable) and volume of h2 on the y axis (this is a dependent variable - as the volume of h2 produced will depend on the point in time during the reaction).

You'll then repeat the experiment using different concentrations of hcl and get more of these graphs. You'll then decide how to determine the rate of reaction for each of these (different volumes of h2 produced?) and plot another graph of concentration of hcl on the x axis (this is the independent variable because this is what you're changing in the experiment) and rate of reaction on the y axis (this is a dependent variable because the rate of the reaction will depend on the hcl conc. - though this is what you're trying to work out!)

Reply 2

Jacob463728

I think the graph would be time on the x and volume of h2 on the y (like you said). There would be multiple lines of the graph, for the different concentrations of hcl. The graph isn't the problem.
I have to state the variables in my write up eg the independent is... the dependent is... the control variables are.... < I just can't decide where time would fit in. Could it be rate of reaction as the dependent, can the dependent be something you've calculated?

(edited 8 years ago)

Reply 3

Dupin

Ok great.
So time is an independent variable. (it does not depend on anything, think of it like when you start your experiment you are changing the time)
The volume of h2 produced is a dependent variable. (it depends on the time, for example if you stop the reaction after a minute there will be less h2 than if you stop after 5 mins)
A control variable would be for example amount of metal, because changing the quantity of metal could change the volume of h2 produced.

Reply 4

Jacob463728

So where would the concentration of hcl fit in, I'm going to change it and it doesn't depend on anything.
The title after all is how does changing the concentration of hcl affect the reaction time
Thanks anyway

(edited 8 years ago)

Reply 5

Dupin

You're right, hcl concentration is an independent variable. This is what I was talking about in the second part of my post.

Your hypothesis is that changing hcl conc will change the rate of reaction. But you are measuring volume of h2. So you need a way of determining the rate of reaction. Perhaps you could divide the total volume of h2 produced by the total reaction time. This will give you a value of volume per time. And you can do this for every conc. experiment. You can then plot these values against the hcl concentration in another graph, and this will help you clearly see how conc affects rate of reaction for your conclusion.

Reply 6

Jacob463728

Thanks, but where should I put time in my variables. I'm thinking of putting it as a control because I'm keeping the time intervals when I record the volume of h2 the same eg 10s - record amount of h2, 20s - record. 10s intervals up to 1 min.
In reply to you, I can calculate the rate of reaction by putting the amount of h2/10s for each interval. I also think I only need 1 graph as h2 against time shows rate of reaction
Thanks again!

(edited 8 years ago)

Reply 7

Kaynine_ghrap

Hi guys, I need help with the ocr chemistry ISA. In my college, we only do one of each practical. Last year I did well in all my theory exams but had C's in my practicals resulting to me getting a B over in physics and chemistry. My grade requirement for uni is A*A*A with an expected A* in chemistry so please help me with any information on the chemistry ISA so that I can do chemical engineering at Cambrigde/ Imperial if I get an offer. Thanks.

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Reply 8

Dupin

I would put it as an independent variable.

Only put time as a control if you are keeping the total time duration the same for every experiment (its not referring to how often you take the measurement - though you should keep this consistent).
You can either:
1) record the volume of h2 every 10 seconds for 1 minute and then STOP recording - for EVERY experiment.
2) record the volume of h2 every 10 seconds until there is NO MORE h2 produced. - so the total time is different for every experiment.
I would recommend the second one.

Reply 9

Jacob463728

my exam board give us a method, which we can change and it said 10s intervals up to 1 min. Idk what to do :Z

(edited 8 years ago)

Reply 10

Dupin

Ok great, they probs did that to make it quicker and easier for you.
Now you can write time as a control because you are keeping the duration of the reaction the same for every experiment.
You can then obtain a value for volume of h2 produced per minute and plot this like we said before.

Reply 11

Jacob463728

Alright thanks man! Very last thing - ik time is a control but I am controlling the time intervals and the overall time or both? and I need to give a reason for controlling it. Thanks you've been very helpful

Reply 12

Dupin

You are controlling the overall time - the duration of the reaction - to calculate the rate (volume per time) so it can be compared between experiments. (the rate may also begin to change inconsistently after 1 minute). Taking measurements at regular intervals is just good experiment practice! No problem man, hope you do well!

Reply 13

Jacob463728

Thanks, it was giving me a headache :smile:

Reply 14

Kaynine_ghrap

Original post by Kaynine_ghrap

Hi guys, I didn't mean someone should tell me what the ISA is, since that's an exam malpractice. I need help or tips on how to improve my practical skills. Cheers

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