Chemical Changes C3

acids in a solution are sources of

acids in a solution are sources of

hydrogen ions

alkalis in a solution are sources of

hydroxide ions

strong acids

completely dissociate into ions in solution

e.g hydrochloric acid is a strong acid. It completely disassociates to form hydrogen ions and chloride ions

HCl → H⁺ + Cl^-

weak acids

only partially dissociate into ions in solution

e.g ethanoic acid is a weak acid. It partially dissociates to form hydrogen ions and ethanoate ions

CH₃COOH ⇌ H⁺ + CH₃COO^-

base

a substance that reacts with an acid to neutralise it and produce a salt - alkalis ares soluble bases

whats an acid/alkali indicator

indicators show whether a solution is acid, neutral or alkaline

recall the effects of acids and alkalis on indcators including litmus, methyl orange and phenolphthalein

the higher the concentration of hydrogen ions in an acidic solution

the lower the ph

the higher the concentration the hydroxide ions in an alkaline solution

the higher the ph

as a hydrogen ion concentration in a solution increases by a factor of 10

the ph of a solution decreases by 1

neutralisation reaction

• neutralisation reaction occurs when an

  Acid + Base → Salt + Water

the hydrogen ions (H⁺) from the acid reacts with the hydroxide ions (OH⁻) from the base to form water (H₂O).

Core practical Aim: investigate the change in Ph on adding powdered calcium hydroxide to a fixed volume of dilute hydrochloric acid

Aim: investigate the change in Ph on adding powdered calcium hydroxide to a fixed volume of dilute hydrochloric acid

Method:

1. Use a measuring cylinder to add dilute hydrochloric acid to a beaker.

2. Dip a clean glass rod into the contents of the beaker and use it to transfer a drop of liquid to a piece of universal indicator.

   • (What Happens): The initial pH will be low (acidic) due to the presence of hydrogen ions (H⁺) from the hydrochloric acid.

3. Wait 30 seconds, then match the color to a pH color chart and record the estimated pH.

4. Add a spatula of calcium hydroxide powder to the beaker.

   • (What Happens): The calcium hydroxide will dissociate into hydroxide ions (OH⁻), which react with the hydrogen ions (H⁺) from the acid, forming water and reducing the acidity. The pH will start to increase.

5. Stir thoroughly, then estimate and record the pH of the mixture.

6. Repeat steps 4 and 5 until there are no more changes in pH.

   • (What Happens): As more calcium hydroxide is added, the pH will continue to rise as more hydrogen ions are neutralized. Eventually, the solution will become alkali after there is an excess of calcium hydroxide in the beaker

dilute

when a solution contains a relatively small amount of dissolved solute

concentrated

when a solution contains a relatively small amount of dissolved solute

What is the general reaction of an aqueous acid with a metal

Acid + Metal → Salt + Hydrogen gas

What is the general reaction of an aqueous acid with a metal oxide

Acid + Metal Oxide → Salt + Water

What is the general reaction of an aqueous acid with a metal hydroxide

Acid + Metal Hydroxide → Salt + Water

What is the general reaction of an aqueous acid with a metal carbonate

Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

chemical test for hydrogen

Collect the gas in a test tube and bring a lit splint near the mouth of the tube. If hydrogen is present, it will burn with a 'squeaky pop' sound.

chemical test for carbon dioxide using limewater

Bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater will turn milky or cloudy

method for getting a dry sample of an insoluble salt (a dry precipitate)

• dissolve the solids

• mix them together

• filter/wash out any impurities

• evaporate

method for getting a soluble salt from an acid and an insoluble reactant

• dissolve the solids

• mix - add the insoluble salt in excess to make sure the solution is completely neutral

• filter - removes the excess and any impurities - only salt + water remains

• evaporate

method for getting a soluble salt from an acid and a soluble reactant

• titrate - to get a precise measurements of the volumes needed for neutralisation

• repeat without indicator - done with perfect proportions to prevent excess of either reactant

• evaporate - and be left with crystals

Core Practical: Investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide including the use of a water bath

• dissolve

• mix

• filter

• evaporate

general rules of solubility

• all common sodium, potassium and ammonium salts are soluble

• all nitrates are soluble

• common chlorides are soluble except silver and lead

• common sulfates are soluble except lead, barium and calcium

• common carbonates and hydroxides and insoluble except sodium, potassium and ammonium

what are electrolytes

ionic compounds in the molten state or dissolved in water

describe what electrolysis is

electrical energy from a direct current supply, decomposes electrolytes

describe the movement of ions during electrolysis

• positively charged cations migrate to the negatively charged cathode

• negatively charged anions migrate to the positively charged anode

oxidisation

loss of electrons (OIL)

reduction

gain of electrons (RIG)

reactivity scale - more reactive than carbon (extracted by electrolysis)

• Potassium

• Sodium

• Calcium

• Magnesium

• Aluminium

• carbon

reactivity scale - less reactive than carbon (extracted by reduction)

• zinc

• iron

• tin

• lead

• hydrogen

reactivity scale - less reactive than hydrogen (mined)

• copper

• silver

• gold

• platinum

electrolysis rule for hydrogen (cathode)

Hydrogen is produced at the cathode if the metal is more reactive than hydrogen

electrolysis rule for halide ions (anode)

oxygen is formed at the anode unless it’s a halide ion (chlorine, bromine, iodine)

Electrolysis of a molten solution steps (Copper chloride)

• dissolved measured amount of copper chloride in water

• attach inert electrode rods using crocodile clips

• put electrodes in copper chloride solution

• turn on power supply begin electrolysis

• Cu²+ ions will go to the cathode and the Cl- ions will go to the anode

• At the Cathode: Copper ions (Cu²⁺) gain electrons and are reduced to copper metal (Cu), which is deposited on the cathode

• At the Anode: Chloride ions (Cl⁻) lose electrons and are oxidized to form chlorine gas (Cl₂), which is released at the anode.

half equations

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