Chapter 9 - Enthalpy Flashcards
What is enthalpy?
A measure of the heat energy in a chemical system
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure
What is standard pressure?
100 kPa
What is standard temperature?
298 K (25c)
How to find ΔH?
H (products) - H (reactants)
What is the law of the conservation of energy?
It states that energy can’t be created or destroyed in a chemical system
What is an exothermic reaction?
Heat is transferred to the surroundings - ΔH is negative
In exothermic reactions, what happens to the temperature of the chemicals?
It increases
Give two examples of exothermic reactions
Combustion
Oxidation, e.g. of carbohydrates
What is an endothermic reaction?
Heat is absorbed from the surroundings - ΔH is positive
In an endothermic reaction, what happens to the temperature of the chemicals?
It decreases
Give two examples of endothermic reactions
Thermal decomposition
Photosynthesis
What is activation energy?
The minimum amount of energy needed to break the bonds in the reactants and start a chemical reaction
What is standard concentration?
1 mol/dm3
What is the standard state?
The state an element is in under standard conditions
What is the standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions, with all substances in their standard states
What is the standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is the standard enthalpy change for the formation of an element?
0 kJ
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, with all substances in standard states
What are the combustion products?
CO2 and H2O
What is the standard enthalpy change of neutralisation/
The enthalpy change that occurs when an acid and a base react together, under standard conditions in their standard states, to produce one mole of water
What is important to remember about Kelvin and Celcius?
A change in temperature will have the same value in kelvin or celcius
What type of process is bond breaking?
Endothermic
What type of process is bond making?
Exothermic
What is bond dissociation enthalpy?
The amount of energy needed per mole of bonds to break
What is the equation for enthalpy changes?
q = m x c x ΔT
What does ‘M’ stand for?
The mass of the surroundings (g)
What does ‘C’ stand for?
The specific heat capacity of the substance (J g-1 K-1)
What is the specific heat capacity of water?
4.18
What is specific heat capacity?
The energy required to raise the temperature of 1g of a substance by 1K
What does ‘ΔT’ stand for?
Change in temperature (K)
What are the units for ‘q’?
kJ mol-1
After using q=mcΔt, what are the units?
Joules, then divide by 1,000 to convert to kJ
What is the definition for average bond enthalpy?
The energy needed to break one mole of bonds in the gas phase averaged over many different compounds
How to measure the enthalpy change of combustion of a flammable liquid?
1) Pour 100 ml of water into a beaker, recording the initial temperature to the nearest 0.5 c
2) Add your liquid to a spirit burner and record the mass
3) Place the burner under the beaker and light
4) Stir the water frequently
5) After three minutes, extinguish the flame and record the new temperature
6) Reweigh the spirit burner
7) You can now use this data to perform calculations
Reasons why your value of ΔH may be different to the textbook value
Heat loss to the surroundings
Incomplete combustion of the flammable liquid
In the time after you have extinguished the burner but not applied the cap, some of the liquid may evaporate
Non-standard conditions
How to calculate the enthalpy change of a reaction
Pippete 25 cm3 of copper sulfate into a polystyrene cup.
Weigh out an excess of your zinc powder
Start a stop clock and take the temperature of the solution every 30 seconds until it stays constant, then add the powder
Record the temperature every 30 seconds and plot this data on a graph
Is energy required to break bonds or make bonds?
Break bonds - it is endothermic
How are enthalpy changes calculated using bond enthalpies?
Enthalpy change = bond enthalpy of reactants - bond enthalpies in products
Is energy released when making or breaking bonds?
Making bonds - it is exothermic
What is Hess’ Law?
The total enthalpy change of a reaction is always the same, no matter what route is taken
What is the standard enthalpy change of formation for all elements?
0
Where do the two routes mentioned in Hess’ Law start?
Where the lines diverge
Where do the two routes mentioned in Hess’ Law end?
Where the lines converge
How to use standard enthalpy of formation to work out enthalpy changes?
ΔHf products - ΔHf reactants
How to use standard enthalpy of combustion to work out enthalpy changes?
ΔcH reactants - ΔcH products
