10 - Enthalpy Flashcards
Define enthalpy
How much heat is stored in a system
symbol: H unit : kJ / mol or kJ mol^-1
Define system
The atoms, ions and particles in the chemical involved in the reaction
Define exothermic and endothermic
An exothermic reaction is one which releases heat energy from the system into the surroundings
An endothermic reaction is one which takes in heat energy from the surroundings into the system
Give an example of an exothermic reaction
Combustion reactions
Give an example of an endothermic reaction
Thermal decomposition of calcium carbonate
What is the difference in enthalpy change for exothermic or endothermic reactions?
Exothermic reactions give a negative enthalpy change
Endothermic reactions give a positive enthalpy change
What is the enthalpy change of the thermal decomposition of calcium carbonate?
+ 178 kJ mol^-1
Define activation energy
Minimum energy required to start a reaction by breaking bonds
symbol: E~a
What equation do we use to find out how much energy is released by / stored in a given amount of substance?
E = nH
Energy = moles * enthalpy
What 3 things is enthalpy affected by?
Temperature
Pressure
Concentration
What are the standard conditions for measuring enthalpy change?
298K temperature
101kPa pressure
1 mol dm^-3 concentration
Define standard enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with substances in standard states
How do you calculate the equations of standard formation?
You write the product (like H~2 0) on the right of the reaction arrow
You then decide how many moles of the elements are needed to make that 1 mole of product
What is the standard formation equation of ammonia
0.5 N~2 (g) \+ 1.5 H~2 (g) --> NH~3 (g)
You would then probably have to write the standard enthalpy change of formation afterwards
What is the enthalpy of formation of just an element?
Always 0 kJ/mol for just elements
What is the first way of measuring enthalpy change?
Transferring heat to and from water or surrounding air
You then measure the temperature of the water / air
Define specific heat capacity
The amount of energy needed to heat 1g of a substance by 1K
What is the specific heat capacity of water?
4.18 J g^-1 K^-1
How do you calculate the heat energy a fuel has released?
Q = mc delta T
Heat energy = mass of surroundings specific heat capacity temperature change in surroundings
What is the density of water / a solution?
1 g cm^-3
Remember that it is grams and centimetres
How do you calculate enthalpy change from heat energy?
Delta H = Q / n
Enthalpy change = heat energy released / number of moles which released that energy
What is direct determination?
When enthalpy change is calculated from one experiment
What are the 4 steps for calculating enthalpy change from experiment results?
Calculate Q using Q = mc Delta T
Calculate n of the surroundings
Do Delta H = Q / n
Make enthalpy change +ve / -ve if reaction is endothermic or exothermic respectively
Define standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with substances all in standard states
Define standard enthalpy change of neutralisation
Enthalpy change that accompanies the reaction of an acid and a base to from 1 mole of water under standard conditions with substances all in standard states
What is the other type of standard enthalpy change?
Standard enthalpy change of reaction
When is standard enthalpy change of reaction used?
When the reaction isn’t any of the others (formation/combustion)
What are the 3 reasons that you can’t measure enthalpy change for a reaction?
There are too many possible products that will be made (we can’t choose a select one)
Rate of reaction is too slow
Activation energy is too high
When do we use Hess’s law?
when dealing with reactions where we can’t measure enthalpy change
State Hess’s law
If a reaction can take place by 2 routes and the start and end conditions are the same, the enthalpy change for each route is the same
Describe what Hess’s law means
If we can do multiple reactions to get the same result as our original reaction (which we can’t do)
We will get the same overall enthalpy change over the multiple alternative reactions than we would doing the original reaction
Describe enthalpy cycles
Reaction A goes straight from reactants to products but we can’t use this reaction
Reaction B and then reaction C get us from the reactants to the same products and we can do these reactions respectively
A = B + C is a rule we need to know
What happens when you go in the opposite direction of an arrow on an enthalpy cycle?
You change the sign of the enthalpy change of that reaction
For combustion reactions, where do the enthalpy cycle arrows point?
How do you remember this?
Both B and C arrows point downwards to the combustion products which are H~2 O and CO~2
combustion=point down
For formation reactions, where do the enthalpy cycle arrows point?
How do you remember this?
Both B and C point upwards
formation=point up
What do you do for a Hess law enthalpy change of reaction?
You are given a table of reactions and their enthalpy changes
You have to assign the reactions to the letters like A, B etc
What happens in terms of energy when bonds are broken and when bonds are formed?
How does this relate to exothermic / endothermic?
Forming bonds releases energy
Breaking bonds requires energy
Making is exothermic and breaking is endothermic
Define average bond enthalpy
Energy required to break one mole of bonds in a gaseous molecule
What is the formula for enthalpy change using bond enthalpy?
Enthalpy change =
Sum of enthalpy changes of bonds broken
-
Sum of enthalpy change of bonds formed
Why are bond enthalpies always positive?
They are the enthalpy change when bonds are formed
Forming bonds releases energy
Describe the practical experiment for determination of enthalpy change of combustion
Measure a known volume of water into a beaker
Weigh the burner containing fuel
Take initial temp of water
Burner is lit and water heated until temperature has risen by a reasonable amount
Final temperature is taken
The burner is weighed to find mass of fuel burnt
When assigning reactions to letters in an enthalpy change of reaction Hess question
What do you need to do in order to see which reaction you are looking for?
You have to cancel out any substances which are in reactants and products in order to find the reaction in the table
BUT DO NOT CROSS THEM OUT because you will need them later
Define standard enthalpy change of reaction
Enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation under standard conditions with all substances in standard states
