Science 1206 Chemistry Unit #10

Transcription

1 Science 1206 Chemistry Unit #10 Introduction to Chemical Equations Communicating Chemical Changes Chemical change occurs when matter changes into a new and different form. The new form has a new make up and a new set of physical properties. For instance when an electric current is passed though liquid water, the water changes into two gasses which are highly flammable; hydrogen and oxygen. Chemical changes involve breaking old chemical bonds and forming new chemical bonds. During a chemical reaction, the atoms are neither created nor destroyed. All that really happens to the atoms is that they are rearranged. In this example the bonds in the water molecule are broken and rearranged to form hydrogen and oxygen. The number of hydrogen and oxygen don't change, all that changes is the way they are bonded. The old substances whose bonds are broken are called reactants, and the new substances that are formed as a result of the reaction are called products. (products are produced) 1

2 Parts of a Chemical Equation In a chemical equation, the reactants are listed on the left side of an arrow symbol, and the products are listed on the right side. Chemical equations have four parts: chemical formulas subscripts for states of matter (s) solid (l) liquid (g) gas (aq) aqueous - dissolved in water numerical coefficients indicates how many atoms/molecules are involved reaction symbols the "+" sign on the reactants (left) side is read as "reacts with" the arrow ( ) is read as "to produce" the "+" sign on the products (right) side is read as "along with". Parts of a Chemical Equation (example) Two molecules of diesel fuel reacts with 49 molecules of oxygen to produce 32 molecules of carbon dioxide along with 34 molecules of water. 2

3 Evidence for Chemical Change When substances actually undergo changes, how do you know if the changes are chemical changes rather than physical changes? Chemical changes involve changes in make up - new substances are formed with new properties. Physical changes involve changes in state without a change in make up. There are four main observations to indicate a chemical change: energy change: the absorption or the release of heat and/or light, sound, electrical energy. exothermic reactions release energy (exo = out) endothermic reactions absorb heat (endo = in) colour change: The formation of a substance whose colour is quite different from the colour of the reactants precipitate formation: in chemistry the word precipitate refers to the formation of solid that settles out of a solution. gas formation: the formation of a gas, gas bubbles etc. sometimes results in a substance with a different odour. You must be careful in how you interpret these changes since these observations alone do not always indicate a chemical reaction. Conservation of Mass The Law of Conservation of Mass: In a chemical reaction the mass of the reactants before a chemical reaction equals the mass of the products after the reaction is complete. mass of reactants = mass products In other words, the mass you end up with equals the mass you started with. 3

4 Balancing Chemical Equations The law of conservation of mass states that during a chemical change or reaction, the mass of the reactants is equal to the mass of the products. Any representation of a chemical change must reflect this law. In order to write a chemical equation obeys the law of conservation of mass, the number of atoms of each element must be the same on both sides of the equation. Balancing Equations (example 1) There is one magnesium and one sulfur on each side of the equation. Balanced! 4

5 Balancing Equations (example 2) If a chemical equation is not balanced, then coefficients have to be placed in front of the chemical formulas in order to balance the equation. Balancing chemical equations with polyatomic ions Equations that involve polyatomic ions at first glance seem difficult to balance. However, if we treat the polyatomic ion as unit it usually works out ok. 5

6 Balancing chemical equations with polyatomic ions (example) KI + Pb(NO 3 ) 2 KNO 3 + PbI 2 consider the nitrate as a single unit, since there are two on the left Pb(NO 3 ) 2 place a 2 in front of KNO 3 KI + Pb(NO 3 ) 2 2 KNO 3 + PbI 2 next balance K, we now have two K on the right 2 KNO 3 place a 2 in front of KI 2 KI + Pb(NO 3 ) 2 2 KNO 3 + PbI 2 Look at the I, ok 2 on each side Look at Pb, ok, one on each side 2 KI + Pb(NO 3 ) 2 2 KNO 3 + PbI 2 Check: Left 2 K, 2 I, 1 Pb, 2 NO 3 Right 2 K, 2 I, 1 Pb, 2 NO 3 How to properly balance equations Balancing equations is an important skill to master. It will take patience and practice. Here are some useful tips that may help you master the skill: list the symbols for atoms or ions that appear on both sides of the chemical equation. treat a polyatomic ion as unit if it appears on both sides of the equation. In other words, do not separate it into its individual atoms if it appears on both sides of the equation. count and compare the numbers of atoms and/or ions of each type listed on the two sides of the equation use coefficients to balance the number of atoms or ions and do a recount of the atoms and/or ions. Equations are balanced using coefficients only. Never balance an equation by changing numerical subscripts. 6