On thi tot nghiep THPT mon Hoa nhu the nao
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| Trao doi sau gio thi tot nghiep THPT nam 2005 |
Thu 1: - Hoc theo sach giao khoa (SGK). - Sach bai tap cua Bo GD-DT: Cac phan ung nen viet don gian nhu SGK.
Vi du: Phan ung trang guong viet theo nhieu cach nhung de phuc vu ky thi, hoc sinh chi can viet don gian:
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Vua de nho, vua phu hop voi dap an de thi.
Thu 2: - Hoc that thuoc ten va cong thuc neu trong sach giao khoa. - Hoc thuoc cac phan ung dac trung cho moi loai nhom chuc la rat can thiet nhung chua du...
Vi du: - Khi de thi cho viet phuong trinh phan ung anilin, phenil amoni clorua phan ung voi dd Br2. Neu hoc theo SGK, hoc sinh chi tra loi duoc anilin + dd Br2 tao ket tua trang. Con de tra loi phenil amoni clorua phan ung duoc voi dd Br2 hay khong, ta can hieu ve hieu ung dien tu (voi hoc sinh gioi moi tra loi duoc vi phan ung nay khong co trong SGK).
- Cung loai cau hoi nhu tren neu cho phenol tac dung voi Na, NaOH thi dung SGK con phenol tac dung voi C2H5OH, Na2CO3. - SGK khong he de cap co phan ung hay khong?
- Vi vay hoc thuoc theo SGK la chua du ma phai luyen tap theo sach bai tap.
+ Cac dinh nghia, nguyen tac, phuong phap, hoc sinh phai hoc ky. Khong duoc thieu sot.
Vi du: - Nuoc cung la nuoc chua "nhieu" ion Ca2+, Mg2+ neu thieu tu "nhieu" co the mat tron so diem vi nuoc chua "it" ion Ca2+, Mg2+ la nuoc mem.
+ Ve an mon kim loai kieu an mon dien hoa.
Vi du: Zn - Cu dd chat dien ly.
- Cuc am luon xay ra qua trinh : Zn - 2e --> Zn2+.
- Cuc duong: 2H+ + 2e --> H2 (dd axit).
2H2O + O + 4e --> 4OH- (dd trung hoa, hoa tan O2)
+ Khi viet phuong trinh phan ung, hoc sinh phai nho: Can bang phan ung, du dieu kien. Nho cac san pham phu (TD: H2O). Kinh nghiem cham thi cho thay nhieu hoc sinh mat diem mot cach dang tiec o diem nay.
Thu 3: Bai toan Hoa thi tot nghiep trung hoc thuong rat don gian. Tuy nhien cung can luu y:
- Neu bai toan huu co nen doc ky de bai de dat cong thuc chung cho chinh xac. - Rat nhieu hoc sinh thuong mac sai lam o phan nay.
Vi du: Khi de thi cho andehit don chuc, khong it hoc sinh viet la CnH2n+1CHO nghia la tu gan them cho andehit mot thuoc tinh la no (mach ho) trong khi viet dung phai la CxHyCHO (voi y <= 2x + 1) hay CnH2n + 1-2k CHO.
Voi cong thuc chat huu co, ngoai viet dung con phai viet hop ly phu hop voi gia thiet.
Vi du: Mot axit don chuc khi gia thiet cho phan ung voi NaOH nen dat cong thuc RCOOH hay CxHyCOOH. Con khi tham gia phan ung dot chay nen dat CxHyO2 hay Cx"Hy"COOH.
Cach viet cong thuc hop ly giup ta can bang phan ung de dang va giai bai toan cung duoc gon nhe hon nhieu.
Voi hoc sinh yeu kem khong co du kha nang lam het bai toan thi co gang lam nhung phan minh lam duoc vao giay.
Vi du: Tinh so mol cac chat, khoi luong phan tu...
- Neu la bai toan vo co ve loai lien quan den:
Kim loai tac dung voi dd kiem trong chuong trinh 12 chi co 3 kim loai Al, Zn, Be. Trong tam la Al. Ta nen thuoc luon phan ung ca he so hop thuc:
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Khong it hoc sinh viet ve 1 khong co H2O roi can bang khong duoc dua den sai ca bai toan.
- Muoi Al3+ tac dung voi dd NaOH tao ket tua.
- Muoi AlO2- tac dung voi dd HCl tao ket tua.
- Khi CO2 tac dung voi dd Ca(OH)2 hay Ba(OH)2 tao ket tua. Hoc sinh luu y co bien luan hay khong?
Phan ung nhiet nhom hoan toan hay khong hoan toan de xac dinh hon hop ran sau phan ung co nhung chat gi? Dau hieu nao de xac dinh Al du...
Khai niem "phan ung hoan toan", "phan ung ket thuc", "hieu suat phan ung 100%" dieu do noi len hai chat tham gia phan ung hoac vua het hoac neu du thi chi du mot chat. Neu khong co 3 khai niem tren thi 2 chat ban dau co the cung du.
Luu y sau cung:
Nhung ngay gan thi hoc sinh nen danh thi gio tu hoc, tu kiem tra, tong ket bai thanh mot dan y ngan gon de nho roi tu do phat trien thanh mot cau tra loi hoan chinh. Neu hoc sinh co ban hoc cung trinh do kiem tra lan nhau thi rat tot vi ca hai cung nho bai. Hon nua viec hoi dap cung giup cac em luyen cach trinh bay van de, hieu bai sau hon, rat co loi khi trinh bay bai thi.
Chuc cac em mot mua thi nhu y.
PHAM THI MINH NGUYET
(Giao vien Hoa 10, 11, 12 va luyen thi DH)
Theo Thanh Nien
