1 of 10© boardworks ltd 2007 reactivity how do you know a chemical reaction has happened?
TRANSCRIPT
1 of 10 © Boardworks Ltd 2007
Reactivity
• How do you know a chemical reaction has happened?
2 of 10 © Boardworks Ltd 2007
What is a chemical reaction?
A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products).
reactants products
For example:
carbon carbon dioxideoxygen
+
+
3 of 10 © Boardworks Ltd 2007
How can you spot a chemical reaction?
Chemical reactions can appear very different.
As you observe a chemical reaction, you may detect:
a colour change
precipitate (solid) forming
energy being produced (fizzing, burning)
an odour being produced.
4 of 10 © Boardworks Ltd 2007
Why doesn’t the mass change?
In a chemical reaction, no atoms are made or destroyed.
The reaction just changes how the atoms are bonded together.
5 of 10 © Boardworks Ltd 2007
Does mass change during a reaction?
6 of 10 © Boardworks Ltd 2007
What is a word equation?
For example, when a piece of sulfur is burned in oxygen gas it produces a white solid called sulfur dioxide.
A word equation uses the names of the reactants and products to show what happens in a chemical reaction.
+ sulfur oxygensulfur
dioxide
The word equation for this reaction is:
7 of 10 © Boardworks Ltd 2007
What is a symbol equation?
A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction.
This equation shows that one atom of sulfur (S) reacts with one molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2).
A symbol equation must be balanced to give the correct ratio of reactants and products.
+ S O2 SO2
+
8 of 10 © Boardworks Ltd 2007
What do state symbols show?
State symbols are added to a symbol equation to show whether the reactants and products are:
With state symbols in place, this symbol equation now shows that the sulfur is a solid, the oxygen is a gas and the sulfur dioxide is also a gas.
solid – symbol is (s)
liquid – symbol is (l)
gas – symbol is (g)
dissolved in water – symbol is (aq).
+ S (s) O2 (g) SO2 (g)
9 of 10 © Boardworks Ltd 2007
Chemical reactions – true or false?
10 of 8 © Boardworks Ltd 2007
What is thermal decomposition?
11 of 8 © Boardworks Ltd 2007
Thermal decomposition – easy or hard?
The more reactive a metal, the harder it is to decompose its carbonate by heating.
incr
ease
in
rea
ctiv
ity
Predict how easy it is to decompose these carbonates:
copper carbonate
calcium carbonate
sodium carbonate
manganese carbonate
zinc carbonate
iron carbonate.
potassium
sodium
calcium
magnesium
aluminium
manganese
zinc
iron
copper
silver
gold
12 of 8 © Boardworks Ltd 2007
Thermal decomposition – activity
13 of 8 © Boardworks Ltd 2007
Thermal decomposition of carbonates
FeOFeCO3 + CO2
ironoxide
ironcarbonate + carbon
dioxide
heat
What are the word and symbol equations for the thermal decomposition of these carbonates?
When the metal carbonate is heated, it decomposes to produce a metal oxide and carbon dioxide.
copper carbonate
manganese carbonate
zinc carbonate.
14 of 8 © Boardworks Ltd 2007
When calcium carbonate is heated, it decomposes to produce calcium oxide and carbon dioxide.
This reaction is carried out in industry to make calcium oxide (quicklime) from calcium carbonate (limestone):
Calcium oxide is used to make concrete and calcium hydroxide (slaked lime).
Heating calcium carbonate
CaOCaCO3 + CO2
calciumoxide
calciumcarbonate + carbon
dioxide
heat
15 of 8 © Boardworks Ltd 2007
Baking powder and self-raising flour contain sodium hydrogencarbonate (also known as sodium bicarbonate).
What effect does this reaction have on dough as it is being baked?
When sodium hydrogencarbonate is heated, it decomposes to make sodium oxide, carbon dioxide and water.
The carbon dioxide released during the reaction creates tiny bubbles, which help the dough to rise.
Heating sodium hydrogencarbonate
16 of 8 © Boardworks Ltd 2007
What is the decomposition equation?
17 of 8 © Boardworks Ltd 2007
What are indicators?
Indicators are chemicals that change colour in the presence of an acid or an alkali.
There are many different indicators. Lots of them come from plants, like red cabbage. Different indicators turn different colours.
Universal indicator is a mixture of indicators. It is useful because it shows a range of colours from pH 1–14.
Geranium plants grown in acidic soil have red flowers, while geraniums grown in alkali soil have blue flowers.
18 of 8 © Boardworks Ltd 2007
What does the pH scale show?
The pH scale runs from 1 to 14. The numbers 1 to 6 represent acidic conditions and 8 to 14 represent alkali conditions. A pH value 7 is neutral.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
What are the pH values of some everyday items?
19 of 8 © Boardworks Ltd 2007
What is the order of pH?
20 of 8 © Boardworks Ltd 2007
What are acids?
Acids are substances that:
Hydrochloric acid is a strong acid.
Ethanoic acid (vinegar) is a weak acid.
Have a pH below 7 and turn universal indicator yellow, orange or red.
Turn litmus red.
Form solutions containing hydrogen ions (H+).
21 of 8 © Boardworks Ltd 2007
What are alkalis?
Have a pH above 7 and turn universal indicator blue or purple.
Alkalis are substances that:
Sodium hydroxide is a strong alkali.
Ammonia is a weak alkali. When ammonia is dissolved in water, it forms ammonium hydroxide (NH4OH), a fertilizer.
Turn litmus blue.
Can neutralize acids
Form solutions containing hydroxide ions (OH-).
22 of 8 © Boardworks Ltd 2007
What are bases?
