GCSEChemistryAQARevision NotesChemical Change: Rate & ExtentRate of ReactionReaction Rates Using Moles

Reaction Rates Using Moles (AQA GCSE Chemistry): Revision Note

Exam code: 8462

Stewart Hird

Written by: Stewart Hird

Updated on

AQA ChemistryAQA Combined Science: Trilogy: Chemistry

Reaction rates using moles

Higher tier only

  • We have seen that the rate of reaction is a measure of a changing property per unit time

  • This will frequently be expressed in units of g/s or cm3/s for a change in mass or volume, respectively

  • However, it is often more useful to be able to express rate of reaction in terms of moles, in which case the unit will be mol/s

  • There is no direct way to measure moles, so first it is necessary to calculate the rate using the rate formula triangle:

Rate of Reaction - Formula Triangle, IGCSE & GCSE Chemistry revision notes

The rate formula triangle

  • After calculating the rate of reaction it can then be converted into mol/s

  • If you are working in mass per unit time then the process is:

moles per unit time = fraction numerator mass space per space unit space time over denominator molar space mass space of space the space substance end fraction

Worked Example

20.0 g of marble chips were added to a flask containing 100 cm3 of 2.0 mol/dm3 hydrochloric acid. The flask was placed on a balance and the mass was measured at the start of the reaction. After 2.5 minutes the mass was found to have decreased by 6.0 g.

Calculate the rates of reaction, in mol/s. 

Answer:

  • Step 1: Convert the time into seconds:

2.5 x 60 = 150 s

  • Step 2: Convert the mass lost to moles:

    • The mass loss is due to carbon dioxide (Mr = 44.0) being released

moles = mass over M subscript straight r

moles = fraction numerator 6.0 over denominator 44.0 end fraction

moles = 0.137 mol

  • Step 3: Calculate the rate of reaction in mol / s:

rRate of reaction = moles / time in seconds

rate of reaction = 0.137 / 150

rate of reaction = 9.1 x 10-4 mol / s

  • If you are working in volume per unit time then you have to use the molar gas volume:

moles per unit time = fraction numerator volume space per space unit space time space over denominator molar space gas space volume space left parenthesis 24 space 000 space cm cubed divided by mol space right parenthesis space end fraction

Worked Example

A piece of magnesium ribbon, 2 cm in length, was added to a flask containing 50.0 cm3 of 1.50 mol/dm3 sulfuric acid. The flask was connected to a gas syringe and the volume of gas measured every minute. 15 cm3 of gas had been collected after 6 minutes.

Calculate the rates of reaction, in mol/s.

Answer:

  • Step 1: Convert the time into seconds:

6.0 x 60 = 360 s

  • Step 2: Convert the volume of gas to moles:

moles = fraction numerator volume space left parenthesis cm cubed right parenthesis over denominator molar space gas space volume space left parenthesis 24 space 000 space cm cubed right parenthesis end fraction

moles = 15 over 24000

moles = 6.25 x 10-4

  • Step 3: Calculate the rate of reaction in mol / s:

rate of reaction = fraction numerator moles over denominator time space in space seconds end fraction

rate of reaction = fraction numerator 6.25 cross times 10 to the power of negative 4 end exponent over denominator 360 end fraction

rate of reaction = 1.7 x 10-6 mol / s

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent

Our rating represented in Trustpilot stars
Read more reviews
Test yourselfFlashcards
Previous:Calculating Rates of ReactionsNext:Rate Graphs
Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.