Physical - Energetics Flashcards
What is enthalpy?
The enthalpy of a chemical system refers to the “heat content” of the system. Enthalpy is given the symbol H.
What is endothermic?
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surrounding decreases. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
What is the law of conservation of energy? (the 1st Law of Thermodynamics)
Energy can be neither created nor destroyed but it can be converted from one form to another. Hess’s law derives from this law.
What are standard conditions?
Enthalpy values vary according to the conditions. Standard state conditions are said to be:
- Pressure: 100 kPa (100,000 Pa) (1 atm)
- A stated temperature (generally 298K, 25 degrees Celsius).
- Any solutions are of concentration 1moldm-3.
What are standard states?
In chemistry, the standard state of a material, be it a pure substance, mixture, or solution, is a reference point used to calculate its properties under different conditions.
Standard state conditions are indicated by ΔHϴ298.
The standard state of an element is the state in which it exists at 298K and 100kPa.
What is the standard enthalpy change of reaction (ΔH°r)?
The standard enthalpy change of a reaction (denoted ΔHr⦵) is the change in heat content that occurs in a system when matter is transformed by a given chemical reaction, when all reactants and products are in their standard states.
What is specific heat capacity?
The amount of energy required to raise the temperature of 1g of the substance by 1 degree C. This is a way of quantifying an increase or decrease in a material’s thermal energy store.
What two main forms (or classes) of energy to the products and reactants in a chemical reaction have?
- chemical potential energy
- kinetic energy
What is calorimetry?
Calorimetry is an experimental method for finding enthalpy change by measuring temperature change over time. When observed and plotted on a graph, data can be extrapolated to give an accurate value for the change in temperature.
What are some examples of exothermic reactions?
- respiration
- combustion of fuels
- neutralisation
What are some examples of endothermic reactions?
- photosynthesis
- thermal decomposition of calcium carbonate
- heating copper sulphate
Why can’t you measure enthalpy directly?
We cannot measure enthalpy directly because we cannot measure absolute gravitational or kinetic energy.
The change in enthalpy is the heat energy change at constant pressure:
enthalpy of products - enthalpy of reactants = enthalpy change
Why are state symbols necessary?
The physical states (gas, liquid, or solid) of the reactants and products also affect the enthalpy change of a reaction. For example, heat must be put in to change liquid to gas and is given out when a gas is changed to a liquid. This means that you must always include state symbols in your equation.
In standard conditions you should assign the correct subscript (g), (l) or (s) to indicate which state the substance is.
What is the standard molar enthalpy change of combustion (ΔH°c)?
The enthalpy change when one mole of a substance is completely burned in excess oxygen with all reactants and products in standard states under standard conditions.
What is the standard molar enthalpy change of formation (ΔH°f)?
The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
What is the standard molar enthalpy change of neutralisation (ΔH°n)?
The enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions.
What is the link between exothermic and endothermic reactions?
It is always the case that a reaction that is endothermic in one direction is exothermic in the reverse direction.
What is a practical application of thermochemistry?
One important practical application of the study of thermochemistry is that it enables us to compare the efficiency of different fuels. Most of the fuels used today for transport (petrol for cars, diesel for cars and lorries, kerosene for aviation fuel, etc.) are derived from crude oil. This is a resource that will eventually run out so chemists are actively studying alternatives. Possible replacements include ethanol and methanol, both of which can be made from plant material, and hydrogen, which can be made by the electrolysis of water.
What does the amount of heat given out or taken in during a chemical reaction depend on? What does it vary with?
It depends on the quantity of reactants.
It varies with the conditions - temperature, pressure, concentration of solutions and so on.
How is the enthalpy change measured in exothermic reactions?
An exothermic reaction is not thought of as being over until the products have cooled back to 298K. The heat given out to the surroundings while the reaction mixture cools is the enthalpy change for the reaction.
In an exothermic reaction the products end up with less energy than the starting materials because their chemical energy has been transferred to heat energy and got lost when they heated up their surroundings. This means that the change in enthalpy is negative, and is given a negative sign.
How is the enthalpy change measured in endothermic reactions?
Some endothermic reactions that take place in aqueous solution absorb heat from the water and cool it down. You don’t think of this reaction as being over until the products have warmed up to the temperature at which they started. In this case, the solution has to take in heat from the surroundings to do this.
In an endothermic reaction the products end up with more energy than the starting material as surrounding heat energy gets transferred into chemical energy, so the change in enthalpy is positive, and is given a positive sign.
What does pressure affect the amount of?
Pressure affects the amount of heat energy given out by reactions that involve gases.
Why is it important to have a standard of pressure for measuring energy changes?
