Chemical changes

Seite 1

Chemical Changes- Acids and bases

Acid= Ph less then 7

Acid= Form H+ ions in water

As hydrogen concentration increases pH concentration decreases

Bases= React with acid forms salt and water

Bases= Alkali with pH more then 7

Form OH- ions in water

Higher concentration of OH- ions higher the concentration of pH

Indicator= dye- changes colour depending on pH

Add few drops of indicator and find the pH

Litmus indicator= red an acidic solution, purple is neutral, blue is alkaline

Methyl orange indicator= red is acidic, Yellow is neutral, Yellow is alkaline

Reaction between acid + base is neutralisation. It produces salt and water

Acid + base --> salt + water

Neutralisation could be shown as ionic equation - H+ + OH- --> H2O

Core practical. Investigating the neutralisation reaction between Calcium oxide-base- and dilute hydrochloric acid

Measure set volume of hydrochloric acid into conical flask. Use pippete/ measuring cylinder

Measure fixed mass of calcium oxide using a mass balance

Add calcium oxide to hydrochloric acid

Allow base to fully react. Record pH using a universal indicator.

Repeat steps 2,3,4 until acids reacted- this would be when the unreacted calcium oxide drops to the bottom

Plot a graph to see how pH changed with mass of the base added

Seite 2

Chemical changes- Reactions of Acids

Salt forms when Acid + base react

Acid + metal oxide ----> Salt + Water

Acid + metal hydroxide ----> Salt + water

Acid reacting with a metal or metal carbonate

Acid + metal ---> salt + hydrogen

Acid + metal ---> salt + water + Carbon dioxide

Test for Hydrogen

Lighted Splint- Squeaky POP

Test for CO2

Water turns milky cloudy white

Seite 3

Chemical changes- Strong and weak acids

Acids split to form hydrogen ions

Strong acids- ionise almost completely in water at a Low pH

Weak ions do not fully ionise in solution

Ionisation of weak acid is a reversible reaction

Acid strength is the proportion of acid ionised in H2O

Concentration is how much acid n a litre of water is concentrated

Increasing concentration of + ions y a factor of 10 or 100pH decrease by factor of 1 or 2

Decreasing H+ ions by a factor of 10 the H+ concentration increases by factor of 1 on pH scale

Seite 4

Chemical changes- Making insoluble salts

First, get a dry sample of insoluble salts, pick 2 soluble salts and react them together

For example, to make lead chloride you get lead nitrate and sodium chloride

Method

Add 1spatul of lead nitrate to test tube. Water dissolves, shake the tube so all nitrate is dissolved. In a separate tube do the same thing with sodium chloride

Tip solutions in small beaker. Stir so content is mixed. Lead should rise out

Folded piece of filter paper into filter funnel, stick funnel into conical flask

Pour content of beaker into middle of filter paper.

Rinse beaker out with deionised water- tip this onto filter paper

Rinse content on filter paper with deionised water to allow all the soluble sodium nitrate to flow through

Scape lead chloride onto fresh filter paper and leave it to dry in the oven

Seite 5

Chemical changes- Making soluble salts

If you are using an alkali - which is a soluble base - then you need to add just enough acid to make a neutral solution (check a small sample with universal indicator paper).

Warm the salt solution to evaporate the water. You get larger crystals if you evaporate the water slowly.

Making a salt from an alkali

1. A solution is placed in an evaporating basin and heated with a Bunsen burner

2. The amount of the solution has reduced through evaporation

3. The solution has evaporated, leaving a crystallised solute

Seite 6

Chemical changes- Electrolysis

Electrolytes are ionic compounds that are:

in the molten state (heated so they become liquids), or

dissolved in water

Electrolysis is a process in which electrical energy, from a direct current (dc) supply, decomposes electrolytes. The free moving ions in electrolytes are attracted to the oppositely charged electrodes, which connect to the dc supply.

Positively charged ions are called cations. They move towards the negatively charged electrode, which is called the cathode.

Negatively charged ions are called anions. They move towards the positively charged electrode, which is called the anode.

cations gain electrons from the negatively charged cathode

anions lose electrons at the positively charged anode

Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis:

Pb2+ ions gain electrons at the cathode and become Pb atoms

Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules

Seite 7

Chemical changes- Electrolysis

During the electrolysis of water:

H+ ions are attracted to the cathode, gain electrons and form hydrogen gas

OH- ions are attracted to the anode, lose electrons and form oxygen gas

At the anode-

for the most common compounds oxygen is produced (from the hydroxide ions)

if halide ions (chloride, bromide or iodide ions) are present, then the negatively charged halide ions lose electrons to form the corresponding non-metal halogen (chlorine, bromine or iodine)

the metal is produced at the cathode if it is less reactive than hydrogen

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

OIL RIG- Oxidation is loss of electrons. Reduction is gain of electrons

Half equations- Positively charged ions gain electrons at the cathode e.g Na+ + e- → Na

A half equation is used to represent the reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:

electrons are shown as e-

the numbers of atoms of each element must be the same on both sides

the total charge on each side must be the same (usually zero)

Seite 8

Chemical changes- Core practical - electrolysis of copper sulfate solution

Part 1 - Investigation using inert electrodes

Pour some copper sulfate solution into a beaker.

