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.