Pressure affects the amount of heat energy given out by reactions that involve gases. If a gas is given out, some energy is required to push away the atmosphere. The greater the atmospheric pressure, the more energy is used for this. This means that less energy remains to be given out as heat by the reaction. This is why it is important to have a standard of pressure for measuring energy changes.
What is the difference between an energy change and an enthalpy change?
Heat is a form of energy so a heat change can also be described as an energy change.
An enthalpy change is still an energy change but it is measured under stated conditions of temperature and pressure.
What are the two ways hydrogen burns in oxygen to form water?
- forming liquid water (greater enthalpy change, more negative)
- forming steam (lower enthalpy change, less negative)
The difference in enthalpy change represents the amount of heat needed to turn one mole of water into steam.
How can you make sure that both reactants are at the same temperature?
One way of making sure that both reactants are at the same temperature is simply to leave them in the same room for some time.
What are enthalpy level diagrams?
Enthalpy level diagrams, sometimes called energy level diagrams, are used to represent enthalpy changes. They show the relative enthalpy levels of the reactants (starting materials) and the products. The vertical axis represents enthalpy, and the horizontal axis, the extent of the reaction. You are usually only interested in the beginning of the reaction, 100% reactants, and the end of the reaction, 0% reactants (and 100% products), so the horizontal axis is usually left without units.
What is the general name for the enthalpy change for any reaction?
The general name for the enthalpy change for any reaction is the standard molar enthalpy change of reaction. It is measured in kilojoules per mole. You write a balanced symbol equation for the reaction and then find the heat change for the quantities in moles given by this equation.
What is temperature?
Temperature is related to the average kinetic energy of the particles in a system. As the particles move faster, their average kinetic energy increases and the temperature goes up. But it doesn’t matter how many particles there are, temperature is independent of the number present. Temperature is measured with a thermometer.
What is heat?
Heat is a measure of the total energy of all the particles present in a given amount of substance. It does depend on how much of the substance is present. The energy of every particle is included. So a bath of lukewarm water has much more heat than a red hot nail because there are so many more particles in it. Heat always flows from high to low temperature, so heat will flow from the nail into the bath water, even though the water has much more heat than the nail.
What do you need to measure the enthalpy change of a reaction?
The enthalpy change of a reaction is the heat given out or taken in as the reaction proceeds. There is no instrument that measures heat directly. To measure the enthalpy change you arrange for the heat to be transferred into a particular mass of a substance, often water. Then you need to know three things:
- mass of the substance that is being heated up or cooled down
- temperature change
- specific heat capacity of the substance.
What is the specific heat capacity of water?
4.18
What is the specific heat capacity equation?
enthalpy change = mass of substance x specific heat capacity x temperature change
How can you find the approximate enthalpy change when a fuel burns?
You can use a simple calorimeter (spirit burner) to find the approximate enthalpy change when a fuel burns. You burn the fuel to heat a known mass of water and then measure the temperature rise of the water. You assume that all the heat from the fuel goes into the water.
Where can a simple calorimeter be used?
The simple calorimeter can be used to compare the enthalpy change of combustion values of a series of similar compounds because the errors will be similar for every experiment.
How can you improve the results from using a simple calorimeter?
cut down the heat loss using mineral wool or a draught screen
What is the flame calorimeter? How is it designed to reduce heat loss even further?
The flame calorimeter is an improved version of the simple calorimeter used for measuring enthalpy changes of combustion.
It incorporates the following features that are designed to reduce heat loss even further:
- the spiral chimney is made of copper
- the flame is enclosed
- the fuel burns in pure oxygen, rather than air
How do you measure the enthalpy changes of reactions in solution?
It is relatively easy to measure heat changes for reactions that take place in solution. The heat is generated in the solutions themselves and only has to be kept in the calorimeter. Expanded polystyrene beakers are often used for the calorimeters. These are good insulators (this reduces heat loss through their sides) and they have a low heat capacity so they absorb very little heat. The specific heat capacity of dilute solutions is usually taken to be the same as that of water.
What are neutralisation reactions?
Neutralisation reactions in solution are exothermic - they give out heat. When an acid is neutralised by an alkali, the equation is:
acid + alkali -> salt + water
To find an enthalpy change for a reaction, you use the quantities in moles given by the balanced equation.
What are displacement reactions?
A metal that is more reactive than another will displace the less reactive one from a compound. If the compound will dissolve in water, this reaction can be investigated using a polystyrene beaker as before.
Why do polystyrene beakers make good calorimeters?
because they are good insulators and have low heat capacities
How can you allow for heat loss?
Although expanded polystyrene cups are good insulators, some heat will still be lost from the sides and top leading to low values for enthalpy changes measured by this method. This can be allowed for by plotting a cooling curve.