Place two graphite rods into the copper sulfate solution. Attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal.

Completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram.

Turn on the power supply and observe what happens at each electrode.

Test any gas produced with a glowing splint and a burning splint.

Record your observations and the results of your tests.

copper metal is formed at the negative electrode

oxygen gas is formed at the positive electrode

Aims

To investigate the electrolysis of copper sulfate solution using inert (unreactive) electrodes.

Seite 9

Chemical changes- Core practical - electrolysis of copper sulfate solution

Part 2 - Investigation using copper electrodes

Pour some copper sulfate solution into a beaker.

Measure and record the mass of a piece of copper foil. Attach it to the negative terminal of a dc supply, and dip the copper foil into the copper sulfate solution.

Repeat step 2 with another piece of copper foil, but this time attach it to the positive terminal.

Make sure the electrodes do not touch each other, then turn on the power supply. Adjust the power supply to achieve a constant current as directed by your teacher.

After 20 minutes, turn off the dc supply.

Carefully remove one of the electrodes. Gently wash it with distilled water, then dip it into propanone. Lift the electrode out and allow all the liquid to evaporate. Do not wipe the electrodes clean. Measure and record the mass of the electrode.

Repeat step 6 with the other electrode. Make sure you know which is which.

Repeat the experiment with fresh electrodes and different currents.

at the positive electrode, copper atoms lose electrons and form copper ions, Cu2+

at the negative electrode, copper ions gain electrons and form copper atoms

The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode. So the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode.

Seite 10

Chemical changes- Questions from spec

Q. What is the effect of acids and alkalis on indicators?

A. Acids and alkalis change the colour of the indicators. An acid turns blue litmus paper red and an alkali turns red litmus paper blue

Q. What are the different indicators?

A. litmus, methyl orange and phenolphthalein

Q. What happens to the pH when there is a high concentration of hydrogen ions in an acidic solution?

A. The pH lowers

Q. What happens to the pH when there is a high concentration of hydroxide ions in an alkaline solution?

A. The pH increases

Q. What happens to the pH if the hydrogen ion in a concentration increases by a factor of 10?

A. The pH decreases by 1

Q. Explain the terms dilute and concentrated

A. A concentrated solution is one where there is a high ratio of dissolved substance (e.g. acid or base) to solvent.

A dilute solution is one where a small amount of a substance has been added to a solvent.

Seite 11

Chemical changes- Questions from spec

Q. Explain the terms weak and strong acids

A. Strong acids dissociate fully in water to produce the maximum number of H+ ions.

Weak acids do not fully dissociate.

Q. What is a base?

A. A base is any substance that reacts with an acid to form a salt and water only

Q. Describe the chemical test for hydrogen?

A. Lighted splint- Squeaky pop

Q. Describe the chemical test for Carbon dioxide?

A. Lime water to CO2. If present cloudy or milky white should form

Q. What is neutralisation reaction?

A. Its a reaction between an acid and a base

Q. What is a acid- alkali neutralisation?

A. Its a reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH–) from the alkali to form water

Seite 12

Chemical changes- Questions from spec

Q. What are electrolytes?

A. Electrolytes are ionic compounds in the molten state or dissolved in water

Q. What does electrolysis mean?

A. Its a process in which electrical energy, from a direct current supply, decomposes electrolytes

Q. Explain the movement of ions during electrolysis

A. Positively charged cations move to the negatively charged cathode. Negatively charged anions move to the positively charged anode

Q. Explain the formation of the products in the electrolysis of copper chloride solution

A Copper metal and chlorine gas. Copper moves to the cathode Chlorine moves to the anode

Q. Explain the formation of the products in the electrolysis of sodium chloride solution

A. Hydrogen gas forms at the negative electrode, chlorine gas forms at the positive electrode

Q. Explain the formation of the products in the electrolysis of sodium sulphate solution

A. Chlorine will be produced. Oxygen will be produced.

Q. Explain the formation of the products in the electrolysis of molten lead bromide solution

A. Lead is formed at the cathode and Bromide is formed at the anode.

Seite 13

Chemical changes- Questions from spec

Q. What does Oxidation mean?

A. Its the loss of electrons

Q. What does Reduction mean?

A. Its the gain of electrons

Q. What electrode does reduction occur at?

A. Cathode

Q. What electrode does oxidation occur at?

A. Anode

Q. Explain the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper

A. Electrolysis is used to extract and purify metals. Copper is purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulfate. The anode (positive electrode) is made from impure copper and the cathode (negative electrode) is made from pure copper. Pure copper forms on the cathode.

Seite 14

Chemical changes- Questions from spec

Q. Investigate the electrolysis of copper sulfate solution with inert electrodes and copper electrodes

A.

Pour some copper sulfate solution into a beaker.

Place two graphite rods into the copper sulfate solution. Attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal.

Completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram.

Turn on the power supply and observe what happens at each electrode.

Test any gas produced with a glowing splint and a burning splint.

Record your observations and the results of your tests.

copper metal is formed at the negative electrode

oxygen gas is formed at the positive electrode