Chp 6 - Chemical Energetics Flashcards
What is an Exothermic reaction
Give an example
A reaction in which energy is given out or exits out to the surroundings (temperature of environment increases)
Examples:
Combustion of fuels
Reaction of acids and metals
Neutralisation reactions
Ask teacher why neutralisation reactions
What is an endothermic reaction
Give an example
A reaction in which energy is taken in or enters in from surroundings (temperature of environment decreases)
The energy change is positive
Examples:
Thermal decomposition of carbonates
Electrolysis
First stages of photosynthesis
How do you decide whether a reaction is Exothermic or endothermic
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed
How do you know if something is endothermic or not
If more energy is absorbed than is released, this reaction is endothermic
More energy is required to break the bonds than that gained from making the new bonds
The change in energy is positive since the products have more energy than the reactants
The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction
Therefore an endothermic reaction has a positive ΔH value
What is enthalpy
This is the total amount of heat energy that a chemical system contains and considers both reactants and products.
How do you know if a reaction is Exothermic or not
If more energy is released than is absorbed, then the reaction is exothermic
More energy is released when new bonds are formed than energy required to break the bonds in the reactants
The change in energy is negative since the products have less energy than the reactants
Therefore an exothermic reaction has a negative ΔH value
What are energy level diagrams
These are graphical representations of the heat changes in chemical reactions
On energy level diagrams where is the enthalpy of reactants and products in
The enthalpy of the reactants and products is displayed on the y-axis
On energy level diagrams where is the reaction pathway shown
The reaction pathway is shown on the x-axis
On a energy level diagram what arrow means an Exothermic reaction
A downwards pointing arrow indicates that a reaction is Exothermic
On a energy level diagram what arrow means an endothermic reaction
An upwards pointing down arrow represents an endothermic reaction
What is an Exothermic reaction Pt 2
During an exothermic reaction, energy is given out
This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative
This is represented on the energy-level diagram above with a downwards arrow as the energy of the products is lower than the reactants
What is an endothermic reaction Pt 2
During an endothermic reaction, energy is absorbed
This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive
This is represented on the energy-level diagram above with an upwards arrow as the energy of the products is higher than the reactants
What is bond energy
This is the energy required to break one mole of covalent bonds between two particular atoms and is measured in kJ per mole.
Or it’s also the amount of energy given out when the bond is formed
What is the energy of reaction calculations
This is the energy that can be used to calculate how much heat would be released or absorbed in a reaction
To do this it is necessary to know the bonds present in both the reactants and products
What is the method of finding out how much heat has been released or absorbed in a reaction.
Equation
Energy change = Energy taken in - Energy given out
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
Add together the bond energies for all the bonds in the products – this is the ‘energy out’
Calculate the energy change: Energy change = energy in – energy out
How do you find out how much how much heat can be release or absorbed in the reaction which is an exothermic reaction
Hydrogen and chlorine react to form hydrogen chloride gas:
H2 + Cl2 → 2HCl
H-H = bond energy = 436 Cl-Cl= bond energy = 243 H-Cl= bond energy = 432
Energy in = 436 + 243 = 679 KJ / Mole
Energy out = 2 x 432 (As there are to HCLs)= 864 KJ / Mole
Energy change = 679 – 864 = -185 KJ / Mole
*The energy change is negative, showing that energy is released to the surroundings so it is an exothermic reaction.
How do you find out how much how much heat can be release or absorbed in the reaction which is an endothermic reaction.
Hydrogen Bromide decomposes to form Hydrogen and Bromine:
2 x ( H - Br ) → H - H + Br - Br
H-Br = bond energy = 366 H-H = bond energy = 436 Br-Br = bond energy = 193
Energy in = 2 x 366 = 732 KJ / Mole
Energy out = 436 + 193 = 629 KJ / Mole
Energy change = 732 – 629 = +103 KJ / Mole
*The energy change is positive, showing that energy is taken in from the surroundings, so is an endothermic reaction.
What H-H + Cl + Cl become
H-H + Cl-Cl —> H-CL + H-CL
What is fuel
A fuel is a substance which releases energy when burned
What happens when a fuel is a hydrocarbon
When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions
Propane for example undergoes combustion according to the following equation:
C3H8 + 5O2 → 3CO2 + 4H2O ΔH = -2219 kJ/mol (Exothermic reaction)
What does the efficiency of fuel refer to
The efficiency of a fuel refers to how much energy is released per unit amount
We can measure the efficiency of fuels by calorimetry
What is a calorimetry
It’s the measurement of heat transfers in chemical reactions.
How do you calculate a substances heat combustion through calimentry
A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
