5.1. Exothermic and endothermic reactions
A subsection of Chemistry, 0620, through 5. Chemical energetics
Listing 10 of 192 questions
Chemical reactions can involve transfer of thermal energy. State the term used for the transfer of thermal energy during a reaction. Tetrachloromethane gas, CCl 4, reacts with steam as shown. CCl 4+ 2H2OCO2+ 4HCl The reaction is reversible. The forward reaction is exothermic. State what happens, if anything, to the rate of the forward reaction if the concentration of CCl 4 is increased. Explain your answer in terms of collision theory. State what happens to the position of equilibrium, if anything, when the pressure is increased. Explain your answer. Fig.5.1 shows an incomplete reaction pathway diagram for the forward reaction. CCl 4+ 2H2OCO2+ 4HCl energy progress of reaction On : ● insert the formulae of the reactants and products ● draw an arrow, labelled Ea, to show the activation energy ● draw an arrow, labelled ∆H, to show the transfer of energy in the reaction.  Define the term activation energy. State one way in which the activation energy of a reaction can be changed. The equation for the reaction between tetrachloromethane gas and steam can be represented as shown in . C + + H = –130 kJ / mol Cl Cl H Cl H Cl H Cl H Cl O H H Cl Cl O H H C O O Table 5.1 shows some bond energies. Table 5.1 bond C–Cl H–O C=O bond energy in kJ / mol Use the bond energies in Table 5.1 and the ∆H value for the reaction to calculate the H–Cl bond energy using the following steps. ● Calculate the energy needed to break the bonds in the reactants.  kJ ● Calculate the energy released when the bonds in carbon dioxide form.  kJ ● Calculate the H–Cl bond energy.  kJ / mol  
0620_w23_qp_42
THEORY
2023
Paper 4, Variant 2
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Use Exothermic reactions produce heat energy. An important fuel is methane, natural gas. The equation for its combustion is as follows. CH4 + 2O2 CO2 + 2H2O In chemical reactions bonds are broken and new bonds are formed. Using this reaction give an example of a bond that is broken, a bond that is formed. Explain, using the idea of bonds forming and breaking, why this reaction is exothermic, that is it produces heat energy. Some radioactive isotopes are used as nuclear fuels. Give the symbol and the nucleon number of an isotope that is used as a nuclear fuel. Give another use of radioactive isotopes. Examiner's Use Cell reactions are both exothermic and redox. They produce electrical energy as well as heat energy. The diagram shows a simple cell. V zinc electrode becomes thinner iron electrode bubbles of hydrogen form dilute sulphuric acid voltmeter Which substance in this cell is the reductant and which ion is the oxidant? reductant oxidant How could the voltage of this cell be increased? What is the important large scale use, relating to iron and steel, of this type of cell reaction? Examiner's Use Cells can be set up with inert electrodes and the electrolytes as oxidant and reductant. V voltmeter carbon electrode carbon electrode potassium manganate(potassium iodide salt bridge (allows ions to move from one beaker to another) electron flow The potassium manganate(is the oxidant and the potassium iodide is the reductant. Describe the colour change that would be observed in the left hand beaker. Write an ionic equation for the reaction in the right hand beaker.
0620_s06_qp_3
THEORY
2006
Paper 3, Variant 0
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Cyclopropane is a colourless gas. Cyclopropane reacts with bromine at room temperature. The chemical equation for the reaction is shown. C H H H H H H C + Br Br C Br Br C H H C H H C H H cyclopropane bromine dibromopropane What is the empirical formula of cyclopropane? What colour change, if any, would you see when cyclopropane is bubbled into aqueous bromine? initial colour final colour The reaction of cyclopropane with bromine is exothermic. Complete the energy level diagram for this reaction by ● adding the product of the reaction, ● labelling the energy change, ∆H. cyclopropane + bromine energy Propene also reacts with bromine. + Br Br H H C Br H C Br H C H H H C C H H H C H H Use the bond energies in the table to calculate the energy change, ΔH, for the reaction. C–H C–C Br–Br C–Br C=C bond energy in kJ / mol energy change = kJ / mol The boiling point of bromine is 59 °C and the boiling point of iodine is 184 °C. Explain why iodine has a higher boiling point than bromine.
0620_s17_qp_43
THEORY
2017
Paper 4, Variant 3
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This question is about phosphorus and compounds of phosphorus. A phosphorus molecule contains four phosphorus atoms only. What is the formula of a phosphorus molecule? Phosphorus reacts with chlorine gas to produce phosphorus(chloride, PCl 3. Write a chemical equation for the reaction between phosphorus and chlorine to produce phosphorus(chloride, PCl 3. Complete the dot-and-cross diagram to show the electron arrangement in a molecule of phosphorus(chloride, PCl 3. Show outer shell electrons only. P Cl Cl Cl  Gaseous phosphorus(chloride, PCl 3, reacts with gaseous chlorine to form gaseous phosphorus(chloride, PCl 5. PCl 3+ Cl 2PCl 5The chemical equation for this reaction can be represented as shown. Cl Cl Cl P + Cl Cl Cl Cl Cl P Cl Cl Use the bond energies in the table to calculate the energy change, in kJ / mol, of the reaction. bond bond energy in kJ / mol P– Cl Cl – Cl ● ● Energy needed to break bonds.  kJ ● ● Energy released when bonds are formed.  kJ ● ● Energy change of reaction.  energy change = kJ / mol  Deduce whether the energy change for this reaction is exothermic or endothermic. Explain your answer. Under certain conditions the reaction reaches equilibrium. PCl 3+ Cl 2PCl 5State and explain the effect, if any, on the position of equilibrium if the pressure is increased. All other conditions are unchanged. Phosphine, PH3, is produced by the reaction between water and calciumphosphide, Ca3P2. Balance the chemical equation for this reaction. Ca3P2 + H2O Ca(OH)2 + PH3 The phosphonium ion, PH4 +, is similar to the ammonium ion. State the formula of the ammonium ion. ����������������������������������������������������������������������� Suggest the formula of phosphonium iodide. ��������������������������������������������������������������� Calciumphosphate contains the phosphate ion, PO4 3–. What is the formula of calciumphosphate? Phosphorus forms another compound with hydrogen with the following composition by mass: P, 93.94%; H, 6.06%. Calculate the empirical formula of the compound.  empirical formula = The compound has a relative molecular mass of 66. Deduce the molecular formula of the compound.  molecular formula = 
0620_w19_qp_43
THEORY
2019
Paper 4, Variant 3
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