Chemical changes 11

What is a physical change

Change in which no new substances are formed (e.g. freezing, boiling)

What is a chemical change

Change in which new substances are formed (in the form of elements and/or compounds)

When a chemical change occurs, what takes place?

A chemical reaction has taken place

Substances that react with each other are called? How about the new substances formed?

Reactants

Products

What are the two groups that chemical changes are classified under?

Reaction in which two or more substances chemically combine to form one or more substances (e.g. carbon burns in the presence of oxygen to form carbon dioxide)

Reaction in which two or more substances breaks down into two or more substances (e.g. when calcium carbonate is heated, it breaks down into carbon dioxide and calcium oxide)

Physical changes can be easily reversed. Can chemical changes do so too?

No, chemical changes cannot be easily reversed, as the reactants cannot be obtained easily from the products, and is usually possible using large amounts of energy.

What is usually given out/taken in during chemical reactions?

Energy. (e.g. large amount of heat is released when carbon burns, or taken in when calcium carbonate is heated)

Are physical changes easily reversable?

Usually can be reversed by using separation techniques

What is inside a chemical equation?

Reactants —-(condition) → Product

What happens to atoms during chemical reactions?

Atoms of the reactants rearrange themselves and combine chemically to form products

Can atoms be created/destroyed in a chemical reaction? So is the mass of the reactants the same as the products?

No. They are neither created nor destroyed in a chemical reaction. Yes they have equal mass. (mass is conserved)

What are the types of chemical changes?

Combustion, thermal decomposition, oxidation, neutralisation

(CONT)

What is combustion?

A chemical reaction in which a substance is heated in the presnece of oxygen to form one or more substances.

What is thermal decompostion

The process in which a substance breaks down into two or more simpler substances upon heating

What is oxidation

A chemical change in which a substance gains oxygen.

e.g. rusting → iron reacts with oxygen and water

OR

Cellular (aerobic) respiration → food molecules (glucose) are broken down in the presence of oxygen to provide cells energy to survive and reproduce

What is neutralisation?

A reaction when acid is mixed with alkali.

What are the characteristics of acids?

Sour taste

Dangerous (may burn the skin)

Turn blue litmus paper red

React with alkalis, metals and carbonates

(Example in laboratory: hydrochloric acid, Sulfuric acid, nitric acid)

(example at home: acetic acid, ascorbic acid, citric acid)

Characteristics of alkali?

Bitter taste

Soapy feel

Dangerous (may burn the skin)

Turn red litmus paper blue

React with acids in neutralisation reactions

(examples in laboratory: sodium hydroxide, calcium hydroxide and potassium hydroxide)

(examples at home: Sodium hydroxide, baking soda, ammonia)

What are the three common reaction of acids?

Neutralisation (acid x alkali)

Forming salt and hydrogen gas (acid x metal)

Forming salt, carbon dioxide gas and water (acid x carbonate)

What is the chemical reaction of neutralisation written as?

Acid + alkali → salt + water

(salt is a general term for a compound containing an element that is a metal, so the “salts” can be different)

What is the chemical reaciton for Forming salt and hydrogen gas (acid x metal)

acid + metal → salt + hydrogen

How do we test for hydrogen gas?

Light a splint and hold it inside the test tube - if it extinguishes with a “pop” sound, hydrogen is present.

But do all metals react in the same way?

No. Some metals like copper, silver and gold do not react with acids.

What is the chemical formula for Forming salt, carbon dioxide gas and water (acid x carbonate)?

acid + carbonate → salt + carbon dioxide + water

How do we test for carbon dioxide gas?

Put limewater in a test tube and connect the test tube with the products with the limewater test tube with a delivery tube. If it turns chalky/cloudy, there is carbon dioxide gas.

What is the pH scale?

A scale to measure how acidic or alkaline a solution is.

Greater, more alkaline

Lower, more acidic

Why are very acidic/alkaline water dangerous to us?

They are usually corrosive and can burn our skin

What if a solution is neither alkaline or acidic?

It is neutral, aka pH 7

How do we test for acidic/alkaline solutions? Why so? How does this method reveal the results?

Use indicators as it is not safe to touch an unknown solution.

It will change colour in the presence of an acid/alkaline. (e.g. litmus paper, methyl orange, bromothymol blue and phenol red and certain vegetables)

What litmus papers do we use for testing alkaline/acidic solutions?

Acidic solutions → blue litmus paper (turns to red)

(if we use red litmus paper it remains red)

Alkaline solutions → red litmus paper (turns to blue)

(if we use blue litmus paper it remains blue)

What is the change for neutral pHs on the litmus paper?

No change on both papers.

What is the universal indicator

An indicator providing the approximate pH values. (paper strips/solution)

How do we turn an acidic solution to pH 7?

