Energetics

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  • Created by: r98
  • Created on: 05-04-16 15:11
Does breaking bonds require energy or give out energy?
Energy must be put in to break bonds.
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Does making bonds require energy or give out energy?
Energy is given out when bonds are formed.
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How can a reaction be described, if at the end of the reaction energy has been given out?
Exothermic.
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How can a reaction be described, if at the end of the reaction energy has been taken in?
Endothermic.
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What is the unit of energy?
Joule, J.
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Is neutralising an acid with an alkali an example of an exothermic or endothermic reaction?
Exothermic reaction.
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Is the breakdown of calcium carbonate to calcium oxide and carbon dioxide an example of an exothermic or endothermic reaction?
Endothermic reaction.
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What is always the case, in relation to endothermic and exothermic reactions?
It's always the case that a reaction is endothermic in one direction and is exothermic in the reverse direction.
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What does the amount of heat given out or taken in during a reaction depend on?
On the quantity of reactants.
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In what units is the heat energy given out/taken in measured in?
Kilojoules per mole: kJ mol^(-1).
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What type of reaction is it when fuels are burnt?
Very exothermic reactions, as there is a large heat output.
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Why is kilojoules per mole used as the unit of measurement?
Because this compares the same number of molecules of each fuel.
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What do we call, measuring a heat change under constant pressure?
An enthalpy change.
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What are the standard conditions for measuring enthalpy changes?
Pressure of 100 kPa & temperature of 298 K.
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What is the standard state of an element?
The state at which it exists at 298 K and 100 kPa.
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In an exothermic reaction, why do the products end up with less heat energy than the starting materials?
Because they've lost heat energy when they heated up their surroundings.
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What is the sign of 'delta H' (enthalpy change) for an exothermic reaction?
'delta H' is negative, so is given a negative sign.
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Do the products of an endothermic reaction end up with more or less energy than the reactants?
The products end up with more energy than the reactants.
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What sign is given to the enthalpy change of an endothermic reaction?
'delta H' is positive, so it's given a positive sign.
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How does pressure affect the amount of heat energy given out by reactions that involve gases?
The greater the atmospheric pressure, the more energy is used to push away the atmosphere, so less energy remains to be given out as heat by the reaction.
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What other factor affects the enthalpy change of a reaction?
The physical states of the reactants and products.
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On an enthalpy level diagram for an exothermic reaction, do products or reactants have less enthalpy?
Products have less enthalpy than the reactants.
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On an enthalpy level diagram for an endothermic reaction, do products or reactants have less enthalpy?
Reactants habe less enthalpy than products.
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What is the definition of standard molar enthalpy of formation?
The enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, all reactions and products in their standard states.
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What is the definition of standard molar enthalpy of combustion?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products in their standard states.
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What is heat?
A measure of the total energy of all the particles present in a given amount of substance.
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What is temperature related to?
It's related to the average kinetic energy of the particles in a system.
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What 3 things need to be known to measure the enthalpy change?
The mass of the substance that is being heated up or cooled down, the temperature, and the specific heat capacity of the substance.
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What is the equation for enthalpy change?
Enthalpy change = mass of substance x specific heat capacity x temperature change
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What's the symbol equation for enthalpy change?
q = m x c x 'delta T'
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What does Hess's law state?
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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What is the value of the enthalpy of formation for an element?
Zero, 0.
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Which allotrope of carbon is the most stable?
Graphite.
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What are an allotropes?
Pure elements which can exist in different physical forms in which their atoms are arranged differently.
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What is the state symbol given to graphite?
s, graphite. So C(s, graphite) represents graphite.
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What do enthalpy diagrams show?
The energy (enthalpy) levels of reactants and products of a chemical reaction on a vertical scale, so we can compare their energies.
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If a substance is of lower energy than another, what is its stability?
It's energetically more stable.
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How can bond dissociation enthalpy be defined?
The enthalpy change required to break a covalent bond with all species in the gaseous state.
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Is breaking a covalent bond an endothermic or exothermic change?
Endothermic change, as energy has to be put in.
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What's the definition for mean bond enthalpy?
The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds.
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How can we calculate the approximate enthalpy change of a reaction?
(energy given out to form bonds) - (energy put in to break bonds). Simple equation: make - break.
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