How can a cooling curve be used in the neutralisation of hydrochloric acid and sodium hydroxide?
The measurement of the heat of neutralisation of hydrochloric acid and sodium hydroxide is repeated using a cooling curve.
Before the experiment, all the apparatus and both solutions are left to stand in the laboratory for some time. This ensures that they all reach the same temperature, that of the laboratory itself.
Then proceed as follows:
- Place 50cm3 of 1.0moldm-3 hydrochloric acid in one polystyrene cup and 50cm3 of 1.0moldm-3 sodium hydroxide solution in another.
- Using a thermometer that reads to 0.1 degrees Celsius, take the temperature of each solution every 30 seconds for four minutes to confirm that both solutions remain at the same temperature, that of the laboratory. A line of ‘best fit’ is drawn through these points. It is likely there will be very small variations around the line of best fit, indicating random errors.
- Now pour one solution into the other and stir, continuing to record the temperature every 30 seconds for a further six minutes.
How else can the neutralisation between hydrochloric acid and sodium hydroxide be done?
The experiment can also be done using an electronic temperature sensor and data logging software to plot the graph directly.
How can you find the best estimate of the temperature immediately after mixing?
On mixing, the temperature rises rapidly as the reaction gives out heat, and then drops slowly and regularly as heat is lost from the polystyrene cup. To find the best estimate of the temperature immediately after mixing, you draw the best straight line through the graph points of a cooling curve after mixing and extrapolate back a graph of temperature against time.
What are mean bond enthalpies?
Different covalent bonds require different amounts of energy to be broken. Values can be found experimentally using calorimetry methods. The bond enthalpy values calculated in this way often differ from the data book values as they are not exact and vary in each situation. This means the data book values are averaged values.
What is the relationship beween change in temperature and energy change?
The measured change in temperature is proportional to the energy change. It allows the energy in or out of a substance of known mass to be calculated, which depends on its specific heat capacity.
Why are enthalpy change values using calorimetry never completely accurate?
Enthalpy change values found using calorimetry are never completely accurate as energy is easily lost from the system through conduction or convection and inaccuracies in measuring temperatures.
What is the cup calorimeter?
The reaction is carried out in an insulated beaker and the temperature change measured. The reaction must be fast so that the maximum temperature is reached quickly (before it starts to cool).
The specific heat capacity of the solution is taken as 4.18 Jg-1K-1 (the same as water) and we usually use the mass of the water (not the solution) in the calculation.
What are the pros of the cup calorimeter?
- Heat generated is from the solutions themselves so only has to be kept in calorimeter.
- Polystyrene cups are very good insulators (often you will use a cup within another cup).
- They have a low heat capacity, so absorb very little heat.
What are the cons of the cup calorimeter?
- Inaccuracy in measuring mass and temperature.
- Solution is assumed to be pure water (therefore c is assumed to be 4.18 Jg-1K-1)
- Some heat is absorbed by the cup.
- Heat is lost (or gained) from the surroundings. Not a completely closed system.
What is a simple calorimeter?
Using apparatus like this:
This equipment follows the basic principle of heat being transferred to the water. As the specific heat capacity of water is known, we can work backwards to calculate how much heat was given out by the burning fuel.
Flame calorimeters and bomb calorimeters:
You can reduce errors from heat loss by measuring the heat capacity of the calorimeter as a whole.
What is a flame calorimeter?
This is a more advanced piece of equipment:
The fuel, which is in a bottle with a wick, is burned so that the heat is passed to water which it heats.
Sometimes the specific heat capacity of water is used, but in more accurate flame calorimeters, the actual heat capacity of the flame calorimeter can be determined and used (by finding the temperature rise when a known amount of a substance with an accurately known enthalpy of combustion is tested).
What is a bomb calorimeter?
The most accurate type of calorimeter.
A known mass of a fuel is placed inside a steel container (the “bomb”) and the container filled with oxygen under pressure. The fuel is electrically ignited and heat evolved used to heat surrounding water. The heat capacity of the calorimeter is found by burning a substance with accurately known ΔHc.
How can a chemical reaction be thought of?
A chemical reaction can be thought of as a journey between reactants and products. There may be many ways to carry out the journey, however the total enthalpy change is constant.
As long as the starting and end points are the same, the energy change must be the same.
What is Hess’s Law?
Hess’s Law states that the enthalpy change for a chemical reaction depends only on the initial and final states and is independent of the path followed. It is the same regardless of the route taken from reactants to products. This is a consequence of the Law of Conservation of Energy.
What does Hess’s Law allow us to do theoretically?
- calculate changes that cannot be measured directly
- calculate enthalpy changes from other data
What is Hess’s First Law?
