Physical - Energetics Flashcards

Question

What are the two ways hydrogen burns in oxygen to form water?

Answer 1

1. forming liquid water (greater enthalpy change, more negative) 2. 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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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: 1. mass of the substance that is being heated up or cooled down 2. temperature change 3. specific heat capacity of the substance.

Answer 8

enthalpy change = mass of substance x specific heat capacity x temperature change

Answer 9

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.

Answer 10

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.

Answer 11

cut down the heat loss using mineral wool or a draught screen

Answer 12

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

Answer 13

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.

Answer 14

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.

Answer 15

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.

Answer 16

because they are good insulators and have low heat capacities

Answer 17

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.

Answer 18

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: 1. Place 50cm3 of 1.0moldm-3 hydrochloric acid in one polystyrene cup and 50cm3 of 1.0moldm-3 sodium hydroxide solution in another. 2. 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. 3. Now pour one solution into the other and stir, continuing to record the temperature every 30 seconds for a further six minutes.

Answer 19

The experiment can also be done using an electronic temperature sensor and data logging software to plot the graph directly.

Answer 20

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.

Answer 21

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.

Answer 22

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.

Answer 23

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.

Answer 24

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.

Answer 25

1. Heat generated is from the solutions themselves so only has to be kept in calorimeter. 2. Polystyrene cups are very good insulators (often you will use a cup within another cup). 3. They have a low heat capacity, so absorb very little heat.

Answer 26

1. Inaccuracy in measuring mass and temperature. 2. Solution is assumed to be pure water (therefore c is assumed to be 4.18 Jg-1K-1) 3. Some heat is absorbed by the cup. 4. Heat is lost (or gained) from the surroundings. Not a completely closed system.

Answer 27

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.

Answer 28

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).

Answer 29

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.

Answer 30

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.

Answer 31

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.

Answer 32

- calculate changes that cannot be measured directly | - calculate enthalpy changes from other data

Answer 33

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.

Answer 34

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.

Answer 35

total enthalpy of reactants - total enthalpy of products

Answer 36

total enthalpy of products - total enthalpy of reactants

Answer 37

downwards (towards CO2 and H2O)

Answer 38

upwards (away from constituent elements)

Answer 39

by definition (already found in its standard state)

Answer 40

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).

Answer 41

1. Write a balanced equation for the reaction. 2. Write down the elements in the two compounds with the correct quantities of each. 3. Put in the enthalpy of formation values with arrows showing the direction - from elements to compounds. 4. Put in the arrows to go from starting materials to products via the elements (the red arrows). 5. Reverse the sign of the enthalpy of formation if the red arrow is in the opposite direction to the black arrow. 6. Go round the cycle in the direction of the red arrows and add up the total enthalpy change values as you go.

Answer 42

1. Draw a line at level 0 to represent the elements. 2. 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. 3. Find the difference in levels between the two compounds. This represents the difference in their enthalpies. 4. The enthalpy of change is the difference in levels taking account of the direction of change. Up is positive and down is negative.

Answer 43

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.

Answer 44

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.

Answer 45

When you put in energy to break a covalent bond.

Answer 46

Defined as the enthalpy change required to break a covalent bond with all species in the gaseous state.

Answer 47

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.

Answer 48

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).

Answer 49

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.

Answer 50

As mean bond enthalpies are averages, calculations using them for specific compounds will only give approximate answers.

Answer 51

The energy required is not the same for each bond.

Answer 52

All mean bond energies are positive because we have to put energy in to break bonds - they are endothermic processes.

Answer 53

The difference between the energy put in to break the bonds and the energy given out to form bonds.

Answer 54

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.

Answer 55

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.

Answer 56

Bond enthalpy is the standard enthalpy change associated with breaking one mole of bonds into one mole of gaseous atoms, under standard conditions.

Answer 57

Substances that have a lower enthalpy.

Answer 58

the environment of a bond within a molecule

Answer 59

bonds broken - bonds formed

Answer 60

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.