3.1.4 Energetics Flashcards
Where is energy transferred in chemical reactions?
Energy is transferred between the reactants, the products and the surroundings.
Does bond breaking require energy or release it?
Requires energy
Does bond forming require energy or release it?
Releases energy
Where is energy released in reactions?
To the surroundings
What is an exothermic reaction?
An exothermic reaction occurs when more energy is released to form new bonds in the products, than to break bonds in the reactants.
Is the overall enthalpy change of an exothermic reaction negative or positive?
Negative
What is an endothermic reaction?
An endothermic reaction occurs when more energy is required to break bonds in the reactants than is released when forming bonds in the products.
Is the overall enthalpy change of an endothermic reaction negative or positive?
Positive
What are the axis of an energy profile diagram?
X- Progress of reaction
Y- Energy
In an endothermic reaction, are the products or reactants more unstable?
Products
Describe the features of an energy profile diagram for an exothermic reaction?
The reactants have more energy than the products
The enthalpy change is negative
Describe the features of the energy profile diagram for an endothermic reaction
The products have more energy than the reactants.
The overall enthalpy change is positive.
In an exothermic reaction, are the products or reactants more unstable?
Reactants
What is the temperature change of an exothermic reaction?
Temperature increases, as heat is transferred to the surroundings.
What is the temperature change of an endothermic reaction?
Temperature decreases, as heat is taken in from the surroundings.
Does the internal energy of the system increase or decrease in an exothermic reaction?
Decreases
Does the internal energy of the system increase or decrease in an endothermic reaction?
Increases
What is enthalpy?
The enthalpy of a system is the total energy content of a system.
How does enthalpy relate to stability?
The higher the enthalpy of a system, the more unstable it is.
What is enthalpy change?
The heat energy change at constant pressure
(The overall energy change in a chemical reaction)
What is standard enthalpy change measured at?
In standard conditions:
100kPa
298K or 25*c
1.0M
And all substances are in their standard states.
What is a standard state?
The physical state of a substance under standard conditions.
What are three types of enthalpy than can be measured?
Enthalpy of formation
Enthalpy of combustion
Standard enthalpy
Define enthalpy of combustion?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.
What must there be one mole of in enthalpy of combustion equations?
The fuel/ substance being burned
How many moles of oxygen are there in enthalpy of combustion equations?
However much oxygen it takes for one mole of the fuel to be burned completely.
Define enthalpy of formation?
The enthalpy change when one mole of a substance is formed from its elements under standard conditions.
What must there be one mole of in enthalpy of formation equations?
The substance being formed
Is combustion endothermic or exothermic?
Exothermic
Is formation exothermic or endothermic?
Either
How many moles of elements are needed in enthalpy of formation equations?
However many moles of elements it takes to form one mole of the product substance.
What is the enthalpy change of formation of an element?
0
What is the unit for enthalpy change?
KJmol^-1
What is temperature?
Describes how hot something is and relates to the average kinetic energy of the particles in a system. If KE increases, the temperature increases and particles move faster.
What is heat and what is it measured in?
A form of energy, measured in joules.
What does the amount of energy in a system depend on?
Temperature
Mass
Material
What is calorimetry?
Used to measure enthalpy changes in chemical reactions, by determining the heat absorbed, evolved or transferred.
What is the equation for heat energy transferred?
Q = M C ^T
What should be controlled in calorimetry?
Volume of water
Height above flame
Time taken
How can calorimetry experiments be improved?
Digital thermometer to improve resolution
Lid on the can to reduce heat loss to the surroundings
What are problems with calorimetry?
Low resolution of thermometer
Heat loss to the surroundings
In complete combustion may occur
The alcohol in the spirit burner may evaporate
What is the equation for enthalpy change?
Q / moles
Describe the steps to find the enthalpy change of combustion?
Find heat energy transferred
Convert joules to KJ
World out the moles of fuel burned
Find enthalpy change q/moles
Change sign to a minus for an exothermic reaction
Describe how to calculate the enthalpy change of reaction?
Find the heat energy transferred
Convert joules to KJ
Write the symbol equation for the reaction
Calculate the moles of both reactants to find the limiting reagent
Calculate the enthalpy change using the moles of the limiting reagent
Determine if it is exothermic or endothermic, and change the sign if it is exothermic.
Why is it not always possible to measure the enthalpy change of a reaction directly?
There may be a high activation energy
There may be a slow rate of reaction
There may be more than one reaction taking place so multiple side products are formed
State Hess’ law
The total enthalpy change for a chemical reaction is independent of the route taken, provided that the initial and final conditions are the same.
What are the possible reaction routes for a reaction?
Reactants -> products
Reactants -> elements ->products
Reactants -> combustion products
Reactants -> products -> combustion products
What are the possible routes for a formation reaction?
Elements- Reactants - products
Elements - products
Can be formed from its elements directly or indirectly
What is the equation for using formation values to calculate enthalpy change of reaction in terms of ABC?
A+B=C
What is the equation for using combustion values to calculate enthalpy change of reaction in terms of ABC?
B=A+C
What is the equation for enthalpy change of reaction using formation values?
Enthalpy change of products - enthalpy change of reactants
What is the equation for enthalpy change of reaction using combustion values?
Enthalpy change of reactants - enthalpy change of products
What is bond dissociation enthalpy?
The amount of energy required to break one mole of a specific covalent bond in the gaseous state.
What is the mean bond enthalpy?
The average bond dissociation enthalpy calculated from a range of compounds for a specific covalent bond.
Is bond breaking exothermic or endothermic and why?
It requires energy, so it is an endothermic process.
Is the enthalpy change for bond breaking positive or negative?
Positive
Is the enthalpy change for bond forming positive or negative?
Negative
Is bond forming an exothermic or endothermic process and why?
Exothermic, as it releases energy
Why are calculations using bond enthalpies different to the literature value?
Because the literature value is a mean bond enthalpy from a range of compounds, not just the one in the question.
When is a reaction endothermic (in terms of bonds)?
When the energy required to break the bonds in the reactants is greater than the energy released when forming bonds in the products.
When is a reaction exothermic (in terms of bonds)?
When the energy required to break the bonds in the reactants is less than the energy released when forming bonds in the products.
Equation to calculate enthalpy change using bond enthalpies?
Energy required to break bonds - energy released when forming bonds
