Chemical Reactions and Metals

Changes to Materials

Physical and chemical changes can be observed in everyday examples like a chocolate bar.

8.1 Simple Chemical Reactions

Learning Objectives:
- Understand the chemical properties of some metals.
- Recognize that a chemical reaction has taken place.

What is a Chemical Reaction?

Examples of chemical reactions include:
- Cooking
- Rusting
- Sticking
- Burning
- Making metals
- Living processes
Chemical reactions occur anywhere new substances are made, not just in the laboratory.

Useful Chemical Reactions

Many chemical reactions are useful or even essential for survival.

Non-Useful Chemical Reactions

Some chemical reactions are a nuisance or can be dangerous.

Reactants and Products

Reactants: The starting substances in a chemical reaction.
Products: The new substances formed in a chemical reaction.
In a chemical reaction, one or more new substances are formed.
The arrow in a chemical equation means "change into."
In a chemical reaction, all the reactants change into the products.
It is usually difficult to reverse a chemical reaction and change the products back into the reactants.

More About Chemical Changes

Chemical changes are usually difficult to reverse.
Example: Magnesium burns in oxygen to form magnesium oxide; it is not possible to "un-burn" the magnesium.
Many reactions need energy to start.
Many reactions, like the burning of magnesium, give out heat energy once started.
Heat energy being given out is one sign of a chemical reaction.

Word Equations

A word equation is a quick, shorthand way of writing a chemical reaction.
Three parts of a word equation:
1. Names of the reactants.
2. An arrow.
3. Names of the products.
Example: Hydrogen reacting with oxygen to form water:
- Hydrogen + Oxygen → Water

Reactant or Product Examples

Magnesium + Oxygen → Magnesium Oxide
- Reactants: Magnesium, Oxygen
- Product: Magnesium Oxide
Copper + Oxygen → Copper Oxide
- Reactants: Copper, Oxygen
- Product: Copper Oxide

Write the word equations

Magnesium + Oxygen → Magnesium Oxide
Calcium Hydroxide + Hydrochloric Acid → Calcium Chloride + Water
Sodium + Hydrochloric Acid → Sodium Chloride + Hydrogen

What is Combustion?

Combustion is the scientific word for burning and is a type of chemical reaction.
Combustion is the reaction when a substance burns and reacts with oxygen to produce heat and light energy.

Using Combustion

Burning fuel like coal, petrol, and natural gas provides > 90\% of the energy needed for transport, factories, and homes.

Equations for Combustion

When a substance burns and reacts with oxygen, the new substances formed are called “oxides.”

Coal (Carbon) + Oxygen → Carbon Dioxide
Hydrogen + Oxygen → Dihydrogen Oxide (Water)
Methane + Oxygen → Carbon Dioxide + Water

Combustion of Methane

Methane (natural gas) is often burnt for cooking.
Methane is made up of carbon and hydrogen.
When methane burns:
- It reacts with oxygen.
- The carbon in methane changes into carbon dioxide.
- The hydrogen in methane changes into water.
Word equation: Methane + Oxygen → Carbon Dioxide + Water

Missing Words About Chemical Reactions

In a chemical reaction, the starting substances are called the reactants.
One or more new substances are always formed, which are called the products.
It is difficult to change products back into reactants.
A chemical reaction can be described in a word equation.
There are often visible changes during a chemical reaction, such as a color change or bubbles.
Heat is given out during many reactions and is often required to start a reaction.

Metals and Water

Some metals react vigorously with water, some react slowly, and some do not react at all.
The Romans used lead for water pipes but didn’t know that lead reacts slowly with water and makes it poisonous.

Metals and Water – General Equation

Potassium and sodium are metals that react vigorously with water.
When a metal reacts with water, the products are a metal hydroxide and hydrogen gas.
General equation: Metal + Water → Metal Hydroxide + Hydrogen

Metals and Water – Equations

Lithium: $2Li + 2H2O \rightarrow 2LiOH + H2$
Sodium: $2Na + 2H2O \rightarrow 2NaOH + H2$
Potassium: $2K + 2H2O \rightarrow 2KOH + H2$

Metals and Water – Observations

Potassium: Immediately produces a lilac flame and skims around the surface making a fizzing noise.
Sodium: Melts and skims over the surface producing a stream of small bubbles; sometimes a yellow-orange flame appeared.
Lithium: Bubbles of gas are given off quite quickly; the water is now alkaline.

Metals and Water – More Observations

Magnesium: Reacts slowly with cold water but reacts quickly with steam.
Copper: No reaction.
Silver: No reaction.
Gold: No reaction.

Metals and Water – Using Unreactive Metals

Copper is used in plumbing and silver and gold in jewelry because they are unreactive.
The words "plumber" and "plumbing" come from "plumbum" (Latin for lead).
Lead reacts very slowly with water, making it poisonous, so it is no longer used in plumbing.
Copper is a much better metal for water pipes because it does not react at all with water.

