2.1.4 Acids

What ion do acids release in aqueous solution?

H+ ions.

What are the common strong acids?

HCl, H2SO4, HNO3.

(hydrochloric acid, sulfuric acid, nitric acid)

What is an example of a weak acid?

Ethanoic acid (CH3COOH).

What is the Brønsted–Lowry definition of an acid?

A proton (H+) donor.

What is a base?

A substance that neutralises an acid.

What are common bases?

Metal oxides, metal hydroxides, ammonia.

What is an alkali?

A soluble base that releases OH– ions in water.

What are common alkalis?

NaOH, KOH, NH3(aq).

(sodium hydroxide, potassium hydroxide, ammonia)

What is the Brønsted–Lowry definition of a base?

A proton (H+) acceptor.

How does OH– act as a base?

Accepts H+ to form H2O.

How does NH3 act as a base?

Accepts H+ to form NH4+.

What is a strong acid?

An acid that fully dissociates in water.

What is a weak acid?

An acid that only slightly dissociates in water.

What is a salt?

A compound formed when H+ in an acid

is replaced by a metal ion or NH4+.

General neutralisation equation

Acid + Base → Salt + Water.

General equation for carbonates

Acid + Carbonate → Salt + Water + CO2.

What observation is seen with carbonates?

Effervescence due to CO2; solid dissolves.

What colour change does phenolphthalein show?

Pink in alkali → colourless in acid.

What colour change does methyl orange show?

Yellow in alkali → red in acid; end point = orange.

Why is a conical flask used in titrations?

Easier to swirl without spilling.

Why is a white tile used in titrations?

Helps see colour change clearly.

Why add acid dropwise near the end point?

To avoid overshooting the end point.

Why must the burette jet space be filled?

Air in the jet gives incorrect larger titres.

Why add only a few drops of indicator?

Indicators are weak acids and too much affects results.

Why can distilled water be added to the flask during titration?

It does not change the moles of reactants.

What are concordant titres?

Two or more titres within 0.10 cm3.

How should titre volumes be recorded?

To 2 dp, ending in .00 or .05.

What is the burette uncertainty per measurement?

±0.05 cm3.

What is the burette uncertainty per titre?

±0.10 cm3 (two readings).

What is the pipette uncertainty?

±0.10 cm3.

What is the volumetric flask uncertainty?

±0.10 cm3.

How do you calculate % uncertainty?

(uncertainty ÷ measurement) × 100.

How can % uncertainty in a burette reading be reduced?

Use a larger titre volume.

How can mass measurement uncertainty be reduced?

Use a more precise balance or weigh a larger mass.

What does it mean if % apparatus uncertainty is smaller than the % difference from literature values?

There are other errors besides equipment uncertainty.

What does it mean if % apparatus uncertainty is larger than the % difference from literature values?

Results agree within equipment limits.