2.1.4 Acids Flashcards
What is an acid ?
(In terms of H+ ions)
Release H+ ions when dissolved in water
H+ is a proton, so they are** proton donors**
What is a base ?
(In terms of H+ ions)
Accept H+ ions when dissolved in water
so they are proton acceptors
-They also dissolve in water to release OH- ions
What is the difference between a base and an alkali ?
Base = neutralises an acid to form a salt
Alkali = dissolves in water, releasing OH- ions
What is a strong acid ?
Give three examples
fully dissociates into H+ ions
(Hcl > H+ + Cl-)
Hydrochloric acid, sulfuric acid, nitric acid
What is a weak acid ?
Give one example
Partially dissociates into H+ ions
(CH3COOH > CH3COO- + H+)
Ethanoic acid
What is a strong base ?
Give two examples
fully dissociates into OH- ions
Sodium hydroxide and Potassium hydroxide
Define neutralisation
How does this occur ?
a reaction between** H+ and OH- forming water.** This may be a reaction between an acid and a base to form a salt (types of bases include carbonates, metal oxides
and alkalis).
H+ ion in the acid is replaced by a metal ion or ammonium ion
What is produced in the reaction between acid and alkali ?
Give an example of an alkali
Salt and water
Sodium** hydroxide** or ammonia
What is produced in the reaction between acid and metal oxide ? (Type of base)
Give an example of a metal oxide
Salt and water
Magnesium oxide
What is produced in the reaction between acid and carbonate ? (Type of base)
Give an example of a carbonate
Salt + water + carbon dioxide
calcium carbonate
What is produced in the reaction between an acid and ammonia solution ? (must be aqueous)
Ammonium salt
Ammonium chloride
Give the formula of hydrochloric acid
Hcl
Give the formula of sulfuric acid
H^2SO^4
Give the formula of nitric acid
HNO^3
Give the formula of ethanoic acid
CH^3COOH
Give the formula of a metal hydroxide (alkali)
NaOH, KOH
Give the formula of ammonia (alkali)
NH^3
Give the formula of a metal oxide (base)
MgO
Give the formula of a metal carbonate (base)
CaCO^3
What is a standard solution ?
solution with a known concentration
How do you prepare a standard solution ?
- Calculate** mass **of solid needed
- Weigh out this mass and transfer to a beaker (include the washings)
- Dissolve in a small amount of distilled water and stir
- Rinse the rod to include the washings
- Transfer to a volumetric flask of the needed volume (include the washings again)
- Add** distilled water** until bottom of the **meniscus **is on the mark
- Add a stopper and invert to mix
How can you use dilution to obtain a desired volume from a stock solution ?
- Calculate moles needed of the desired concentration
- Calculate the volume of stock needed to achieve these moles ( Volume = Moles just calculated / concentration of stock solution)
How do you carry out an acid-base titration ?
- add measured volume of one solution to a conical flask with a pipette
- white tile and indicator
- Rinse the burette using acid solution to remove air bubbles
- Add second solution to the burette, record initial reading from the botton of the meniscus at eye level (to nearest 0.05 cm3)
- TRIAL TITRE (used to find rough amount of solution needed)
- Run the solution into the conical flask and swirl
- The END POINT is when the solution changes colour (Neutral) - Stop adding acid and read final burette reading
- Do final-initial volume to find volume added
9.** Repeat, adding solution dropwise** until two titres are **concordant ** - **Average **the accurate and concordant titres to obtain the mean titre
What is the value needed for concordant results ?
within 0.10 cm cubed
Why is a conical flask used instead of a beaker in titration ?
easier to swirl the mixture without spilling the contents
Why must we not add too much indicator ?
they are generally weak acids, if we add too much it could impact the titration
What will happen if the jet space is not filled properly/contains air bubbles ?
it will fill during the titration, leading to a larger than expected titre reading
How can you ensure all the acid on the sides of the flask enters the reaction mixture ?
swirl distilled water around the side of the flask
This will not impact the results as water does not react with the acid or change the moles of acid present
Why is it sometimes necessary to do titrations on several samples ?
the concentration of the chemical tested may vary between samples
How do you calculate percentage error ?
(final/measured) x 100
-Error is written on most apparatus
-Total error is the sum of the errors of all equipment
How do you carry out a titration calculation ?
- Calculate moles of known solution (Conc = moles/volume)
- Use **ratio **to find moles of unknown
- Calculate second concentration
(Concentration = moles/volume)
Define reading
the values found from a
single judgement when
using a piece of equipment
Define measurement
the values taken as the difference between the judgements of two values (e.g. using a burette in a titration)
What are the uncertainty values for a balance, volumetric flask, pipette and burette ?
*balance = 0.001 g (if a 3 d.p. balance)
*volumetric flask = 0.1 cm3
*25 cm3 pipette = 0.1 cm3
*burette (start & end readings and end point ) = 0.10 cm3
Burette is actually 0.05 but as you always take two readings this is multiplied by two
How can you reduce uncertainty of the apparatus ?
- use apparatus with a greater resolution
How can you reduce uncertainty in a titration ?
- Use a pipette or burette (greater resolution)
- titrate a larger volume (increase volume and concentration in flask, or decrease concentration in burette)
How can you reduce uncertainty in measuring mass ?
- use a balance with a higher resolution
- use a larger mass
- weigh sample before and after, then calculate the difference
How can you calculate percentage difference between actual value and calculated value ?
- Find the difference between the two Mr
- Calculate: ( Difference/real ) x 100
- If the uncertainty is less than the % difference than there is discrepancy due to errors other than the apparatus