All alkalis are bases. Bases are substances that react with acids by absorbing hydrogen ions (H+).
Some bases are soluble in water – these are called alkalis.
All alkalis contain hydroxide ions (OH–). The more OH– ions in the solution, the stronger the alkali.
The oxides, hydroxides and carbonates of metals, such as sodium hydroxide, are bases.
Ammonia is a base that does not contain a metal.
alkalis(solublebases)
bases (react with acids)
23 of 8 © Boardworks Ltd 2007
Acids and bases – true or false?
24 of 16 © Boardworks Ltd 2007
+acid base a salt
How are salts made and named?
When an acid reacts with a base, a neutralization reaction occurs and produces a chemical called a salt.
The name of the salt depends on the names of the reactants.
The first part of the salt’s name comes from the base:
e.g. sodium hydroxide sodium…
The second part of the salt’s name comes from the acid:
e.g. sulfuric acid sulfate…
For example, if sodium hydroxide neutralizes sulfuric acid, the product is a salt called sodium sulfate.
25 of 7 © Boardworks Ltd 2007
Sometimes when two solutions are mixed, they react to form an insoluble solid product.
For example, mixing solutions of lead nitrate and sodium chloride produces a yellow precipitate of lead chloride.
What is a precipitation reaction?
The insoluble solid product is called a precipitate.
You can spot a precipitate because the mixture goes cloudy.
leadnitrate
Pb(NO3)2 (aq)
sodiumchloride
2NaCl (aq)
leadchloride
PbCl2 (s)
sodiumnitrate
2NaNO3 (aq)
+
+
+
+
26 of 7 © Boardworks Ltd 2007
Identifying metals
27 of 16 © Boardworks Ltd 2007
What is the name of the salt?
28 of 16 © Boardworks Ltd 2007
What are salts used for?
Table salt is sodium chloride. This is the salt used to flavour and preserve food.
Indigestion remedies often contain magnesium salts.
Salts can also be used as coloured pigments in paints, and to help fuels burn better.
The colours of fireworks are formed when certain salts burn. Calcium chloride, for example, burns a bright red colour.
29 of 16 © Boardworks Ltd 2007
Water is formed because OH– ions from the alkali react with H+ ions from the acid to produce molecules of water (H2O).
Making salts 1: acid + alkali
When an acid reacts with an alkali, the products are a salt and water.
acid alkali salt water+ +
For example:
+ +
++
sodiumhydroxide
NaOH (aq)
sodiumchloride
NaCl (aq)
water
H2O (aq)
hydrochloricacid
HCl (aq)
30 of 16 © Boardworks Ltd 2007
When is the reaction complete?
There is no obvious sign when this reaction is complete, so an indicator is used to show when the solution is neutral.
This process is called titration.
The reaction between sodium hydroxide and hydrochloric acid produces sodium chloride, which is soluble in water.
+ +
++
sodiumhydroxide
NaOH (aq)
sodiumchloride
NaCl (aq)
water
H2O (aq)
hydrochloricacid
HCl (aq)
31 of 16 © Boardworks Ltd 2007
What is a titration?
32 of 16 © Boardworks Ltd 2007
Making salts 2: acid + metal
When a metal is added to an acid, the products are a salt and hydrogen gas.
For example:
This method of making salts only works with some metals.
hydrochloricacid
2HCl (aq)
+
+
+
+
zinc
Zn (aq)
zincchloride
ZnCl2 (aq)
hydrogen
H2 (aq)
acid metal a salt hydrogen+ +
What would happen if potassium was used? Or copper?
33 of 16 © Boardworks Ltd 2007
Which metal will react the most?
34 of 16 © Boardworks Ltd 2007
Making salts 3: acid + metal oxide
When a metal oxide is mixed with an acid, the products are a salt and water.
calciumchloride + water
calciumoxide
CaO (aq)
hydrochloricacid
2HCl (aq) CaCl2 (aq) +
+
+ H2O (aq)
For example:
acid metal oxide salt water+ +
Water forms because oxygen ions from the oxide join up with H+ ions from the acid to produce molecules of water (H2O).
35 of 16 © Boardworks Ltd 2007
Obtaining salts from metal oxides
When a metal oxide is added to an acid, it dissolves as it reacts.
How could the solid copper sulfate salt be separated from the water?
You know when you have added enough of the metal oxide because it stops dissolving.
+ +copperoxide
CuO (s)
sulfuricacid
H2SO4 (aq)
coppersulfate
CuSO4 (aq)
water
H2O (aq)
36 of 16 © Boardworks Ltd 2007
Obtaining salts from copper oxide
37 of 16 © Boardworks Ltd 2007
Making salts 4: acid + carbonate
When a carbonate is mixed with an acid, the products are a salt, carbon dioxide and water.
coppernitrate + water +
carbondioxide
coppercarbonate
CuCO3 (s)
nitricacid
2HNO3
(aq)
Cu(NO3)2
(aq)+
+
+ H2O(aq)
+ CO2
(g)
For example:
What would you expect to observe in this reaction?
acid carbonate salt water+ +carbondioxide+
Water and carbon dioxide are formed because the carbonate ions (CO3
2-) react with H+ ions from the acid.
38 of 16 © Boardworks Ltd 2007
Making salts – summary
Salts can be made by reacting acids with bases.
acid + alkali a salt + water
acid + metal a salt + hydrogen
acid + metal oxide a salt + water
acid + carbonate a salt + carbon dioxide + water
There are four ways of making salts from acids:
39 of 16 © Boardworks Ltd 2007
Matching reactants and salts
40 of 16 © Boardworks Ltd 2007
Complete the neutralization reaction