Metals and Oxygen – General Equation

Most metals will react with oxygen.
Some metals react faster than others, and some may react very slowly or not at all.
When a metal reacts with oxygen, the product is a metal oxide.
General equation: Metal + Oxygen → Metal Oxide
Example: Magnesium burns in oxygen with a bright flame to produce magnesium oxide.

Metals and Oxygen – Equations

Magnesium: $2Mg + O_2 \rightarrow 2MgO$
Copper: $2Cu + O_2 \rightarrow 2CuO$
Iron: $4Fe + 3O2 \rightarrow 2Fe2O_3$

Metals and Oxygen – Observations

Magnesium: The ribbon burned with a dazzling white flame giving grey-white smoke and ash.
Copper: The copper turnings went through reds and oranges and then slowly got a permanent coating of black.
Iron: The iron filings glowed red and sparkled leaving a brown-black looking solid.

Metals and Oxygen – Order of Reactivity

Order of reactivity (most to least): Magnesium > Iron > Copper

Metals and Acid – The Acid Test

Alchemists were interested in finding ways of changing cheap metals into gold.
Acid was used to find out if gold objects were made of real or fake gold.
Gold is such an unreactive metal that it does not react with strong acids.
The phrase "the acid test" is used today to mean any process that will reveal fakes.

Metals and Acid – General Equation

Gold is an unreactive metal that does not react with acid.
When a metal reacts with acid, the products are a metal salt and hydrogen.
General equation: Metal + Acid → Metal Salt + Hydrogen
The type of acid affects the type of salt produced.
Other metals, such as magnesium and zinc, react with acid producing bubbles of gas (hydrogen).

Metals and Hydrochloric Acid – Equations

Magnesium: $Mg + 2HCl \rightarrow MgCl2 + H2$
Aluminium: $2Al + 6HCl \rightarrow 2AlCl3 + 3H2$
Zinc: $Zn + 2HCl \rightarrow ZnCl2 + H2$

Metals and Sulfuric Acid – Equations

Magnesium: $Mg + H2SO4 \rightarrow MgSO4 + H2$
Aluminium: $2Al + 3H2SO4 \rightarrow Al2(SO4)3 + 3H2$
Zinc: $Zn + H2SO4 \rightarrow ZnSO4 + H2$

Metals and Nitric Acid – Equations

Magnesium: $Mg + 2HNO3 \rightarrow Mg(NO3)2 + H2$
Aluminium: $2Al + 6HNO3 \rightarrow 2Al(NO3)3 + 3H2$
Zinc: $Zn + 2HNO3 \rightarrow Zn(NO3)2 + H2$

Metals and Acid – Observations

Magnesium: Fizzed rapidly with cold acid.
Aluminium: Bubbled quickly with cold acid.
Copper: No reaction with hot or cold acid.
Iron: No reaction with cold acid, slow bubbling with hot acid.
Lead: No reaction with cold acid, occasional bubble with hot acid.
Calcium: Really fast with cold acid.
Zinc: Moderate bubbling with cold acid.

8.2 Neutralization Reactions & 8.3 Investigating Acids and Alkalis

Learning Objectives:
- Learn how to make a neutral solution.
- Learn why neutralization is important.

What is an Indicator?

An indicator is a special chemical that changes to a different color in an acid or an alkali.
Litmus paper and litmus solution are examples of indicators.
Litmus is red in acid and blue in alkali.
Litmus is used to show if a solution is acid or alkali but does not show if the acid or alkali is weak or strong.

What is Universal Indicator?

Universal indicator has a range of colors that show how weak or strong the acid or alkali is.

What is the pH Scale?

The strength of an acid or alkali is measured by the pH scale.
Each universal indicator color is given a number called the pH value.
Universal indicator can tell you the pH of a solution.
pH scale ranges from 0-14:
- Strong acid: 1-3
- Weak acid: 4-6
- Neutral: 7
- Weak alkali: 8-10
- Strong alkali: 11-14

Acid/Base Neutralization

Neutralization: Acid + Base -> Salt + Water

Mixing an Acid and an Alkali

Mixing an acid and an alkali causes a chemical change called a chemical reaction.

What is Neutralization?

The chemical reaction between an acid and an alkali is called neutralization.
The pH value of the reaction mixture becomes closer to 7 during neutralization.

Neutralizing Stings

A bee sting is acidic; a wasp sting is alkaline.
Bicarbonate of soda (pH 9) can be used to treat a bee sting because it is a weak base.
Vinegar (pH 3) can be used to treat a wasp sting because it is an acid.

Neutralizing Soil

Soils in different places can be acidic, alkaline, or neutral.
Many plants do not grow well in acidic soil.
Farmers test soil to check the pH.
Farmers often add an alkali to acidic soil to make their plants grow better.

