Topic 5: Energetics/Thermochemistry Flashcards
Heat vs Temperature
Heat is a form of energy that flows from something at a high temperature to something at a lower temperature
Temperature is a measure of the average kinetic energy of particles
Define internal energy
Internal energy is the name given the total amount of energy (kinetic and potential) in a sample of a substance.
If a 100g block of iron at 100˚C is placed in contact with a 50g block of iron at 50˚C, heat will flow from the hotter block to the colder one until they are the same temperature. When they are at the same temperature, the 100g block of iron will have higher internal energy simply because there is more of it, but the average kinetic energy of the particles in the two blocks will be the same because they are the same temperature.
What is systems and surrounding in terms of energy flow in chemical reactions?
The ‘system’ is a chemical reaction and the ‘surrounding’ is everything else in the universe
What can chemical reactions be classified as?
Exothermic or endothermic
What happens in an exothermic reaction?
In an exothermic reaction, heat energy is transferred from a system (chemical reaction) to the surrounding – the surrounding gets hotter
Release heat into the environment
What happens in an endothermic reaction?
In an endothermic reaction, a system (chemical reaction) takes in heat energy from the surroundings - the surroundings get cooler
To absorb heat form energy
What is enthalpy change (∆H) and what does it tell us?
Enthalpy changes to heat energy exchange with the surroundings at constant pressure
It tells us when the system moves from an initial state to some final state
What does ∆ mean?
∆ means a change in a quantity
What is ∆H of a system?
The ∆H is basically the amount of heat energy taken in/ given out in a chemical reaction. Enthalpy is given the symbol H and enthalpy change is ∆H
Which is negative, which is positive ∆H? Exothermic or endothermic?
∆H for an exothermic reaction is negative (Because the enthalpy of the products is less than the reactants).
∆H for an endothermic reaction is positive
What does an enthalpy diagram for the combustion of methane look like?
What is the stability of these reactions?
The term stability is usually used to describe the relative energies of reactants and products in a chemical reaction. If the products have less energy than the reactants than they are more stable.
Less energy = More stable
In an exothermic reaction the products are at a lower energy (enthalpy) level than the reactants, and we say that the products are more stable than the reactants
How can the enthalpy change be measured with other enthalpy?
enthalpy change = enthalpy of products - enthalpy of reactants
What does this symbol mean ⦵?
It means ‘under standard condition’
What does standard enthalpy change of reaction mean? ΔHr°?
Standard enthalpy change is the change when molar amounts of reactants react together under standard condition to give products with everything else also in standard condition
What does the standard enthalpy change of combustion mean? ?ΔH°c? Is it positive of negative?
Standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. It is always negative because combustion is always an exothermic process.
What is the pressure at standard condition?
100kPa (1.00 x 10⁵Pa)
What does specific heat capacity mean?
Is the energy required to raise the temperature of 1g/kg of substance by 1˚/1K.
The higher the specific heat capacity the more difficult it is to heat up the substances than substances with low specific heat capacity. It also applies when a substance cools.
What is specific heat capacity represented by?
(c)
What is the formula to calculate heat energy? q? What does each component stand for?
q=mcΔT
q= heat energy
m= mass
c=specific heat capacity
ΔT= change in temperature
How do we use the basic technique calorimetry?
The idea is that the heat given out in a combustion reaction is used to heat another substance of no specific heat capacity such as water.
What is the specific heat capacity of water?
4.18J g-1˚C-1
What are some systematic errors in the experiment using a calorimetry? How can it be improved?
The most major problem is that of heat lost to the surroundings. Of the heat energy released when the ethanol burns, only some of it goes into heating the water – the rest goes into heating the copper can and the surrounding air. There can also be evaporation of the water and alcohol.
The experiment could’ve been improved by determining the specific heat capacity of the can and taking this into account when doing the calculations, insulating the can so that less he is lost through the can to the surrounding, using some sort of draught shield to reduce convection currents around experiment
What is incomplete combustion?
When there is not a sufficient supply of oxygen to the flame, some of the ethanol can burn to produce carbon monoxide and soot as well as water. It gives out less heat than complete combustion. Incomplete combustion causes the flame to be yellow/orange rather than blue because of the presence of soot particles, and the soot can also be seen on the bottom of the cooper can.
