Energetics

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  • Created by: sydjow17
  • Created on: 08-12-19 11:18
Define the term endothermic.
Describes a reaction in which heat energy is taken in as the reactants change to products - the temperature therefore drops.
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Define the term exothermic.
Describes a reaction in which heat energy is given out as the reactants change to products - the temperature therefore rises.
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Why do most reactions involve an energy change?
Energy must be put in to break bonds and energy is given out when bonds are formed.
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Define enthalpy change.
A measure of heat given out or taken in when a chemical or physical change occurs at constant pressure.
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What are the standard conditions for measuring enthalpy change?
pressure of 100kPa and a temperature of 298K (25˚c)
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A reaction is not over until..
the products are back at the temperature at which they started.
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How does pressure affect the amount of heat energy given out by reactions?
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.
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How does the physical states of the reactants and products affect the enthalpy change of a reaction?
For example, energy must be put in to turn a liquid to a gas, and energy is given out when a gas turns into a liquid. This means you must always include state symbols in your equations.
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What does an enthalpy level diagram represent and what are the axis labelled|?
Enthalpy changes; they show the relative enthalpy levels of the reactants and products. The y-axis represents enthalpy and the x-axis represents the extent of the reaction.
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Define the standard molar enthalpy of formation.
The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.
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Define the standard molar enthalpy of combustion.
The enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states.
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Describe the meaning of temperature.
Related to the AVERAGE kinetic energy of the particles in a system. It is independent of the number of particles present.
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Describe the meaning of heat.
A measure of the TOTAL energy of all the particles present in a given amount of substance. It is dependent on the number of particles present.
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How do you measure the enthalpy change of a reaction?
Arrange for the heat to be transferred into a particular mass of a substance, often water.
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Define specific heat capacity.
The amount of heat needed to raise the temperature of 1g of substance by 1K.
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What is the unit for specific heat capacity?
J/g/K.
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What is the unit for temperature in the specific heat capacity formula?
Degrees celsius or kelvin, depending on what is given in the question.
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What is a calorimeter?
An instrument for measuring the heat changes that accompany a chemical reaction.
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State Hess's law.
Hess's law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
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Describe what a thermochemical cycle is.
A sequence of chemical reactions that convert a reactant into a product. The total enthalpy change of the sequence of reactions will be the same as that for the conversion of the reactant to product directly (or by any other route).
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What is the standard molar enthalpy of formation for an element?
Zero.
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Define bond dissociation enthalpy.
The enthalpy change required to break a covalent bond with all species in the gaseous state.
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Define mean bond enthalpy.
The average value of the bond dissociation enthalpy for a given type of covalent bond taken from a range of different compounds.
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Other cards in this set

Card 2

Front

Define the term exothermic.

Back

Describes a reaction in which heat energy is given out as the reactants change to products - the temperature therefore rises.

Card 3

Front

Why do most reactions involve an energy change?

Back

Preview of the front of card 3

Card 4

Front

Define enthalpy change.

Back

Preview of the front of card 4

Card 5

Front

What are the standard conditions for measuring enthalpy change?

Back

Preview of the front of card 5
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