Hey everyone and welcome back to my second HSC Chemistry blog!
Today’s blog will provide insight into module 5 including how to correctly structure a response to give yourself the best shot at full marks. This blog will also contain a list of goals you should try achieving this week!
Let’s get into it!
From module 5, there are many different types of application-based questions that appear frequently in the HSC. Some common examples include:
Deriving and calculating the equilibrium constant (Keq).
Predicting the direction a reaction will proceed by calculating the reaction quotient Q.
Deriving and calculating the solubility product (Ksp).
Predicting the formation of a precipitate given the standard reference values of Ksp.
These types of questions can appear in both multiple choice and short responses.
Where do students normally lose their marks with these types of questions?
Responses are not structured and do not appear in a logical order.
Students don’t address all parts of the question.
Calculation or substitution errors. For example, this can occur when substituting your concentrations into your equilibrium expression or when inputting the values into your calculator.
However, to make sure you guys don’t make these mistakes I’m going to provide all the steps required to write a perfect response!
To do this, let’s look at a question based on the solubility product:
Question: A 100ml saturated solution of calcium hydroxide at 25 degrees Celsius contains 0.173g of calcium hydroxide. Calculate the solubility product of this salt. (4 marks)
In order to obtain full marks, this is how you would structure your response:
1. Write a balanced chemical equation! (1 mark)
This is a very important step because it helps you derive the correct solubility product. It also shows the marker that you have the skill of writing balanced chemical equations and that you have an in depth understanding of what occurs during the solubility of an ionic compound.
Sample Answer: Ca(OH)2(s) <---> Ca2+(aq) + 2OH-(aq)
2. Deduce the solubility product expression. (1 mark)
This step is crucial! Without the solubility expression, it will become very difficult to obtain the correct solubility product at the end.
Sample Answer: Ksp = [Ca2+][OH-]2
3. Calculate the concentration for each substance. (1 mark)
In order to calculate the Ksp value, we need to find the concentration of the products. If the question doesn’t provide us with the concentration and gives us the number of grams, it’s important to follow the correct steps to get to the concentration. This includes first finding the number of moles by dividing the number of grams by the molar mass. Then dividing the number of moles by the volume (in litres) to find the concentration (mol/L).
n(Ca(OH)2 = 0.173/74.093g
n(Ca(OH)2 = 0.00233 moles
[Ca(OH)2] = 0.00233/0.1
[Ca(OH)2] = 0.0233 mol/L.
Therefore, the concentration of calcium hydroxide is 0.0233 mol/L. Since the molar ratio between calcium hydroxide and calcium ions is 1:1, the concentration of calcium ions is 0.0233 mol/L. Since the molar ratio between calcium hydroxide and hydroxide ions is 1:2, the concentration of hydroxide ions is 0.0466 mol/L.
4. Calculate the solubility product. (1 mark)
This is done by substituting the concentration of products into the solubility expression.
Ksp = [Ca2+][OH-]2
Ksp = (0.0233)(0.0466)2
Ksp = 5.06 x 10-5
What should you do this week?
Start the week by creating a timetable/schedule for each day in the week. I would highly recommend creating an hourly timetable for the day instead of a to do list. For example, Monday morning/afternoon:
10:00 am – 1:00pm: complete the 2019 HSC chemistry paper (timed conditions)
1:00pm – 2: 00pm: break/lunch
2:00pm – 4:00pm: revise module 5 biology content.
This will make you feel more organised and less flustered. This method helped me manage my time effectively whilst reducing my stress as I had clear outline of what my day and week will consist off.
Complete at least 3 full chemistry papers this week. The more papers you do, the better! Once you finish the practise paper, mark your answers, and write your mistakes down into your ‘mistakes book’.
If there are topics that you are not confident in, reread your class notes and create flash cards or summary sheets to help you better understand and remember the topics. Also, pick out questions based on those topics from past HSC papers and have a go at them!
Quote of the week:
“Keep your face always toward the sunshine and shadows will fall behind you”
- Walt Whitman
That’s it for today!
Look out for next week’s blog which will consist of more valuable insights!
Good luck and have a great week!