Using Hess’s Law 1: Heat of Combustion
If you burned all the products, you should get the same amounts of CO2 and H2O as if you burned all the reactants. This is called a thermochemical cycle.
How do you measure enthalpy changes?
The enthalpy changes for some reactions cannot be measured directly. To find these, you can use an indirect approach. Chemists use enthalpy changes that they can measure to work out enthalpy changes that they cannot. In particular, it is often easy to measure enthalpies of combustion. To do this, chemists use Hess’s law, first states by Germain Hess, a Swiss-born Russian chemist, born in 1802.
How do you work out the enthalpy of combustion?
total enthalpy of reactants - total enthalpy of products
How do you work out the enthalpy of formation?
total enthalpy of products - total enthalpy of reactants
Which direction so the arrows point in the combustion cycle?
downwards (towards CO2 and H2O)
Which direction do the arrows point in the formation cycle?
upwards (away from constituent elements)
Why do elements not have an enthalpy value?
by definition (already found in its standard state)
What are enthalpies of elements in their standard states taken as?
The enthalpies of all elements in their standard states (i.e., the states in which they exist at 298K and 100kPa) are taken as zero. (298K and 100kPa are approximately normal room conditions).
How would you use a thermochemical cycle?
- Write a balanced equation for the reaction.
- Write down the elements in the two compounds with the correct quantities of each.
- Put in the enthalpy of formation values with arrows showing the direction - from elements to compounds.
- Put in the arrows to go from starting materials to products via the elements (the red arrows).
- Reverse the sign of the enthalpy of formation if the red arrow is in the opposite direction to the black arrow.
- Go round the cycle in the direction of the red arrows and add up the total enthalpy change values as you go.
How would you use an enthalpy diagram?
- Draw a line at level 0 to represent the elements.
- Look up the values of the enthalpy of formation for each compound and enter these on the enthalpy diagrams, taking account of the signs - negative values are below 0, positive values are above.
- Find the difference in levels between the two compounds. This represents the difference in their enthalpies.
- The enthalpy of change is the difference in levels taking account of the direction of change. Up is positive and down is negative.
What is shown when you plot the enthalpy change of combustion against the number of carbon atoms?
For straight chain alkanes, you get a straight line graph. The straight lines means that the enthalpy change of combustion changes by the same amount for each extra carbon atom in the chain.
What is bond enthalpy?
Each alkane differs from the previous one by one CH2 group, that is, there is one extra C-C bond in the molecule and two extra C-H bonds. This suggests that you can assign a definite amount of energy to a particular bond. This is called the bond enthalpy.
What is an endothermic change?
When you put in energy to break a covalent bond.
What is bond dissociation enthalpy?
Defined as the enthalpy change required to break a covalent bond with all species in the gaseous state.
What is an exothermic change?
The energy given out when a bond is formed. For the same bond, this will be the same amount of energy as needed to break the bond.
What is the mean bond enthalpy?
The same bond may have slightly different bond enthalpies in different molecules, so you usually use the average value. This value is called the mean bond enthalpy (often called the bond energy).
What is an example of Hess’s law?
The fact that you get out the same amount of energy when you make a bond, as you put in to break it is an example of Hess’s law.
Why will you only get approximate answers?
As mean bond enthalpies are averages, calculations using them for specific compounds will only give approximate answers.
What happens if the bonds in (e.g. methane) are broken one at a time?
The energy required is not the same for each bond.
Are bond energies positive or negative?
All mean bond energies are positive because we have to put energy in to break bonds - they are endothermic processes.
What is the enthalpy change of a reaction?
The difference between the energy put in to break the bonds and the energy given out to form bonds.
How do you work out if the enthalpy change is positive or negative?
If more energy was put in than was given out, the enthalpy change is positive (the reaction is endothermic) because more energy was needed to break the bonds than form them.
If more energy was given out than was put in, the enthalpy change is negative (the reaction is exothermic) because more energy was needed to form the bonds than break them.
What do bond enthalpies give a measure of?
Bond enthalpies give a measure of the strength of bonds, and can help to predict which bond in a molecule is most likely to break. However, this is not the only factor, the polarity of the bond is also important.
What is the exact definition of bond enthalpy?
Bond enthalpy is the standard enthalpy change associated with breaking one mole of bonds into one mole of gaseous atoms, under standard conditions.
Which substances are energetically more stable?
Substances that have a lower enthalpy.
What does the actual strength of a bond depend on?
the environment of a bond within a molecule
What is the linear equation for calculating bond enthalpies?
bonds broken - bonds formed
What is meant by the term average bond enthalpy?
The standard enthalpy change associated with breaking one mole of bonds, averaged over a number of molecules containing that bond, into one mole of gaseous atoms, under standard conditions.
