Energetics Flashcards
What does the first law of thermodynamics state?
Energy can not be created or destroyed
What is enthalpy?
A measure of the heat content of a substance at constant pressure
Can’t measure actual enthalpy only enthalpy change
A thermodynamic value that is equal to the systems internal energy
What is enthalpy change?
The heat energy change at constant pressure
What equation is used to calculate energy transfer?
Mass x specific heat capacity x change in temperature
How do you calculate temperature change?
Final temperature - initial temperature
Temperature change is equal to energy change
How do you measure the change in energy of the surroundings?
By measuring the change in temperature
What is the most common value for specific heat capacity?
4.18
Which is given in most exam questions
What does the term exothermic mean?
Energy leaves the reaction
External temperature increases - energy of the surroundings increases
Enthalpy change decreases so is a negative value
Energy transfers from the system to the surroundings
What does the term endothermic mean?
Energy goes into the reaction
External temperature decreases - energy of the surroundings decreases
Enthalpy change increases
Energy transfers from the surroundings to the system
What symbol is enthalpy change written as?
Delta H
Delta = a little triangle
What is energy change equal to?
Enthalpy change
What is the equation for enthalpy change ?
-q/n
n= number of moles
-q= energy change
What does a calorimeter do?
Measure temperature change
What is one gram equal to?
One centimetre cubed
What measurements do you need to be able to calculate enthalpy of combustion?
Mass of the cold water
Temperature rise of the water(= temperature rise if the fuel)
Specific heat capacity which is 4.18
What is the enthalpy change of combustion?
The enthalpy change observed when a substance is burned
Enthalpy change is always negative
Endothermic reaction
What is the enthalpy change of solution?
Can be endothermic or exothermic
Enthalpy change can be positive or negative
Describe the reaction profile of an exothermic reaction
The reactants start with a higher energy than the finishing products
Describe the reaction profile for an endothermic reaction
The reactants start off with lower amounts of energy and finish with a higher amount of energy in their products
What are chemicals known as in a reaction?
The system
What are things like air and water known as in a reaction ?
The surroundings
What is internal energy?
Heat energy which is stored inside chemicals
How do you calculate enthalpy change?
Enthalpy of products - enthalpy of reactants
What are enthalpy changes per amount of?
For one mole of reactants or per mole of reactants
Must use limiting reagent only after identifying it
What are the steps in calculating enthalpy change if a solution ?
Calculate energy Change
Calculate moles of limiting reagent
Calculate enthalpy change
What is energy calculated in?
Joules that needs to be converted into kilojoules
What does a combustion reaction do?
Release heat that can heat water
The heated water is the surroundings of the reaction
What are the steps in calculating enthalpy changes in combustion?
Calculate the energy change
Calculate the moles of fuel burnt
Calculate enthalpy change
What does Hess’s law state?
The enthalpy change of a chemical reaction is always the same even if you carry out that chai cal reaction by a different route
What is the enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions in their standard states
What does the Hess cycle for formation look like?
Arrows going up to show the formation reactions taking place
What is the enthalpy of formation of an element?
Always zero
Data needed is always supplied in the question
What are the steps in calculating the enthalpy of formation?
Always draw out the Hess cycle
Identify the enthalpies of formation from the question and add them in
Calculate the total enthalpy of reactants
Calculate the total enthalpy of products
Calculate enthalpy change
How do you calculate the enthalpy change of formation?
Products - reactants
What s the enthalpy of combustion?
The enthalpy change when one mole of a compound reacts fully with oxygen under standard conditions in their standard states
What are the steps in calculating enthalpy of combustion?
Draw out a Hess cycle
Identify the enthalpies of combustion
Calculate the total enthalpy of reactants
Calculate the total enthalpy of products
Calculate overall enthalpy change
What does a Hess cycle for combustion look like?
Arrows always go downwards to the products of carbon dioxide and water
How do you calculate the enthalpy change for a combustion reaction?
Reactants - products
What does a Hess cycle actually show?
The alternative route
What is the process of breaking bonds?
An endothermic process
Energy has to go In to break the bonds apart
What is the process of forming bonds?
Forming bonds is an exothermic process
Energy is released when atoms are joined together as bonds
What is bond enthalpy?
The enthalpy change required to break one mole of bonds under standard conditions
What is the equation for this reaction?
Bonds broken - bonds formed
What are the steps in calculating bond enthalpies?
Draw out the structures
Identify all the bond enthalpy data in the question and add it in
Calculate the total bonds broken
Calculate the total bonds formed
Calculate the overall enthalpy change
What is special about bond enthalpies?
Bond enthalpies are averages of that bond that are found in different molecules
Actual bond enthalpies in a molecule that you calculate will not be the same as a bond enthalpy listed in a data book
Will be close but not exact
What is a bond enthalpy?
Breaking one mole of bonds
What is the extra definition of bond enthalpy?
The average enthalpy needed to break one mole of that type of chemical bond th form individual atomsAll compounds are in the gaseous state
What is enthalpy of ionisation?
The enthalpy change when one mole of electrons is removed to form one mole of atoms in their gaseous state
What type of bond is broken in a bond enthalpy?
Covalent bond
Describe the reaction taking place in a solution
In an insulated beaker and the temperature change is measured
Reaction must be fast so the maximum temperature is reached quickly and before it starts to cool
When are flame calorimeters used?
Used to find enthalpy of combustion of a fuel
Fuel in a bottle with a wick is burned so heat is passed to the water which it heats
What is a major problem with calorimeters?
Heat loss which leads to errors in results
What is hess’s law in simple terms?
The enthalpy change for a reaction is independent of the route taken