Do complete neutralisation, so it has a pH of 7 and is neither alkali and acidic.

Applications of neutralisation

Antacids → alkaline compounds neutralise excess acid produced by the stomach to relieve discomfort

Toothpaste → weakly alkaline neutralises any acid secreted by bacteria that can cause tooth decay

Soil → adding suitable substances for neutralisation as too acidic/alkaline soil may not be optimal for growth

What causes a chemical change to occur?

Mixing

Heating

Exposure to light

Interaction with oxygen

Using an electric current

HE (L) IUM

Explain mixing

Mixing two or more reactants can cause them to combine irreversibly to form one or more products

Explain heating

Heat/increase in temperature is needed for chemical reactions (combustion, thermal decomposition)

Explain exposure to light

Photosynthesis (a chemical reaction) occurs in green plants in the presence of light. Green plants uses chlorophyll to trap the light to combine it with carbon dioxide and water to produce oxygen and glucose.

Explain exposure to oxygen

Exposure to oxygen can cause substances to gain oxygen and cause a chemical reaction.

(Rusting, cellular respiration etc.)

Explain using an electric current (the process and how it works)

A chemical reaction called electroplating. (E.g. using it to make recycled silver gold-coated medals)

When electroplating silver with gold, we pass an electric current through a solution containing gold particles from one point (i.e gold metal) to another point (i.e, the silver core.) This allows the gold particles to coat the silver core layer by layer and form a gold-plated object.

How do we stop iron from rusting?

Electroplate it with a material that cannot oxidise (e.g. chromium)

How does chemical reactions help us in our daily lives?

Cooking

Respiration

Decay

Rusting

Combustion

CRDRC

Explain how chemical reactions occurs during cooking, and its effects on the food and our body.

Cooking causes chemical reactions in food. Heat from cooking softens food and improves its flavour. It also makes it easier for the body to absorb nutrients from food, but some may be destroyed in the process

Explain how chemical reactions occur during respiration and its effects on living organisms

During respiration glucose molecules react with oxygen to produce carbon dioxide and water. The energy released allows each cell in an organism to survive and reproduce.

Explain what is decay, its effects and how do we prevent it

Decay or decomposition is the process of breaking down organic matter into simpler substances, allowing nutrients to be returned to the environment. It can cause food to spoil, harming our health.

We can prevent it by vacuum packaging - removes oxygen and limits the growth of bacteria and other microorganisms

Explain what is rusting and why it occurs and how we can prevent it

Rusting is an oxidation reaction that is unwelcome. It occurs rapidly in humid climates (large amount of water vapour).

Ways to prevent:

Painting

Oiling

Galvanising (coating iron with a protective layer of zinc)

Drying

this prevents contact with oxygen and water

POGD

Explain how does combustion come about and how it affects us

Combustion of fuels can produce air pollutants (carbon monoxide, sulfur dioxide oxides of nitrogen) when inhaled this can cause harmful health effects

Removing forests by burning them -> increases carbon dioxide in the atmosphere as it releases carbon and less carbon is absorbed, trapping heat into the atmosphere causing global warming (indirectly, climate change)

Carbon dioxide released can also cause acidification of oceans

What are the effects of carbon monoxide and its source

Source:

Burning of fuels in factories and motor vehicles in the presence of insufficient oxygen

Effects:

A colourless, odourless and highly toxic gas

Reduces amount of oxygen taken up by red blood cells, thus leading to headaches and death by suffocation

What are the effects of sulfur dioxide and its source

Source:

Burning of sulfur-containing fuels (such as coal and petroleum in power stations and factories)

Volcanic eruptions

Effects:

Causes eye, nose and lung irritation and worsen symptoms of asthma, heard disease and respiratory problems (even at low levels)

dissolves in rainwater to form acid rain, which harms aquatic life and damages buildings

What are the effects of oxides of nitrogen and its source

Source:

burning of fuels at high temperatures in motor vehicles, factories and power stations

Reaction of nitrogen and oxygen in the air during lightning strikes

Effects:

Causes eye, nose and lung irritation and worsen symptoms of asthma, heard disease and respiratory problems (even at low levels)

dissolves in rainwater to form acid rain, which harms aquatic life and damages buildings

Why does burning certain stuff release carbon dioxide? Why is the carbon dioxide released harmful?

Those substances contain carbon

It can cause acidification of the ocean

How does ocean acidification occur? What are its effects?

When large amounts of carbon in the atmosphere dissolves in seawater, carbonic acid is formed. (cause)

Shells and skeletons of small marine organisms(e.g. oysters, corals) contain calcium carbonate. They are broken down when it reacts with the acidic seawater. Marine organisms are not able to form new shells or skeletons easily as the carbonate compounds would break down easily.

Fish and plants may die as they cannot survive under conditions of a lower pH, affecting other organisms that depend on them