8.4 Detecting Chemical Reactions

Learning Objectives:
- Learn about different ways of telling if a chemical reaction has happened.
- Test gases given off in reactions so that you can identify them.

What is a Compound?

A compound is the substance produced when two or more elements combine in a chemical reaction.
A compound is always made up of two or more different types of atom.
Example: Hydrogen (H) and oxygen (O) combine to make the compound water.

Making a Compound – Carbon Dioxide

A compound has very different properties to the elements from which it is made.
Carbon: A black solid which can be used as a fuel.
Oxygen: A colorless gas which is essential for life.
Carbon Dioxide: A colorless gas which is used to put out fires.

Making a Compound – Water

Hydrogen: A colorless gas which is used in hot air balloons.
Oxygen: A colorless gas which is essential for life.
Water: A liquid which is essential to our lives and has many different uses.

Chemical Reaction to Make Water

Elements combine with each other to make new substances called compounds.
In a chemical reaction, new substances are formed.
The substances which combine in a reaction are the reactants.
The new substances produced in a reaction are the products.
Compounds can also combine with elements or other compounds to make new compounds.
The products have different properties to the reactants from which they are formed.

Atoms Joining in a Chemical Reaction

In a chemical reaction, the atoms in the reactants become joined in different ways, and this is how new substances are formed.

Recap…What is a Chemical Reaction?

It is a chemical change in which one or more substances are destroyed and one or more new substances are created.

Parts of a Chemical Reaction

Reactants → Products
Reactants: Substances that are destroyed by the chemical change (bonds break).
Products: Substances created by the chemical change (new bonds form).
The arrow (→) is read as “yields.”

Other Symbols in Chemical Reactions

(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous solution (the substance is dissolved in $H_2O$ )
"+" separates two or more reactants or products
"→" yield sign separates reactants from products

Evidence for a Chemical Reaction

Evolution of light or heat.
Temperature change (increase or decrease) to the surroundings.
Formation of a gas (bubbling or an odor) other than boiling.
Color change (due to the formation of a new substance).
Formation of a precipitate (a new solid forms) from the reaction of two aqueous solutions.

Exothermic and Endothermic Changes

Exothermic change: Transfers energy to the surroundings (gets hotter).
Endothermic change: Transfers energy from the surroundings (gets cooler).

How do we know a chemical reaction has happened?

Bubbles / fizzing – A gas being released
A color change
Bits / Precipitate
A temperature change

Burning Metals in Air

When metals such as copper are burned they combine with oxygen in the air to form Copper Oxide.
Copper + Oxygen -> Copper Oxide

Testing for Hydrogen

Many metals react with acids. When this happens, the metal fizzes as bubbles are produced.
A gas is produced. To test if the gas is hydrogen: Place a burning splint next to the mouth of the test tube. A ‘squeaky pop’ as the gas ignites shows that hydrogen is the gas produced in this reaction.
Magnesium + Hydrochloric acid  Magnesium chloride + Hydrogen

Test for Carbon Dioxide

When a metal carbonate reacts with acid, it fizzes and then seems to disappear. The carbonate and the acid have reacted and changed into a salt, water, and carbon dioxide.
When carbon dioxide gas is bubbled through limewater, it turns the limewater cloudy or milky white.
Carbon dioxide can be tested for using the limewater test.
Metal Carbonate + Acid -> Salt + Water + Carbon Dioxide

Testing for Carbon Dioxide

The salt produced depends on the metal carbonate and the type of acid involved in the reaction:
- Metal carbonate + Hydrochloric acid -> Metal chloride + Carbon dioxide + Water
- Metal carbonate + Sulfuric acid -> Metal sulfate + Carbon dioxide + Water
- Metal carbonate + Nitric acid -> Metal nitrate + Carbon dioxide + Water

Testing for Carbon Dioxide Equation

$CO2 (g) + Ca(OH)2 (aq) \rightarrow CaCO3 (s) + H2O (l)$

Materials for Testing Carbon Dioxide

A test tube
Limewater
A straw
A funnel

Procedures for Testing Carbon Dioxide

Pour limewater into the test tube.
Blow bubbles into the cup for at least one minute (caution—do not drink the solution).

Results of Testing Carbon Dioxide

When carbon dioxide reacts with limewater, it turns white/cloudy/milky.

Testing for Oxygen

Oxidation is the name given to a chemical reaction in which oxygen is added to a substance.
When a metal such as copper is heated in air, it reacts with oxygen.
Black copper oxide is formed.

Testing for Oxygen - Elephant Toothpaste

The chemical reaction that produces the ‘Elephant Toothpaste’ effect is – decomposition of Hydrogen Peroxide.
Hydrogen Peroxide, in the presence of a catalyst, decomposes rapidly into Water and Oxygen.
The chemical equation is: $2H2O2 \rightarrow 2H2O + O2$