AS - Unit 2 - Energy Flashcards
What is enthalpy?
The heat content that is stored in a chemical system
What does exothermic mean?
It refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings
What determines enthalpy change?
When the products of a reaction have more or less chemical energy than the reactants, resulting in a change of enthalpy
What is enthalpy change?
The heat exchange with the surroundings during a chemical reaction, at constant pressure
The difference between the enthalpy of the products and the enthalpy of the reactants
Give three things with occur in an exothermic reaction
The enthalpy of the products is smaller than the enthalpy of the reactants
There is heat loss from the chemical system to the surroundings
delta H has a negative sign because heat has been lost by the chemical system
What does endothermic mean?
It refers to a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings
Give three things that happen in an endothermic reaction
The enthalpy of the products is greater than the enthalpy of the reactants
There is a heat gain to the chemical system from the surroundings
delta H has a positive sign because heat has been gained by the chemical system
Is combustion endo or exothermic?
It is exothermic, and the energy given out is often used for everyday purposes such as powering a car by combusting fuels
Give two lifestyle changes that people can make to cut down on the production of CO2
Walk or cycle distances that don’t necessarily need a car
Avoid long haul flights to holidays abroad
What reaction is possibly the most important exothermic reaction for life?
Respiration
Give an example of an endothermic reaction
Thermal decomposition of limestone
What is an enthalpy profile diagram?
A diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products
In exothermic reactions, which way does the enthalpy profile diagram begin and end?
The reactants begin at the top, the arrow points down and the products end at the bottom
In endothermic reactions, which way does the enthalpy profile diagram begin and end?
The reactants begin at the bottom, the arrow points up and the products end at the top
What is activation energy?
The minimum energy required to start a reaction by the breaking of bonds
Why is activation energy important in exothermic reactions?
Natural gas needs a spark to overcome its activation energy, once the energy barrier has been overcome the net output of energy provides more energy that can be used to overcome the activation energy for the reaction to continue.
Without this barrier exothermic reactions would take place spontaneously and fuels would not exist as they would spontaneously combust
What are standard conditions?
A pressure of 100kPa
Temperature usually of 298K (25 degrees C)
Concentration of 1.0 mol dm-3
What is a standard state?
The physical state of a substance under the standard conditions of 100kPa and 298K
What is the standard enthalpy change of a reaction?
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
What is the standard enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states
What is the standard enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states and under standard conditions
What is the value for the enthalpy change of formation of an ELEMENT?
0kJ mol-1
What is specific heat capacity?
The energy required to raise the temperature of 1g of a substance by 1 degree C
Heat loss in a chemical system = ???
Heat gain by surroundings
Heat gain in a chemical system = ???
Heat loss by surroundings
To determine the heat exchange during a reaction what do you need to know?
m - the mass of the surroundings involved in the heat exchange
c - the specific heat capacity of the surroundings
delta T - the temperature change of the surroundings
delta T = T(final) - T(initial)
How do you calculate the value of heat exchange to the surroundings (Q)?
Q = mc delta T
What is combustion?
A reaction involving oxygen to form oxides
How do you calculate the enthalpy change of combustion experimentally?
Burn a known mass of a substance in air
Heat a known mass of water
Measure the temperature change in water
e.g. Measure a volume of water into the beaker
1cm3 of water weighs 1g so the mass of water is easy to deduce
The burner containing the fuel is weighed
the initial temperature of the water is taken
the burner is lit and the water heated until the temperature has risen by a reasonable amount
max temperature is taken and the temp change (delta T) can be determined
Flame is extinguished and the burner re-wighed to find the mass of the fuel that has been burnt
Give two reasons for experimental and theoretical enthalpy values being different when combusting
May have been incomplete combustion
May have been heat loss to the surroundings
What is bond enthalpy?
The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species
What is average bond enthalpy?
The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species
What do bond enthalpies tell you?
The amount of energy needed to break a certain bond