CHEMISTRY- ENERGY CHANGES IN INDUSTRY Flashcards
What happens in exothermic reactions
Energy is released from reactants to surroundings
Temp of surroundings increases
What is the symbol for enthalpy change
Delta H
What happens in endothermic reactions
Energy takes from surroundings to reactions
Temp of surroundings decreases
Name diatomic elements
O2
H2
I2
Br2
Cl2
F2
N2
What is the standard enthalpy of combustion
Enthalpy change when 1 mole of a substance and it’s standard state burns completely in oxygen under standard conditions of 298k and one bar pressure.
Give an example of standard enthalpy of formation
H2 + 1/2 O —> H2O
What is the standard formation of water
L
What is the standard enthalpy of formation in their standard states
Zero
Why is the enthalpy of formation of Na zero
Na is an element in its standard state
State why the enthalpy of formation of liquid Na is not zero
Liquid is not sodium’s standard state
What is the standard enthalpy of formation
Enthalpy change when one mole of a substance is formed from its element under standard conditions of 298k and one bar pressure
What do standard enthalpies of combustion have
Almost always have negative values
Is burning a substance in oxygen exothermic or endothermic
Exothermic
What happens to substance that cannot be combusted
They have zero enthalpy of combustion
What types of substances cannot be combusted
Water
Carbon dioxide
And other oxides
Why may the enthalpy of combustion of a compound be difficult to measure
Incomplete combustion may occur
What do ions bond with
Water molecules
What is the hydration of ions
The ions bond with water molecules
Positive ions will be attracted to the slightly negative oxygen in water molecules
Whilst negative ions will be attracted to the slightly positive hydrogens in water molecules
What is Hess law
Reaction may have a very high activation energy
Reaction rate may be too slow
Possibility of competing reactions
What is hess law used for
To calculate an enthalpy change indirectly
name the type of reaction taking place when pentane burns in oxygen
combustion
explain how exothermic processes affect their surroundings
releases heat from the system to the surroundings and the tempreture will increase
state what is meant by the term standard enthalpy change of formation
when 1 mole of compound is formed from its elements under standard condition
state what is meant by the term enthalpy change of combustion
when 1 mole of a substance is combusted completely in oxygen
give one reason why the standard molar enthalpy change of formation of ethanol can not be measured directly
there are potentially other products and activation energy is too high
What is the definition of an enthalpy change?
The measure of heat given out or taken in during a process
Why is enthalpy change used instead of energy?
Because we cannot easily measure energy changes
Why is enthalpy easy to use?
They are measured under constant pressure
That atmosphere is at a constant pressure, so we require no extra equipment
What type of enthalpy change does exothermic reaction have?
Negative
What type of enthalpy change does endothermic reaction have?
Positive
What is combustion?
An exothermic process that gives out heat
What are the standard conditions for enthalpy change
Pressure of one bar
Temperature of 298 Kelvin
Substances in its most stable state under the above conditions
What type of process is bond breaking?
Endothermic
What type of process is bond making?
Exothermic
What is the process used to extract aluminium from aluminium oxide?
Electrolysis
Explain why the structure of aluminium oxide makes the compound resist to high temperature
Because it has a strong intermolecular forces
Has high, melting point
Contains strong electrostatic forces between the ions
Has a giant ionic structure
What is the temperature condition for Kelvin for standard conditions
298
What is the standard condition for pressure?
100 Kpa
Explain why the standard enthalpy change of formation for hydrogen gas is 0 kgMol-1
Hydrogen gas is an element on standard conditions so no change occurs
Name an organic compound that can be dissolved in water to manufacture hydrogen by electrolysis
Sodium chloride
State one use of short chain alkanes
Combustion
Explain how vanadium oxide acts as a catalyst
The vanadium oxide is regenerated by the end of the reaction
Vanadium oxide is oxidised by oxygen
