Chemistry summary ‘reactions’
1.1 introduction: what is a reaction?
Chemical reaction -> in a chemical reactions the reactants disappear and reaction products
with different properties are formed
Reactants -> the substances you have before the reaction
Reactions products -> the substances you end up with after the reaction
Combustion reaction -> when a substance reacts with oxygen to produce heat and light in
the form of a flame
Example: combustion of hydrogen (H2)
- when H2 is mixed with O2 and ignited the mixture will burn and form a new substance H2O
Reaction equation = Hydrogen (g) + oxygen (g) -> water (l)
The bonds between the H and O atoms have been broken and rearranged themselves
Each O atom is now bonded to 2 H atoms
The substances H2 (g) and O2 (g) have been transformed into a new substance H2O (l)
The existing bonds between atoms are broken and new bonds are formed to make new
substances with different properties
1.2 changes and chemical reactions
It’s difficult to see the difference between a phase transition and a chemical reaction
When ice is heated up it will change into liquid form and further into gas but nothing changes
to the molecules itself, the bonds are not broken and the water molecules stay the same
Therefor this process can be repeated endlessly
Egg white will go from liquid to solid when heated, but once an egg has solidified, it’s
impossible to turn it back into liquid
A new substance is formed; a chemical reaction has taken place
Chemical reactions are often but not always irreversible
When you dissolve a substance you can see a change too
When salt is added to water it seems to have disappeared but it’s still there and you can see
that when you evaporate the water, nothing has happened to the salt molecules
So phase transitions and dissolution of a substance are examples of physical changes
The molecules themselves don’t change as they would in a chemical reaction
1.3 how do you recognise a chemical reaction?
Here are several types of changes that may indicate that a chemical reaction has occurred
1. Development of gas
When base metals are brought in contact with water a gas is produced
You can identify if it’s hydrogen by igniting it with a match
You will hear a small explosion with a sound called a ‘squeaky pop’
2. A change in appearance
Like a change in colour or formation of a precipitate
Precipitate -> when the reaction product is insoluble a suspension will be formed, eventually
the small solid particles will sink to the bottom
, 3. A change in smell or taste
You can notice a certain smell during a chemical reaction like the odor of a struck match
(smell of sulfur oxide (SO2)) or when you prepare food the smell changes
But taste can also change for example by frying potatoes or foods that contain sugar will
caramelise
4. A change in tempature
In most chemical reactions energy is either taken up or released -> the energy effect
In many chemical reactions energy is transferred to the surroundings in the form of heat or
light
With burning a substance you have both light and heat
Exothermic reactions -> a reaction in which energy is released
In other reactions energy is taken up from the surroundings often in the form of heat
When citric acid and soda are mixed the resulting mixture becomes cold
Endothermic reaction -> a reaction in which energy is taken up
Activation energy -> when reactants need to be heated for the reaction to get started
This shouldn’t be confused with an endothermic reactions
Because when you lit paper to burn the paper more energy is released than the small
amount needed to lit it, so it’s still an exothermic reaction
Reaction profiles -> graphs that can be used to show how the energy in a reaction changes as
the reaction proceeds
In the first graph an exothermic reaction is shown
The reactants contain more energy than the reaction products so energy is released
The energy effect is negative
In the second graph an endothermic reaction is shown
The reactions contain less energy than the reaction products so energy is taken up
The energy effect is positive
The activation energy can be found by looking at the difference between the energy of the
reactants and the highest point in the graph (activated state)
2.1 Lavoisier’s law
The mass before and after a reaction stays the same
1.1 introduction: what is a reaction?
Chemical reaction -> in a chemical reactions the reactants disappear and reaction products
with different properties are formed
Reactants -> the substances you have before the reaction
Reactions products -> the substances you end up with after the reaction
Combustion reaction -> when a substance reacts with oxygen to produce heat and light in
the form of a flame
Example: combustion of hydrogen (H2)
- when H2 is mixed with O2 and ignited the mixture will burn and form a new substance H2O
Reaction equation = Hydrogen (g) + oxygen (g) -> water (l)
The bonds between the H and O atoms have been broken and rearranged themselves
Each O atom is now bonded to 2 H atoms
The substances H2 (g) and O2 (g) have been transformed into a new substance H2O (l)
The existing bonds between atoms are broken and new bonds are formed to make new
substances with different properties
1.2 changes and chemical reactions
It’s difficult to see the difference between a phase transition and a chemical reaction
When ice is heated up it will change into liquid form and further into gas but nothing changes
to the molecules itself, the bonds are not broken and the water molecules stay the same
Therefor this process can be repeated endlessly
Egg white will go from liquid to solid when heated, but once an egg has solidified, it’s
impossible to turn it back into liquid
A new substance is formed; a chemical reaction has taken place
Chemical reactions are often but not always irreversible
When you dissolve a substance you can see a change too
When salt is added to water it seems to have disappeared but it’s still there and you can see
that when you evaporate the water, nothing has happened to the salt molecules
So phase transitions and dissolution of a substance are examples of physical changes
The molecules themselves don’t change as they would in a chemical reaction
1.3 how do you recognise a chemical reaction?
Here are several types of changes that may indicate that a chemical reaction has occurred
1. Development of gas
When base metals are brought in contact with water a gas is produced
You can identify if it’s hydrogen by igniting it with a match
You will hear a small explosion with a sound called a ‘squeaky pop’
2. A change in appearance
Like a change in colour or formation of a precipitate
Precipitate -> when the reaction product is insoluble a suspension will be formed, eventually
the small solid particles will sink to the bottom
, 3. A change in smell or taste
You can notice a certain smell during a chemical reaction like the odor of a struck match
(smell of sulfur oxide (SO2)) or when you prepare food the smell changes
But taste can also change for example by frying potatoes or foods that contain sugar will
caramelise
4. A change in tempature
In most chemical reactions energy is either taken up or released -> the energy effect
In many chemical reactions energy is transferred to the surroundings in the form of heat or
light
With burning a substance you have both light and heat
Exothermic reactions -> a reaction in which energy is released
In other reactions energy is taken up from the surroundings often in the form of heat
When citric acid and soda are mixed the resulting mixture becomes cold
Endothermic reaction -> a reaction in which energy is taken up
Activation energy -> when reactants need to be heated for the reaction to get started
This shouldn’t be confused with an endothermic reactions
Because when you lit paper to burn the paper more energy is released than the small
amount needed to lit it, so it’s still an exothermic reaction
Reaction profiles -> graphs that can be used to show how the energy in a reaction changes as
the reaction proceeds
In the first graph an exothermic reaction is shown
The reactants contain more energy than the reaction products so energy is released
The energy effect is negative
In the second graph an endothermic reaction is shown
The reactions contain less energy than the reaction products so energy is taken up
The energy effect is positive
The activation energy can be found by looking at the difference between the energy of the
reactants and the highest point in the graph (activated state)
2.1 Lavoisier’s law
The mass before and after a reaction stays the same
