10881162
Description
Flashcards by Tom Dawson, updated more than 1 year ago
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Created by Tom Dawson
over 6 years ago
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| Question | Answer |
| ENTHALPY CHANGE | The change in heat energy measured under conditions of constant pressure. |
| ENTHALPY NOTATION | Enthalpy change is written as ∆H. This is the heat energy transferred in a reaction at a constant pressure. The units of ∆H are kJ mol-1. ∆Hθ shows that the reaction took place under standard conditions. |
| STANDARD CONDITIONS | Pressure:100kPa Temperature: 298K or stated |
| EXOTHERMIC AND ENDOTHERMIC REACTIONS | Exothermic reactions give out energy to their surroundings. Therefore ∆H will be negative. Endothermic reactions take in energy from their surroundings. Therefore ∆H will be positive. |
| BOND ENTHALPY | The energy needed to break a bond. Bond enthalpies are dependent on which atoms are attached to either side of the bond. |
| BREAKING AND MAKING BONDS | Energy needs to be put into a bond to break it, breaking bonds is therefore endothermic (∆H is positive). Energy is released when a bond is formed, making bonds is therefore exothermic (∆H is negative). |
| MEAN BOND ENTHALPIES | The average energy needed to break a certain type of bond. This value can be found in a data book. |
| CALCULATING ENTHALPY CHANGE (EQUATION) | Enthalpy change of a reaction = Total energy absorbed - Total energy released. |
| CALCULATING ENTHALPY CHANGE (METHOD) | 1. Calculate the total energy needed to break bonds in reactants - multiply mean bond enthalpy by the number of each bond present. The total will be the total energy absorbed in the reaction. 2. Find the total energy needed to form all the bonds in the products - use the same method as before. 3. Calculate overall enthalpy change of the reaction - subtract total energy released by total energy absorbed. |
| DIFFERENT TYPES OF ∆H | ∆Hfϴ: Standard enthalpy change of formation - the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions. ∆Hcϴ: Standard enthalpy of combustion - the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions in standard states ∆Hrϴ: Standard enthalpy of reaction - the enthalpy change when a reaction occurs in the molar quantities shown in the chemical reaction under standard conditions in standard states. |
| q = mc∆T | q = mc∆t q = enthalpy change in joules (J). m = mass of solution/water in grams (g). c = specific heat capacity of solution/water in Jg-1K-1. For water this is 4.18Jg-1K-1. Specific heat capacity is the amount of heat energy needed to raise the temperature of 1g of a solution by 1 Kelvin. ∆T = the change in temperature of the solution/water in Kelvin (k). |
| CALCULATING ENTHALPY OF COMBUSTION | 1. |
| HESS' LAW | q |
| USING ENTHALPIES OF FORMATION | |
| USING ENTHALPIES OF COMBUSTION |
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