đŸ”´đŸ”¸2.1 -thermochemistry Flashcards
Define an exothermic reaction
A reaction that releases energy to the surroundings, there is a temperature rise and đŸ”¼H is negative
Define an endothermic reaction
A reaction that takes in energy from the surroundings, there is a decrease in temperature and đŸ”¼H is positive
What is the kinetic energy of matter?
The energy of motion at a molecular level
What is the cause potential energy of matter?
The positions of atoms relative to one another
What requires a change in potential energy?
Bond-breaking and bond-making
What is the internal energy of a system?
The sum of the kinetic energy of all of the particles in a system and their potential energy
Give 3 examples of exothermic reactions
- reacting acids with metals
- thermite reaction
- combustion
- respiration
Give 3 examples of endothermic reactions
- melting ice
- photosynthesis
- thermal decomposition of group 2 carbonates
Define enthalpy (H)
The heat content of a system at constant pressure
Define enthalpy change (đŸ”¼H)
The heat added to a system at a constant pressure
What is the standard pressure?
101KPA
Name an equation to calculate enthalpy change
Enthalpy of products - enthalpy of reactants
Which line is higher in an exothermic enthalpy profile, The products or reactants.
The reactants line is higher than the products therefore (đŸ”¼H) is negative
Which line is higher in an endothermic enthalpy profile, The products or reactants.
The products line is higher than the reactants therefore (đŸ”¼H) is positive
Give an example of an exothermic reaction
H2 (g) + 1/2O2 (g) —> H20 (g)
đŸ”¼H = -242
Define the principle of the conservation of energy
States that energy can’t be created or destroyed only changed from one form to another
Name the standard conditions
- all substances in standard states
- temperature of 298K (25 degrees)
- A pressure of 1 atm (101,000 Pa)
What Is enthalpy change of formation?
Enthalpy change when one mole of a substance is formed from its constituent elements
What is the standard enthalpy change of formation of carbon dioxide?
C(s) + O2 (g) —> CO2 (g)
What is the standard enthalpy of formation of carbon monoxide
C(s) + 1/2O2 (g) —-> CO (g)
Name the essential point to consider when looking at enthalpy change equations
That enthalpy change occurs at 1 mole therefore the molecule in the substance must remain at 1 mole and the other molecules must be balanced around this.
What is the enthalpy change of all elements in their standard state?
Why?
0 KJmol -1
Because there is no chemical change occurring
What is the standard enthalpy change of combustion
The enthalpy change when one mole of a substance is completely combusted in oxygen
What is the standard enthalpy change of combustion of Hydrogen
H2 (g) + 1/2 O2 (g) —> H2O (l)
What is the standard enthalpy change of combustion of methane
CH4 (g) + 2O2 (g) —> CO2 (g) + 2H2O (l)
What is the enthalpy change of reaction
Enthalpy change In a reaction between the number of moles of reactants shown in the equation for the reaction
E.g double the moles = double the enthalpy
How do you calculate the enthalpy change of formation?
Enthalpy change of reaction = sum of the enthalpy change of formation of the reactants - the sum of the enthalpy change of formation of the products
What is Hess’s law?
Hess’s law states that the total enthalpy change for a reaction is Independent of the route taken from the reactants to the products
How do you demonstrate Hess’ law?
Enthalpy change cycle diagrams
What do you break the bottom of the enthalpy cycle into during a enthalpy change of formation
Carbon dioxide + water
NEED TO PRACTICE QUESTIONS
What do you break the bottom of the enthalpy cycle into during a enthalpy change of reaction
Into its elements
NEED TO PRACTISE QUESTIONS
Define bond enthalpy
The enthalpy required to break a covalent X-Y bond into X atoms all in the gas phase
Why is the value of bond enthalpies always positive
Because breaking a bond requires energy
Define average bond enthalpy
The average value of the enthalpy requires to break a given type of covalent bond in the molecules of a gaseous species
Why is the average bond enthalpy calculated
Because bond enthalpy can vary depending on the structure of the rest molecule, e.g. C-C in ethane (C2H6) has a different value to C-C in pentane (C5H12)
What is the specific heat capacity?
Energy requires to raise the temperature of 1g if a substance by 1k
What is the specific heat capacity of water?
4.18 jg-1K-1
What letter is the amount of heat transferred represented by?
q
Name the equation showing the relationship between temperature change and amount of heat transferred
q=mcđŸ”¼T
Amount of heat transferred = mass of solution x specific heat capacity x difference in temperature
Name the equation to show the enthalpy change of a reaction per mole
đŸ”¼H = -q/n
Enthalpy change = - value of heat transferred/ moles reacted
(NEED TO DO EXAMPLE QUESTIONS)
What do we always assume the density of a solution is?
1.00gcm-3 (same as water)
What are the 3 steps to calculate the molar enthalpy change in displacement, neutralisation, dissolving a solid to form an aqueous solution and combustion reactions ?
Step 1: calculate the value of heat transferred in experiment using q=mcđŸ”¼T
Step 2: calculate the amount in moles of the reactants using n = m/Mr
Step 3: Calculate the molar enthalpy change using đŸ”¼H = -q/n
