Chem unit 2.1

• In what kind of reactors do chemical reactions with energy changes occur?

  Laboratory or nuclear reactor
• Why are standard conditions necessary when measuring energy changes in chemical reactions?

  So the results can be compared
• What does the Principle of Conservation of Energy state?

  Energy can neither be created nor destroyed
• According to the Principle of Conservation of Energy, what happens to the total energy during a chemical reaction?

  It remains unchanged
• During a chemical reaction, what may be exchanged between the materials and the surroundings?

  Energy
• What is another name for the heat content of a substance?

  Enthalpy (H)
• What often accompanies chemical changes in relation to a substance's enthalpy?

  Changes in heat content or enthalpy
• What type of energy is associated with a substance that leads to changes in enthalpy during a reaction?

  Potential energy
• What does enthalpy indicate about a substance?

  Its total energy content
• How is a change in heat content or enthalpy change shown?

  By a change in temperature
• Under what conditions are reactions generally performed when measuring enthalpy change (ΔH)?

  Constant pressure
• What symbol represents the change of enthalpy?

  ΔH
• What is another name for the change of enthalpy?

  Heat of reaction
• In the equation ΔH = (Hproducts - Hreactants), what do Hproducts and Hreactants represent?

  Enthalpy of products and reactants
• How does the temperature change in an exothermic reaction?

  The temperature increases
• How does the temperature change in an endothermic reaction?

  The temperature decreases
• What is the unit for enthalpy change?

  Kilojoule (kJ)
• What does the enthalpy change represent in a reaction at constant pressure?

  The difference in energy between products and reactants
• In an exothermic reaction, how does the enthalpy of the products compare to that of the reactants?

  Lower than that of the reactants
• What is the sign of ΔH for an exothermic reaction?

  Negative
• What is the sign of ΔH for an endothermic reaction?

  Positive
• In an exothermic reaction, what happens to the heat?

  Heat is lost to the surroundings
• In an endothermic reaction, what happens to the heat?

  Heat is gained from the surroundings
• In the reaction $2Mg(s) +$$O_2(g) \rightarrow 2MgO(s)$, is ΔH positive or negative?


  Negative
• What type of energy change occurs in both exothermic and endothermic reactions according to the principle of conservation of energy?

  Change in form of energy
• What are the standard conditions of temperature and pressure for enthalpy changes?

  298 K (25°C) and 1 atmosphere
• Under standard conditions, what physical state should substances be in?

  Their normal physical state
• Under standard conditions, what concentration should any solution have?

  1 mol dm⁻³
• What does the standard enthalpy change refer to?

  Molar quantities
• What is the enthalpy change that occurs as a result of a reaction called?

  Enthalpy change of reaction
• What symbol is used to represent the enthalpy change of a reaction?

  ΔH
• What is the usual unit of enthalpy change?

  Kilojoule (kJ)
• According to the equation ΔH(…)= H(…) – H(…), what is being calculated?

  Enthalpy change
• What symbol represents the standard enthalpy change of a reaction?

  ΔH°rxn
• What is defined as the heat change when a reaction takes place between the quantities of reactants indicated by the equation at standard conditions?

  Standard enthalpy change of a reaction
• In the reaction $C(s) +$$O_2(g) \rightarrow CO_2(g)$ with $ΔH°_{298} =$$-393.4 \text{kJ mol}^{-1}$, what does this mean?


  1 mole of graphite reacts with oxygen
• What is the standard molar enthalpy change of formation denoted by?

  ΔH°f
• What does the standard molar enthalpy of formation (ΔH°f) of a compound represent?

  Enthalpy change for forming one mole
• What are standard conditions for ΔH°f?

  298 K and 1 atmosphere
• Given $H_2(g) +$$\frac{1}{2} O_2(g) \rightarrow H_2O(l)$, what is being described by $ΔH°f (H_2O) =$$-285.9 \text{kJ mol}^{-1}$?


  Formation of one mole of water