Enthalpy Changes Flashcards
What happens to bonds when reactions take place?
Some bonds are broken and some are made.
What does the making and breaking of bonds in a reaction cause?
This causes a change in energy.
What is enthalpy change?
This is the heat energy transferred in a reaction at constant pressure.
What are the standard conditions?
100kPa and 298K
What are the units of enthalpy change?
kJmol-1
What type of reaction gives out energy?
Exothermic
What is the value of delta H in an exothermic reaction?
It is negative.
What is the symbol of enthalpy change?
Delta H
What happens to the temperature in an exothermic reaction?
It increases.
What type of reaction is usually exothermic?
Oxidation
Give two examples of oxidation reactions.
The combustion of a fuel.
The oxidation of carbohydrates.
What type of reaction absorbs energy?
Endothermic
What is the value of delta H like in endothermic reactions?
It is positive.
What happens to the temperature in endothermic reactions?
The temperature decreases.
Give two examples of endothermic reactions.
Thermal decomposition of calcium carbonate.
The main reactions of photosynthesis.
What do enthalpy profile diagrams show?
They show energy changes during reactions.
What is Ea?
Activation energy
What is meant by Ea?
This is the minimum amount of energy required to begin breaking reactant bonds and start a chemical reaction.
On an enthalpy profile diagram for an endothermic reaction, which way should the delta H arrow point?
It should point upwards as delta H is positive and energy is absorbed from the surroundings.
On an enthalpy profile diagram for an exothermic reaction, which way should the delta H arrow point?
It should point downwards because delta H is negative as energy is released to the surroundings.
How can you find enthalpy changes?
In books or through experiments.
Where can you find standard enthalpy changes?
In books.
What two things can enthalpy changes be affected by?
They can be affected by temperature and pressure.
What are the 4 types of standard enthalpy change?
Reaction
Formation
Combustion
Neutralisation
What is the standard enthalpy change of reaction?
This is the enthalpy change when the reaction occurs in molar quantities shown in the chemical equation, under standard conditions.
What is the standard enthalpy change of formation?
This is 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?
This is the enthalpy change when 1 mole of a substance is completely burned in loxygen under standard conditions.
What is the standard enthalpy change of neutralisation?
This is the enthalpy change when an acid and an alkali react together, under standard conditions to form 1 mole of water.
What is needed to break bonds?
Energy
Is breaking bonds endothermic or exothermic?
It is endothermic.
What is released when making bonds?
Energy
Is making bonds endothermic or exothermic?
This is exothermic.
What is attracted in ionic bonding?
Positive and negative ions.
What is attracted in covalent bonding?
Positive nuclei are attracted to the negative charge of the shared electrons.
What does the energy break?
The attraction between the ions or nuclei and electrons.
What is meant by bond dissociation enthalpy?
This is the energy needed per mole to break the attraction.
What do bond dissociation enthalpies always involve?
Bond breaking in gaseous compounds?
Why do bond dissociation enthalpies always invole gaseous compounds?
To make comparisons fair.
Is the average bond enthalpy exact?
No
Why isn’t the average bond enthalpy exact?
It is the average over a large range of molecules.
What is the average bond enthalpy?
This is the energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds.
What two things do you need to measure the enthalpy change in the lab?
You need the number of moles in the reactants and the temperature change.
How do you find the enthalpy of combustion of a flammable liquid?
You burn it so that it heats a mass of water. You can work out the heat absorbed by knowing the mass of water and the temperature change. You will also need to know the specific heat capacity of the water.
What happens to some of the heat in experiments?
It is lost to the surroundings.
What is the specific heat capacity of a substance?
This is the amount of heat energy required to raise the temperature of 1g of that substance by 1 K.
What other methods can be used to calculate enthalpy change?
Neutralisation
Displacement
How do you find the enthalpy change in a neutralisation reaction?
Combine known quanities of acid and alkali in an insulated container and measure the temperature change.
What equation can be used to find the amount of heat given out?
q=mcAT
What does q stand for?
Heat loss or gained.
What is q measured in?
J
If the pressure is constant, what does q stand for?
Enthalpy change
What does m stand for?
Mass of solution
What is m measured in?
g
What is c?
This is the specific heat capacity of water.
What is c measured in?
Jg-1K-1
What does A(delta)T stand for?
The temperature change of solution
What is AT measured in?
K
