balanced equations + associated calculations

Why must chemical equations be balanced?

So the reacting ratios are correct → so can then be used to calculate reacting masses, percentage yield, atom economy

What are the rules for balancing equations?

• Not ok to change any small numbers of a formula

• Add large numbers in front of a formula

• Large numbers can be x/2 or halves if in front of a diatom e.g ½ O₂ is okay but not ½ H₂O

What are the state symbols?

s) = solid

(l) = liquid

(aq) = aqueous

(g) = gas

STATES OF MATTER

metals are..

diatoms are mostly..

ionic compounds are…

most simple molecules are..

acids are..

when in solution, ionic compounds are..

• solids

• gases

• solids

• gases

• aqueous

• aqueous

What does aqueous mean?

Dissolved in water

* when compounds are dissolved in water, they dissociate into separate ions

What do ionic equations only show?

The ions / species that change in a reaction

What is the ionic equation for:

Explain how to form an ionic equation for a precipitate reaction

• The simplest ionic equation will always be:

  ____ (aq) + _____ (aq) → _____ (s)

• The only product present is the precipitate and the only two reactants are the aqueous ions that make the precipitate

Write the ionic equation for magnesium nitrate solution + sodium hydroxide solution → solid magnesium hydroxide (precipitate)

Mg²⁺ (aq) + 2OH⁻ (aq) → Mg(OH)₂ (s)

Define atom economy

A measure of what proportion of the products of a reaction are the desired product + how much is waste

Higher the atom economy, the less…

waste that is produced

What is the equation for atom economy?

What does the % yield tell us?

The practical efficiency of the process

Reasons why percentage yield is less than 100%

• Incomplete reaction

• Side reactions

• Loss of product during washing

• Loss of product transferring product

What is the equation for percentage yield?

Rearrange the formula to make actual yield the subject

(percentage yield x theoretical yield) / 100

Rearrange the formula to make theoretical yield the subject

actual yield x 100 / percentage yield

The theoretical mass is also the…

expected mass

10.0g of calcium carbonate was decomposed to give 3.60g of calcium oxide. What is the % yield of calcium oxide in this reaction?

What is the equation for percentage purity? ADD PIC

The concentration of a solution is measured as what?

the number of moles of solute per decimetre cubed of solution (moldm⁻³)

What is concentration also known as?

Molarity

What is the triangle that links moles, concentration + volume?

How to convert from cm³ to dm³

divide by 1000

7.55g of sodium chloride is dissolved in 500cm³ of water - what is the concentration of the solution in moldm⁻³

• calculate the moles of NaCl

  mass/ Mr → 7.55/58.5 = 0.1291

• convert volume from cm³ to dm³

  500/1000 = 0.5 dm³

• calculate concentration

  0.1291/0.5 → 0.258moldm

How to calculate concentration in g dm⁻³

conc in mol dm⁻³ x Mr

What is a standard solution?

A solution whose concentration is known accurately → its concentration is usually given in mol dm⁻³ or g dm⁻³

When making a standard solution, what is important?

• Mass of substance is accurately measured

• All substance weighed out is successfully transferred to volumetric flask used to make up solution

What is the procedure that will make that happen?

What is a titration?

A technique where a solution of a known concentration is used to determine the concentration of an unknown solution

Briefly explain what an acid base titration involves

• The standard solution is added to a burette to a known volume in the conical until the reaction is complete

• Often an indicator is used to signal the end of the reaction → the endpoint

What colours do these indicators make?

• Phenolphthalein

• Methyl orange

The accuracy of the results of the titration will be a reflection of what?

The care you have taken whilst performing it → when done carefully, titrations give a very accurate, precise results

Titrations of unknown solutions are done in what 2 steps?

• A rough titration used to determine the approximate amount of standard solution needed to neutralise an unknown solution

• Subsequent more accurate titrations that you will use for your calculations

Explain in detail the method of the rough titration

• Rinse burette with deionised water, then rinse with the standard solution + fill burette incl. area below tap with the solution → take initial reading + record to 2.d.p, ending in a 5 or 0

• Rinse pipette with deionised water, then rinse with unknown concentration solution + transfer 25cm³ of the solution from your beaker into your conical flask using pipette

• Add 3-4 drops of indicator to your conical flask + place under your burette → don’t add more than 4 drops as indicators are weak acids so anymore will affect the end point

• The solution in conical flask will change colour depending on its pH + indicator used

• Remove filter funnel + add solution from burette, with constant swirling of flask until indicator just changes colour → indicates neutralisation

• Write down final burette reading to 2.d.p - ending in 0 or 5 → this is the volume of solution from burette required for neutralisation (titre value)

• Empty conical flask + wash thoroughly with deionised water

• Refill burette if necessary → if there’s less than 25cm³

What can you add to make sure all your solution has reacted

Add deionised water

Why should we always overfill the burette + let some solution out into waste beaker?

It ensures that the area below the tap (jet) is also filled → prevents titre volume from being too high

Why is it important to rinse pipette + burette with the solution that will be in the apparatus?

This removes any drops of deionised water that would lower the concentrations of the solutions + effect the end point - titre value

Why is a conical flask used?

Easy to swirl so there are no spillages

Why do we remove the filter funnel from burette when carrying out the titration?

Ensures no additional drops enter from the burette + lower titre

Explain the method of subsequent titrations

• At start of titration, instead of adding a small amount of solution from burette at a time, add 2cm³ less than your previous titre value to your conical flask

• Then add dropwise from burette whilst swirling until you see the endpoint

• Continue with titration until you have concordant titre values → only use the concordant values to calculate average titre which is used in the calculations

Close to the endpoint, what can you do to ensure all reagents are in solution

Use deionised water to wash the sides of your conical flask → won’t affect the end point as water isn’t a reagent

What are concordant values?

Values that are within 0.1cm³ of each other

Give an example of a titration + its method

Every time you make a measurement with a piece of apparatus, there is a small..

margin of error in that measurement due to the apparatus itself

What are errors like this called?

Apparatus error + can’t be avoided → can be reduced by using the most precise equipment available

What is the equation for calculating the apparatus error for each piece of equipment?

When you calculate apparatus error, what does that actually mean?

The result of the experiment should be within __% of the correct value

When you design experiments, you should aim to ensure that the total apparatus error is minimised by doing what?

Working on a suitable scale + with suitable apparatus

For apparatus that is read more than once, what should you do when calculating the apparatus error?

Multiply the error you are given by the no. of readings

How can you reduce the amount of uncertainty without changing the equipment used?

By increasing the measured amount → decrease the % uncertainty

In a titration, how could we increase the titter volume + lower burette’s % uncertainty?

• Decrease conc. of solutionI in burette

• Increase volume / conc. of solution in conical flask

Explain how to complete a back titration question asking to calculate mass / percentage yield :