Enthalpy Changes Flashcards
Define enthalpy change & give its units
Enthalpy change (delta H) is the heat energy transferred in a reaction at constant pressure
the units are kJmol^-1
How do you show measurements of enthalpy change were made under standard conditions?
Delta H^ (circle with a horizontal line through)
What does it mean when an element is in its standard state?
It is in the state it’s at when the pressure is 100kPa
What is change in h?
Change in enthalpy/ energy
The energy transferred to or from the surroundings when the reaction occurs
Is the enthalpy change positive or negative for an exothermic reaction?
Negative
Is the enthalpy change positive or negative for an endothermic reaction?
Positive
What happens to the reactants, products and surroundings in an exothemic reaction?
Reactants lose energy
Products are lover in energy (then the reactants)
Energy is given out to the surroundings so they heat up
What happens to the reactants, products and surroundings in an endothermic reaction?
Reactants gain energy
Products are higher in energy (than the reactants)
Energy is taken in from the surroundingsso they cod down (may not notice if it’s being heated eg with a Bunsen)
What are the units of enthalpy change?
kJmol^-1
What are the units of temperature?
Kelvin
How do you go from degrees Celsius to kelvins?
Add 273
What are the standard conditions?
P = 1atm (=1.05 x 10^5 Nm^-2)
T=25°C = 298K
Any solution=1moldm^-3 concentration
All substances are in standard states the state of the substance at standard conditions)
What are the two common types of experiments used to measure change in temperature?
- Burning a fuel
- Doing a reaction in a polystyrene cup
What’s the most accurate way to measure change in temp by burning a fuel?
By using a bomb calorimeter
Why is change in temp measured instead of directly measuring enthalpy change?
Enthalpy change can’t be measured directly, so temp change must be measured, then q = m c (change) T must be used.
Which measurements must be made during the burning a fuel practical?
- Mass of water being heated (in practice its usually volume - for water, 1cm^3 = 1g)
- starting and final temp of water
- Starting and final mass of spirit burner
Draw a diagram of the apparatus needed for the burning fuel practical (with labels)
Labels:
- drought shields
- copper calorimeter
- (known volume of) water
-thermometer
- spirit burner(containing liquid fuel)
How can you calculate enthalpy change from the burning a fuel practical results?
Use q=m x c x change temp
Then / by 1000
Then / by number of moles that reacted (number of moles of fuel that burned)
Think about the sign - burning fuel is an exothermic reaction so put a - in front of the answer
What does each component of q=m x c x change T mean?
Q = energy transferred
M = mass of water being burned (not mass of fuel)
C = specific heat capacity of water (4.18)
Change T = temperature change of the water
Why doesn’t the burning fuel experiment give a perfect answer?
(What are the sources of error in the experiment)
- Heat loss to the surroundings
- Incomplete combustion
- heat loss to the calorimeter
(Those are the 3 most important ones, but there’s also:) - same fuel may evaporate
- enthalpy change for standard states means water should be produced as liquid, but in this expt water is produced as a gas
What’s the 6-step method for doing an enthalpy change experiment in a polystyrene cup with solutions?
- Use known volumes (measure with pipettes) of solutions of known concentrations
- take starting temperature (use an average for 2 separate solutions
- then mix them in a polystyrene cup (to minimise heat loss ( exo) or gain (endo))
- stir the mixture & record the max temperature reached (exo) or min ( endo)
- then use q = mc x change t where m= total mass of the solution after mixing
- then /by 1000 and / again by number of moles
What are some common exothermic reactions?
Oxidation (combustion of fuel & oxidation of carbohydrates in respiration)
What are some common endothermic reactions?
Thermal decomposition & photosynthesis
Is bond breaking endo or exo?
Endothermic
Is bond making endo or exo?
Exothermic
Define bond enthalpy
The energy needed to break a bond or the energy given out when a bond forms (stronger bonds have higher bond enthalpies)
How do you find enthalpy change of reaction using avrg bond enthalpies?
Enthalpy change of reaction = total energy absorbed to break bonds - total energy released in making bonds
Define average bond enthalpy?
