Chapter 8 Flashcards
When bonds are broken and formed, what is often caused?
A change in energy
What does enthalpy change mean?
A change in energy
What is the scientific definition of enthalpy change?
The heat energy change in a reaction at constant pressure
What is the units of enthalpy change?
kJ / mol
When describing enthalpy change, why do you write delta H?
To show that the measurements were made under standard conditions and that the elements were in there standard states.
What are standard conditions?
100kPa pressure, and 298k
What do exothermic reactions do?
They give out energy
For exothermic reactions, what is the value of delta H?
It is a negative value
For endothermic reactions, what is the value of delta H?
It is a positive value
What are endothermic reactions?
Reactions that take in energy from the surroundings
What is the relationship between stability and the enthalpy of a substance?
The less enthalpy a substance, the more stable it is
What is the activation energy?
The minimum amount of energy needed to begin breaking bonds and start a chemical reaction
In endothermic reactions, are the reactants or products more stable?
The reactants are more stable
In exothermic reactions, are the reactants or products more stable?
The products are more stable
What is standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
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 of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.
What is the standard enthalpy change of neutralisation?
The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water.
When writing an equation for the standard enthalpy change of combustion, what must the amount of moles for the compound reactant be?
1 mole always, as the standard enthalpy change of combustion is when 1 MOLE of a substance is completely burned in oxygen.
What 3 reactants are required for the formation of methanol (CH3OH)?
Hydrogen, oxygen and carbon.
What is the equation for the standard enthapy change of combustion of carbon?
C + O2 —> CO2
When trying to find the enthalpy change for a reaction, what 2 things do you need to know?
- The number of moles of the reactants
- The change in temperature of a reaction
For reactions carried out using a constant pressure, what is the heat change the same as?
The enthalpy change
What is the main problem with mixing reactants together, using a thermometer and measuring the overall temperature change?
Some heat will be lost to the surroundings, so the temperature change you measure will be less than the actual temperature change
Describe an experiment that could be used to find the enthalpy change of the endothermic reaction between citric acid and sodium bicarbonate:
1) Add a set volume of citric acid to a polystyrene cup
2) Put a lid on the beaker and measure the temperature until stable
3) Add a set mass of sodium bicarbonate to the beaker, and stirred the mixture
4) Measure the temperature of the reaction every 30s until it reaches a minimum value (exothermic) for a couple of minutes.
5) Draw a graph of temperature against time
6) Find the extrapolated minimum temperature and find the total temperature change.
How do you find the extrapolated minimum tempreature?
Draw a tangent on the time-temp graph at the point where the temperature is at its lowest value. Find the point where this tangent intersects the time where the reaction started
To find the enthalpy changes of combustion, what do you need?
A calorimeter
Describe an experiment to find out the enthalpy changes of comustion?
1) Burn a flammable liquid
2) As the fuel burns, it heats the water. If you know the mass of the water you can work out the heat absorbed by the water
What is the main equation used to calculate enthalpy changes?
q = mcT
In the equation q = mcT, what is q?
q=heat lost or gained
If the pressure is constant, what is the heat lost or gained equal to?
The enthalpy change
In the equation q = mcT, what is m?
The mass of the water in the calorimeter, or solution in the insulated container ( in grams).
In the equation q = mcT, what is c?
The specific heat capacity of water
What is the specific heat capacity of water?
4.18 J g-1 K-1
In the equation q = mcT, what is T?
The change in temperature
What does ‘calorimetry’ mean?
Measuring heat change
What is located at the bottom of a calormeter?
A spirit burner containing the fuel
What does Hess’s Law state?
The total enthalpy change of a reaction is always the same, no matter of what route is taken.
By using Hess’s law, what does the enthalpy change of route 1 equal?
The enthalpy change of route 2
Is bond breaking exothermic or endothermic?
Bond breaking is endothermic, so enthalpy change is positive.
Why is bond breaking enothermic?
You need to supply energy to break the bonds
Is bond forming exothermic or endothermic?
Bond formic is exothermic, so the enthalpy change is negative
Why is bond forming exothermic?
Because energy is released when bonds are formed
If you need more energy to break bonds than is released when bonds are made, what is the value of delta H?
Positive
What is required to break bonds in ionic bonding etc?
Energy is required to break bonds
What is Bond Enthalpy?
The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase.
Why are mean bond enthalpies not exact?
Its the average for a bigger range of molecules. For example the O-H bond in water is in alcohols and carboxylic acids as well.
What is the mean bond enthalpy?
The energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds.
What is the equation for enthalpy change of reaction?
Enthalpy change of reaction = Bond enthalpies of reactants - Bond enthalpies of products