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
What is the bond length?
The distance between the two nuclei in a covalent bond
What’s the relationship between attraction, bond enthalpy and bond length?
The stronger the attraction, the higher the bond enthalpy and the shorter the bond length
How is the enthalpy of a substance related to its stability?
The less enthalpy a substance has, the more stable it is
What is activation energy?
The minimum amount of energy needed to begin breaking reactant bonds & start a chemical reaction
What is different about finding the enthalpy change for reactions involving a solid than a reaction with 2 solutions?
With 2 solutions the reaction is almost instantaneous, so all the heat is given out at once. But using a solid in a reaction is slower so heat is being lost from the first bit before the last bit has reacted, meaning maximum temperature recorded is not a true value (would be too small)
How do you find a more correct value for temperature change when using a solid in the reaction?
Take the temperature several times over several minutes and plot a temperature against time graphi then find a more accurate value for temp change using extrapolation
Define standard enthalpy change for a reaction
The enthalpy change when molar quantities of the reactants, as stated in the equation, react together under standard conditions, with all substances in their standard states
Define standard enthalpy change of combustion
The enthalpy change when one mole of the substance burns completely in oxygen under standard conditions, with all substances in their standard states
Define standard enthalpy change of formation
The enthalpy change when one mole of the substance is made from its elements in their standard states under standard conditions
What will the standard enthalpy change of formation be for an element?
Zero
Define the standard enthalpy change of neutralisation
The enthalpy change when one mole of H+ ions (from an aqueous acid) react with one mole of OH- ions (from an aqueous alkali) to form one mole of water under standard conditions and in solutions containing 1 moldm^-3
What are the steps for enthalpy change calculations?
- use q=mc(change)T
- /1000
- /number of moles reacted
- think about the sign
What are bond enthalpy values?
The energy needed to break a particular type of a covalent bond, for one mole of bonds in the gas state, is called the bond enthalpy & is always positive
When writing equations for bond enthalpy, why are the products the separate atoms and not diatomic molecules?
The only thing happening in the reaction is the bond breaking therefore the products ave the separate atoms
What are the 3 main uses of bond enthalpy values?
- To compare bond strengths
- To explain where the enthalpy change of a reaction comes from
- Using bond enthalpy values to calculate a new value for enthalpy change of a reaction
Are double (& triple) bonds longer or shorter than single bonds?
Shorter because double bonds are stronger than single bonds
Which state are bond enthalpy values measured in?
bond enthalpy values ave always measured for breaking bonds when the compound is gaseous (fair test)
What are the steps for using bond enthalpy values to calculate a value for enthalpy change of a reaction?
- Write a balanced equation & draw the full structural formula of each substance so yon can see & count all the bonds involved
- Imagine breaking all the bonds in the reactants. List the number of each type of bond tube broken & total up the bond enthalpy values ( bond breaking is endothermic so give the number a positive sign)
- Imagine making all the bonds in the products. List the number of each type of bond being made & total up all the bond enthalpy values (bond making is exothermic, so give this total a negative sign)
- Add the 2 numbers together to get the overall enthalpy change
Why will enthalpy change values using bond enthalpies not match the data book values?
- bond enthalpy values are averages
- for bond enthalpy values everything is in its gaseous state, but for enthalpy change, everything is in their standard states
Explain in terms of bond breaking & making, why a fuel gives out energy when it burns.
More energy is given out when bonds in the product molecules are made than is taken in to break bonds in the reactant molecules
Why will the enthalpy change of combustion values of isomers be very similar?
The same number &type of bonds need breaking and the same number and type of bonds are being made so the calculated enthalpy change values will be the same. The actual values will be very close, but not exactly the same as bond enthalpy values vary slightly in different molecules
Why does an exothemic reaction give out energy?
The products have stronger bonds than the reactants so more energy is given out making their bonds than had to be put in breaking reactant bonds so the reaction will be exothermic overall
Why does an endothermic reaction take in energy?
The energy required to break the bonds of the reactants is greater then the energy released when new bonds form in the products
